Epidemiology:

Because P. hominis is not known to have a cyst stage, transmission probably occurs in the trophic form. If

ingested in a substance such as milk, these organisms apparently can survive passage through the stomach and

small intestine in patients with achlorhydria.

Pathogeneis and Spectrum of Disease:

P. hominis is considered nonpathogenic and does not cause disease.

Laboratory Diagnosis:

P. hominis trophozoites can sometimes be seen on a permanent stained smear, but definitive identification can

be difficult. However, it is important to report the presence of the organism if seen.

Therapy:

Specific treatment is not recommended for this nonpathogen.

Prevention:

Prevention depends on adequate disposal of human excreta and improved personal hygiene, preventive

measures that apply to most of the intestinal protozoa.

CILIATES:

The class Ciliata, or ciliates, includes species that move by means of cilia, or short extensions of cytoplasm that

cover the surface of the organism. The ciliates also have two different types of nuclei, one macronucleus and

one or more micronuclei. This group includes only one organism that infects humans, Balantidium coli, which

infects the intestinal tract and may produce severe symptoms.

Balantidium Coli:

General Characteristics:

The life cycle of B. coli includes both the trophozoite and cyst stages (Figures 16).

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 The cyst form is the infective stage. After ingestion of the cysts and excystation, trophozoites secrete

hyaluronidase, which aids the invasion of the colonic tissue.The trophozoite is quite large, oval, and covered

with short cilia. It measures approximately 50 to 150 μm long

and 40 to 70 μm wide. The organism can be seen wet preparation on lower power. The anterior end is

somewhat pointed and has a cytostome (primitive mouth opening); in contrast, the posterior end is broadly

rounded. The cytoplasm contains many vacuoles with ingested bacteria and debris. The trophozoite has two

nuclei: one very large, bean-shaped macronucleus and a smaller, round micronucleus. The organisms live in the

large intestine. The trophozoites have a rapid, rotatory, boring motion because of the movement of the cilia.

The cyst is formed as the trophozoite moves down the intestine. Nuclear division does not occur in the cyst;

therefore, only two nuclei are present, the macronucleus and the micronucleus. The cysts measure 50 to 70 μm

in diameter.

Epidemiology:

B. coli is widely distributed in hogs, particularly in warm and temperate climates, and in monkeys in the

tropics. Human infection is found in warmer climates, sporadically in cooler areas, and in institutionalized

groups with low levels of personal hygiene.

Pathogenesis and Spectrum of Disease:

Some individuals with B. coli infection are asymptomatic, whereas others have severe dysentery, similar to that

seen in patients with amebiasis. Symptoms include diarrhea or dysentery, tenesmus, nausea, vomiting, anorexia,

and headache. Insomnia, muscular weakness, and weight loss also have been reported. Diarrhea may persist for

weeks to months, with or without subsequent development of dysentery. Tremendous fluid loss may occur,

with diarrhea similar to that seen in cholera or in some coccidial or microsporidial infections. B. coli can invade

tissue. It may penetrate the mucosa on contact, with cellular infiltration in the area of the developing ulcer.

Some of the abscess formations may extend to the muscular layer. The ulcers may vary in shape, and the ulcer

bed may be full of pus and necrotic debris. Although the number of cases is small, extraintestinal disease

(peritonitis, urinary tract infection, inflammatory vaginitis) has been reported

Figure 16 A, Balantidium coli trophozoite. B, B. coli trophozoite.

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Laboratory Diagnosis:

Routine stool examinations, particularly wet preparation examinations of fresh and concentrated material,

demonstrate the presence of organisms. Organism recognition and identification on a permanent stained smear

is usually difficult. These protozoa are large and stain very darkly, which obscures any internal morphology. B.

coli organisms may be confused with helminth eggs or debris because of their size.

Therapy:

Tetracycline is the drug of choice for treating B. coli infection, although it is considered investigational for this

infection. Iodoquinol or metronidazole may be used as an alternative. Nitazoxanide, a broad-spectrum

antiparasitic drug, may be another alternative.

Prevention

In areas where pigs are raised, the incidence of human infection can be quite high in pig farmers and

slaughterhouse workers. Human infection is fairly rare in temperate areas, although infections can develop into

an epidemic, particularly in areas of poor environmental sanitation and personal hygiene. This situation has

been seen in mental hospitals in the United States. Preventive measures involve increased attention to personal

hygiene and sanitation measures, because the mode of transmission is ingestion of infective cysts through

contaminated food or water.

Sporozoa (APICOMPLEXA):

 All the Apicomplexa are unicellular and have an apical complex. These structures can be seen in electron

microscopy studies and are used to help classify the various organisms. Genera that develop in the

gastrointestinal tract of vertebrates throughout their entire life cycle include Isospora, Cyclospora, and

Cryptosporidium. Genera capable of or requiring extraintestinal developmentare referred to as cyst-forming

coccidia; they include Sarcocystis and Toxoplasma spp. The genera that cause disease in humans include

Cryptosporidium, Cyclospora, Isospora, Sarcocystis, and Toxoplasma .

Cryptosporidium SPP.:

General Characteristics

Cryptosporidium spp. are intracellular parasites that primarily infect epithelial cells of the stomach, intestine,

and biliary ducts. The organism previously called Cryptosporidium parvum, thought to be the primary

Cryptosporidium species infecting humans, now is classified as two species, C. parvum (mammals, including

humans) and C. hominis (primarily humans). Differentiation of these two species based on oocyst morphology

is not possible. Currently, more than 20 established Cryptosporidium species have been identified in

vertebrates, and more than 10 Cryptosporidium spp. have been reported in humans. Cryp

Cryptosporidium species infecting humans, now is classified as two species, C. parvum (mammals, including

humans) and C. hominis (primarily humans). Differentiation of these two species based on oocyst morphology

is not possible. Currently, more than 20 established Cryptosporidium species have been identified in

vertebrates, and more than 10 Cryptosporidium spp. have been reported in humans. Cryptosporidium infections

begin with ingestion of viable oocysts. Upon contact with gastric and duodenal fluid, each oocyst releases four

sporozoites, which invade.

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Figure 17 Life cycle of Cryptosporidium. (a) Sporulated oocyst infeces. (b) Excystation in intestine. (c) Free

sporozoite in intestine. (d)Type I meront (six or eight merozoites). (e) Recycling of type I merozoite.(f) Type II meront

(four merozoites). (g) Microgametocyte withapproximately 16 microgametes. (h) Microgamete fertilizes

macrogamete(i) to form zygote (j). Approximately 80% of the zygotes form thick-walled oocysts (k), which sporulate

within the host cell. About 20% of the zygotes do not form an oocyst wall; their sporozoites are surrounded only by a

unit membrane (l). Sporozoites in “autoinfective,” thin-walled oocysts (l) are released into the intestinal lumen (m) and

reinitiate the endogenous cycle (at c).

Figure 18 Cryptosporidium. A, Oocysts recovered from a Sheather’s sugar flotation; organisms measure 4 to

6 μm. B, Scanning electron microscopy view of organisms at brush border of epithelial cells.

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the epithelial cells and develop into trophozoites surrounded by a parasitophorous vacuole (layers of

endoplasmic reticulum around an intracellular parasite). In the epithelial cells, trophozoites undergo two or

three generations of asexual amplification, called merogony, leading to the formation of different types of

meronts containing four to eight merozoites. The merozoites differentiate into sexually distinct stages in a

process called gametogony. New oocysts are then formed in the epithelial cells in a process called sporogony.

About 20% of the oocysts are thin walled and may excyst in the digestive tract of the host, leading to the

infection of new cells (autoinfection). The remaining 80% of the oocysts are excreted into the environment; are

resistant to low temperature, high salinity, and most disinfectants; and can initiate infection in a new host.

Cryptosporidium oocysts in humans measure 4 to 6 μm in diameter.

Epidemiology:

Humans can acquire cryptosporidiosis through several transmission routes, such as direct contact with infected

people or animals or consumption of contaminated water (drinking or recreational) or food. The interval

between ingestion of infective oocysts to completion of the life cycle and excretion of new oocysts usually is 4

to 10 days. The only extracellular stage in the Cryptosporidium life cycle is the oocysts; these are the

environmental stage of the parasite and are immediately infectious when passed in the stool.

Cryptosporidiosis is common in immunocompromised individuals, such as those with AIDS or primary

immunodeficiency and cancer and transplant patients undergoing immunosuppressive therapy.

Pathogenesis and Spectrum of Disease:

Immunocompetent Individuals: Clinical symptoms include nausea, low-grade fever, abdominal cramps, anorexia, and five

to 10 watery, frothy bowel movements per day, which may be followed by constipation. Some patients may have diarrhea,

and others may have few symptoms, particularly later in the course of the infection.

Immunocompromised Individuals. Hemodialysis patients with chronic renal failure and renal transplant patients

Figure 19 Cryptosporidium oocysts and Giardia cysts stained with monoclonal antibody–conjugated

fluorescent reagent.

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with cryptosporidiosis can have chronic, life-threatening diarrhea. In individuals infected with the human

immunodeficiency

virus (HIV), cryptosporidiosis increases as the CD4+ lymphocyte count falls, Cryptosporidium infections are not always

confined to the gastrointestinal tract; additional symptoms (respiratory problems, cholecystitis, hepatitis, and pancreatitis)

have been associated with extraintestinal infections.

Laboratory Diagnosis:

Oocysts in clinical specimens are difficult to see without special staining techniques, such as the modified acid-fast,

Kinyoun’s, or Giemsa method, or the newer immunoassay methods. The four sporozoites may be seen in the oocyst wall

in some of the organisms, although they are not always visible in freshly passed specimens.

Antigen Detection.Immunoassays are very helpful, because they are a more sensitive method of detecting organisms in

stool specimens. A direct fluorescent antigen (FA) procedure with excellent specificity and sensitivity has been developed

and results in a significantly increased detection rate over conventional staining and microscopy methods. Some of these

reagents, particularly the combination direct FA product used to identify both Giardia spp. cysts and Cryptosporidium

spp. oocysts, are being widely used in water testing and outbreak situations. Most antibodies in commercial direct

fluorescent antibody (DFA) kits react with oocysts of almost all Cryptosporidium species, making identification to the

species level impossible. Enzyme immunoassay (EIA) tests also provide excellent specificity and sensitivity for

laboratories using this approach, as do the immunochromatographic cartridge rapid test formats. It is important to

remember that if a patient is in the carrier state or undergoing self-cure, the number of oocysts may drop below the

sensitivity levels of these kits, producing a false-negative result.

Nucleic Acid-Based Methods. Molecular techniques, especially polymerase chain reaction (PCR) and PCRrelated

methods, have been used to detect and differentiate Cryptosporidium spp., and a few of the PCR assays are commercially

available. Several genus-specific PCRrestriction fragment length polymorphism–based genotyping tools have been

developed for detecting and differentiating Cryptosporidium organisms at the species level. Other genotyping techniques

are designed mostly for differentiation of C. parvum and C. hominis and cannot detect and differentiate other

Cryptosporidium spp. Or genotypes.

Histology.In the examination of histologic preparations, developmental stages (sporozoites, trophozoites, merozoites, and

oocysts) in the life cycle of Cryptosporidium spp. can be found at all levels of the intestinal tract, with the jejunum being

the most heavily infected site. Routine hematoxylin and eosin staining is sufficient to demonstrate these parasites. Under

regular light microscopy, the organisms are visible as small, round structures (about 1 to 3 μm in diameter) aligned along

the brush border. They are intracellular but extracytoplasmic and are found in parasitophorous vacuoles. In clean wet

mounts, Cyclospora organisms are seen as nonrefractile spheres, which are difficult to recognize as parasites. Unless a

high number of oocysts are present, they may easily be mistaken for artifacts. They are acidfast variable with the modified

acid-fast stain; those that are unstained appear as glassy, wrinkled spheres (wrinkledcellophane). The oocysts are twice the

size of those of Cryptosporidium spp. and measure 8 to 10 μm in diameter. Because it takes 10 days to 2 weeks for the

oocysts to sporulate, no internal structures are visible (sporozoites), as can be seen in Cryptosporidium organisms.

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Therapy:

 Oral or intravenous rehydration and antimotility drugs are used whenever severe diarrhea is associated with

cryptosporidiosis. Nitazoxanide is the only drug approved by the U.S. Food and Drug Administration (FDA) for the

treatment of cryptosporidiosis in immunocompetent individuals. This drug can shorten the clinical disease and reduce the

number of parasites present.

Cyclospora cayetanensis

 These organisms are acid-fast variable and have been found in the feces of immunocompetent travelers to developing

countries, immunocompetent individuals with no travel history, and patients with AIDS.

The life cycle of C. cayetanensis involves only humans as hosts. Oocysts are passed in the feces unsporulated , At room

temperature (23° to 25°C), small numbers of oocysts may sporulate within 10 to 12 days. In clean wet mounts,

Cyclospora organisms are seen as nonrefractile spheres, which are difficult to recognize as parasites. Unless a high

number of oocysts are present, they may easily be mistaken for artifacts. They are acidfast variable with the modified

acid-fast stain; those that are unstained appear as glassy, wrinkled spheres (wrinkled cellophane). The oocysts are twice

the size of those of Cryptosporidium spp. and measure 8 to 10 μm in diameter. Because it takes 10 days to 2 weeks for the

oocysts to sporulate, no internal structures are visible (sporozoites), as can be seen in Cryptosporidium organisms.

Epidemiology:

Transmission of C. cayetanensis is thought to be by the fecal-oral route. However, direct person-to-person transmission

has not been well documented and may not be a factor, because sporulation takes a number of days. Outbreaks linked to

contaminated water and various types of fresh produce (raspberries, basil, baby lettuce leaves, and snow peas) have been

reported. Information on reservoir hosts is not well defined; however, in some areas humans appear to be the only host.

Pathogenesis and Spectrum of Disease:

Although some patients are asymptomatic, others report a flulike illness, marked by nausea, vomiting, anorexia, weight

loss, and explosive diarrhea lasting 1 to 3 weeks.

The incubation period is not yet known. However, the onset of symptoms after infection generally averages 7 to 8 days,

and the symptoms last 2 to 3 weeks. Oocyst shedding in the feces is highly variable and may range from 7 days to several

months. Indigenous infections are confined primarily to tropical, subtropical, or warm temperate regions of the world.

Outbreaks occur in other areas of the world as a result of contaminated

foodstuffs.

In immunocompromised and immunocompetent patients, C. cayetanensis infection can be associated with biliary disease.

With light and transmission electron microscopy developmental stages have been seen in the gallbladder epithelium of

AIDS

Laboratory Diagnosis:

Special Stains. With modified acid-fast stains, the oocysts appear light pink to deep red, and some contain granules or

have a bubbly appearance (described as wrinkled cellophane).

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Flow Cytometry. Flow cytometry is another diagnostic option. This approach appears to be a useful alternative to

microscopy, particularly for screening large numbers of clinical specimens for Cyclospora oocysts in an outbreak

situation. However, it is not commonly used. Other Diagnostic Methods. Although culture, antigen detection, nucleic acid

detection, and serologic tests for antibody have been developed, none of these methods is routinely available for most

clinical laboratories.

Therapy:

Patients have been treated symptomatically with antidiarrheal preparations and have obtained some relief; however, the

disease appears to be self-limiting within a few weeks. Trimethoprim (TMP-SMX), currently the drug of choice, is given

orally twice daily for 7 days. Elimination of parasites, a decrease in diarrhea, and diminished abdominal pain occur within

2 to 3 days after treatment. Patients with AIDS may need higher doses and long-term maintenance treatment. However,

more than 40% of patients have a recurrence of symptoms in 1 to 3 months after treatment.

Prevention:

Individuals in endemic areas should wear gloves when gardening to prevent exposure to oocysts of C. cayetanensis

Thorough washing of produce may help remove oocysts. Most of the produce items implicated in the transmission of C.

cayetanensis are consumed raw; thus cooking as a means of prevention is not relevant.

ISOSPORA (CYSTOISOSPORA) BELLI

General Characteristics:

 Although isosporiasis is found worldwide, certain tropical areas in the Western Hemisphere have specific

locations where endemic infections occur. These organisms infect both adults and children, and intestinal

involvement and symptoms are generally transient unless the patient is immunocompromised. I. belli has also

been implicated in traveler’s diarrhea. However, unlike with Cryptosporidium spp. and C. cayetanensis, large

outbreaks of isosporiasis have not been reported I. belli oocysts are passed in the stool. They are long and

oval, measuring 20 to 33 μm long by 10 to 19 μm wide. Usually the oocyst contains one immature sporont, but

two may be present. Continued development occurs outside the body, with the development of two mature

sporocysts, each containing four sporozoites, which can be recovered from the fecal specimen. The sporulated

oocyst is the infective stage that excysts in the small intestine, releasing the sporozoites, which penetrate the

mucosal cells and initiate the life cycle.

Epidemiology

I. belli oocysts are passed in the feces unsporulated or partially sporulated. Oocysts complete sporulation

within72 hours, although it may take longer, depending on the temperature. The time required for unsporulated

oocysts to appear in the feces after ingestion of sporulated oocysts is 9 to 17 days. Oocyst shedding is variable

and depends on the immune status of the infected individual. Oocysts can be found for 30 to 50 days in

immunocompetent patients, and immunosuppressed patients may continue to shed oocysts for 6 months or

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longer. Chronic infections can occur, and oocysts can be shed for months to years. In one particular case, an

immunocompetent individual had symptoms for 26 years, and I. belli was recovered in stool a number of times

over 10 years. I. belli is thought to be the only species of Isospora that infects humans, and no other reservoir

hosts are recognized

for this infection. Transmission occurs through ingestion of water or food contaminated with mature, sporulated

oocysts. Sexual transmission by direct oral contact with the anus or perineum also occurs, although this mode

of transmission is probably much less common.

 The oocysts are very resistant to environmental conditions and may remain viable for months if kept cool and

moist; oocysts usually mature within 48 hours after stool passage and are then infectious. Pathogenesis and

Spectrum of Disease Symptoms include diarrhea (most common), weight loss, abdominal colic, and fever.

Stools (usually six to 10 per day) are watery to soft, foamy, and offensive smelling, suggesting a malabsorption

process. Many patients have eosinophilia, recurrences are quite common, and the disease is more severe in

infants and young children. Patients who are immunosuppressed, particularly those with AIDS, often present

with profuse diarrhea associated with weakness, anorexia, and weight loss. Biopsies reveal an abnormal mucosa

with short villi, hypertrophied crypts, and infiltration of the lamina propria with eosinophils, neutrophils, and

round cells. Physicians should consider I. belli in AIDS patients with diarrhea who have immigrated from or

traveled to Latin America, are Hispanics born in the United States, are young adults, or who have not received

prophylaxis with TMP-SMX for Pneumocystis infection. It has also been recommended that patients with AIDS

traveling to Latin America and other developing countries be advised of the waterborne and food-borne

transmission of I. belli and that chemoprophylaxis should be considered.

Extraintestinal infections in patients with AIDS have been reported. At autopsy, microscopic findings

associated with I. belli infection were seen in the lymph nodes and walls of the small and large intestines,

mesenteric and mediastinal lymph nodes, lymphatic channels, liver, and spleen. I. belli infections in the

gallbladder epithelium and endometrial epithelium have also been reported, and oocysts have been recovered in

bile specimens.

Laboratory Diagnosis:

 Examination of fresh material, either as the direct smear or as concentrated material, is recommended rather

than the permanent stained smear. The oocysts are very pale and transparent and can easily be overlooked. The

light level should be reduced, and additional contrast should be obtained with the microscope for optimal

examination conditions. On the permanent stained smear, the organisms may take up excess stain and resemble

helminth eggs or artifacts.

 It is possible to have a positive biopsy specimen but not recover the oocysts in the stool because of the small

numbers of organisms present. The oocysts are acid-fast and can also be demonstrated by using auramine

rhodamine stains. Organisms tentatively identified by using auramine rhodamine stains should be confirmed by

wet smear examination or acid-fast stains, particularly if the stool contains other cells or excess artifact material

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(more normal stool consistency). Currently, there are no commercially available nucleic acid-based methods for

the detection of I. belli. However, PCR assays have been developed for the detection of the organism in stool

samples.

Histology:

Developmental stages of I. belli have been reported for intestinal biopsy specimens of the duodenum, jejunum,

and occasionally ileum. Intestinal development tends to occur in epithelial cells, although developing stages are

occasionally reported from the lamina propria or submucosa. Extraintestinal infections in patients with AIDS

have been reported; the organisms become dormant as cysts in a variety of tissues, including the intestine,

mesenteric lymph nodes, liver, and spleen;

these cysts are called unizoite cysts. In histologic sections, these cysts are thick walled and measure 12-22 × 8-

10 μm, and each contains a single dormant sporozoite/merozoite of about 8-10 × 5 μm. As immunity declines,

these cysts can reactivate patent infections.

Therapy:

The drug of choice to treat I. belli infection is trimethoprimsulfamethoxazole, which is given two to four times

a day for 10 to 14 days. With this approach, the parasites are

eliminated, the diarrhea stops, and the abdominal pain decreases within a few days.

Prevention:

Because transmission occurs through the infective oocysts, prevention includes improved personal hygiene

measures and sanitary conditions to eliminate possible fecal-oral transmission from contaminated food, water,

and possibly environmental surfaces.

Sarcocystis spp.:

General Characteristics:

 Two well-described Sarcocystis species include S. bovihominis (cattle) and S. suihominis (pigs). (Some

publications refer to S. bovihominis as S. hominis.) When uncooked meat from these infected animals is

ingested by humans, gamogony (fission resulting in the production of sporozoan gametes) can occur in the

intestinal cells, with eventual production of the sporocysts in stool. Sarcocystis spp. have an obligatory twohost life cycle. Intermediate hosts (herbivores and omnivores) become infected through ingestion of sporocysts

excreted in the feces of the definitive hosts (carnivores and omnivores).

The definitive hosts become infected through ingestion of mature cysts found in the muscles of the intermediate

hosts. In some intermediate hosts, such as cattle and sheep, all adult animals may be infected. Extraintestinal

human sarcocystosis is rare, with a much lower incidence than is seen with the intestinal infection. Humans

who have ingested meat containing the mature sarcocysts serve as the definitive hosts. Fever, severe diarrhea,

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abdominal pain, and weight loss have been reported in immunocompromised hosts, although the number of

patients with these symptoms has been quite small.

 The sporocysts found in the stool are broadly oval and slightly tapered at the ends. They measure 9 to 16 μm

long and contain four mature sporozoites and the residual body . Normally, the oocyst contains two sporocysts

(similar to I. belli); however, in Sarcocystis infections, the sporocysts are released from the oocyst and normally

are seen singly. These sporocysts tend to be larger than Cryptosporidium oocysts that contain four sporozoites,

so they look totally different. The oocysts are fully sporulated when passed in the stool.

Pathogenesis and Spectrum of Disease:

 When humans (intermediate host) ingest oocysts from other animal stool sources, the sarcocysts that develop

in human muscle are 7 to 16 μm long and cause few, if any, problems. Basically, no inflammatory response to

these organisms occurs in the muscle, and no evidence of pathogenicity is seen. Patients demonstrate symptoms

related to the disintegration of the sarcocysts and death of intracystic bradyzoites. Painful muscle swellings

measuring 1 to 3 cm in diameter are associated with erythema of the overlying skin; these occur periodically

and last 2 days to 2 weeks. Symptoms also include fever, diffuse myalgia, muscle tenderness, weakness,

eosinophilia, and bronchospasm. Different types of skeletal and cardiac muscle sarcocysts have been found in

humans. No specific therapy is required for this type of infection. Corticosteroids can reduce allergic

inflammatory reactions.

Infections in humans can manifest primarily as intestinal disease if infected meat is ingested or as muscular

disease if sporocysts are ingested. Intestinal disease occurs within a few hours after consumption of infected

meat and is characterized by nausea, abdominal pain, and diarrhea. However, in both situations patients may be

infected and asymptomatic.

Laboratory Diagnosis:

 A presumptive diagnosis of intestinal disease may be based on the patient’s symptoms, particularly with

documented ingestion of raw or poorly cooked meat. Confirmation of the diagnosis may depend on finding

human fecal specimens containing sporocysts, which are passed in the stool 11 to 18 days after ingestion of

beef or pork.

 Sporocysts of the two Sarcocystis species are very difficult to differentiate. A muscle biopsy is appropriate for

suspected symptomatic intramuscular infection in a patient with a history of travel to or residence in a tropical

location. Sarcocysts in biopsy specimens can be identified by microscopy on routine histologic sections stained

with hematoxylin and eosin. Most sarcocysts in humans have been found inskeletal and cardiac muscle

however, muscles in the larynx, pharynx, and upper esophagus have also been involved. No molecular assays

are currently available for the detection of sarcocystis in humans. However, several amplification methods have

been used to detect sarcocystis in intermediate hosts.

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Therapy:

No known treatment or prophylaxis is available for intestinal infection, myositis, vasculitis, or related lesions

caused by human sarcocystosis. Supportive therapy for patients with severe diarrhea is indicated. It is also

unclear whether immunosuppressives are effective at reducing the inflammatory reactions seen in vasculitis or

myositis. Without more definitive data, no course of therapy currently can be recommended.

Prevention:

Cooking meat to an internal temperature higher than 67°C kills Toxoplasma gondii tissue cysts in meat; this

temperature should also kill Sarcocystis tissue cysts in meat.

Preventing cattle, buffalos, and swine from consuming human feces shedding infective oocysts also prevents

animal infection. Most cases of human muscular Sarcocystis

infection have been reported from the Far East. When humans are intermediate hosts, preventive measures

involve careful disposal of animal feces that may contain the infective sporocysts. This may be impossible in

wilderness areas, where wild animals may serve as reservoir hosts for many Sarcocystis spp.

Microsporidia:

 Microsporidia are obligate intracellular, spore-forming parasites. More than 140 microsporidial genera and

1200 species have been identified. To date, seven genera (Anncaliia, Encephalitozoon, Enterocytozoon,

Nosema, Pleistophora, Vittaforma, and Trachipleistophora) and unclassified microsporidia (Microsporidium)

have been identified as causing human infections.

 Although the microsporidia are true eukaryotes, they also have molecular and cytologic characteristics of

prokaryotes. Microsporidia evolved from the fungi and are most closely related to the Zygomycetes. Features

shared with fungi include the presence of chitin and trehalose, similarities in cell cycles, and certain gene

organizations.

 Microsporidia are now considered highly derived fungi that underwent genetic and functional losses, resulting

in one of the smallest eukaryotic genomes known.

However, the life cycle of microsporidia is unique and unlike that of any fungal species. At this point, clinical

and diagnostic issues and responsibilities may remain with the parasitologists, and we may be in a transition

stage, similar to that seen with Pneumocystis jirovecii as it was moved from the parasites to fungi in terms of

classification status.

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General characteristics:

 Human microsporidial infections have been documented worldwide. The spore, the only life cycle stage able

to survive outside the host cell, is the infective stage (see Figures 20).

Infection occurs with ingestion or inhalation of the infective spores, from which the infective sporoplasm (spore protoplasm)

enters the host cell through the polar tubule. The microsporidia multiply extensively in the host cell cytoplasm; the life

cycle includes repeated divisions by binary fission (merogony) or multiple fission (schizogony) and spore

production (sporogony). Both merogony and sporogon

can occur in the same cell at the same time. During sporogony a thick spore wall is formed, providing environmental

protection for the spore. Microsporidial spores measure 0.7 to about 4 μm in diameter. Mature spores contain a tubular

extrusion apparatus (polar tube or tubule) for injecting infective spore contents (sporoplasm) into the host cell.

Epidemiology:

Transmission possibilities include human-to-human and animal-to-human routes. Many questions relating to

reservoir hosts and possible congenital infections are still unanswered. Primary infection occurs through

inhalation or ingestion of spores from environmental sources or by zoonotic transmission. The presence of

Encephalitozoon intestinalis has been confirmed in tertiary sewage effluent, surface water, and groundwater;

Figure 20 Life cycle diagram of the microsporidia. A to G, Asexual development of sporoblasts. H, Release of spores.

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Enterocytozoon bieneusi has been confirmed in surface water; and Vittaforma corneae has been confirmed in

tertiary effluent.

This study represents the first confirmation, to the species level, of human-pathogenic microsporidia in water,

indicating that these parasites are probably waterborne pathogens. Ingestion of the environmentally highly

resistant spores is probably the normal mode of

transmission. E. bieneusi, an intestinal pathogen, serves as an example of infection potential. The spores are

released into the intestinal lumen and are passed in the stool. These spores are environmentally resistant and can

be ingested by other hosts. Zoonotic transmission of microsporidia infecting humans has not been verified but

appears likely, because many microsporidial species can infect both humans and animals.

Pathogenesis and spectrum of disease:

 Microsporidia were recognized as causing disease in animals as early as the 1920s but were not recognized as

agents of human disease until the AIDS pandemic began in the mid-1980s. Before then, several earlier human

cases had been reported but were thought to be very unusual.

Enterocytozoon bieneusi:

A number of cases of E. bieneusi infection have been reported in patients with AIDS. Chronic intractable

diarrhea, fever, malaise, and weight loss are symptoms of E. bieneusi infections, and these symptoms mimic

those seen with cryptosporidiosis or isosporiasis. Often these patients have four to eight watery, nonbloody

stools each day, accompanied by nausea and anorexia. Dehydration and D-xylose and fat malabsorption also

may develop. These patients tend to be severely immunodeficient, with a CD4 count almost always below 200

cells/mm3 and often below 100 cells/mm3. Mixed infections with E. bieneusi and E. intestinalis have also been

reported. E. bieneusi infection has been implicated in AIDS-related sclerosing cholangitis. However,

demonstration of E. bieneusi spores in extraepithelial tissues does not always appear to be associated with

subsequent development of systemic infection.

E. bieneusi spores have been identified in sputum and bronchoalveolar lavage fluid in addition to stool

specimens. E. bieneusi can colonize the respiratory tract, and clinical specimens from these specimens may

reveal the presence of spores. Multiorgan microsporidiosis caused by E. bieneusi has been diagnosed in patients

infected with HIV; organisms have been recovered in stools, duodenal biopsy specimens, nasal discharge, and

sputum. Infection with E. bieneusi has also been reported in immunocompetent individuals; symptoms were

selflimited, and diarrheal disease resolved within 2 weeks. E. bieneusi may be more commonly associated with

sporadic diarrheal disease than was previously suspected, and the immune system may play a role in the control

of this intestinal infection. It is also quite possible that E. bieneusi may persist as an asymptomatic infection in

immunocompetent individuals.

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Encephalitozoon spp.:

Both Encephalitozoon cuniculi and Encephalitozoon hellem have been isolated from human infections.

The spectrum disease in patients with AIDS, organ transplant recipients, and otherwise immunocompromised

patients includes keratoconjunctivitis, intraocular infection, sinusitis, bronchiolitis, pneumonitis, nephritis,

ureteritis, cystitis, prostatitis, urethritis, hepatitis, sclerosing cholangitis, peritonitis, diarrhea, and encephalitis.

Clinical manifestations may vary, ranging from an asymptomatic carrier state to organ failure.

Encephalitozoon (Septata) intestinalis:

 Encephalitozoon (Septata) intestinalis infects primarily small intestinal enterocytes, but infection does not

remain confined to epithelial cells. E. intestinalis is also found in lamina propria macrophages, fibroblasts, and

endothelial cells. Dissemination to the kidneys, lower airways, and biliary tract appears to occur through

infected macrophages.

Fortunately, these infections tend to respond to therapy with albendazole, unlike infections caused by E.

bieneusi.

Other Microsporidia

 Different microsporidial species have been isolated from immunocompetent individuals who presented with

keratoconjunctivitis, severe keratitis, or corneal ulcers. Also, keratoconjunctivitis has been found in an

immunocompetent contact lens wearer.

In immunocompromised patients, myositis has been seen in infections caused by Pleistophora sp., Pleistophora

ronneafiei, Trichomonas hominis, Anncaliia vesicularum, and Anncaliia algerae. Trachipleistophora

anthropophthera has been identified at autopsy in cerebral, cardiac, renal, pancreatic, thyroid, hepatic, splenic,

lymphoid, and bone marrow tissue of patients with AIDS. Disseminated infection caused by Anncaliia connori

was found at autopsy in a 4-month-old athymic male infant.

Laboratory diagnosis:

 The most commonly used stains are chromotrope-based stains (modified trichrome) and chemofluorescent

optical brightening agents, including calcofluor white and other chemofluorescent stains. Regardless of the

staining technique selected, the use of positive control material is highly recommended. Detection of the small

microsporidial spores requires adequate illumination and magnification (i.e., magnification using the oil

immersion objective [×100] for a total magnification of ×1000).

 Microsporidia do not tend to stain predictably, if at all, in tissues. However, spores occasionally can be seen

very well with use of the periodic acid-Schiff (PAS) stain, silver stains, or acid-fast stains. Modified Gram

stains also have proved sensitive. The spore has a small, PAS-positive posterior body; the spore coat stains with

silver, and the spores are acid-fast variable. Tissue examination by electronmicroscopy (EM) techniques is still

considered the best approach for differentiation of genera; however, this option is not available to all

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laboratories, and the sensitivity of EM may not be equal to that of other methods when examining stool or

urine.

Therapy:

 Albendazole therapy can result in clinical cure of HIVassociated infection with Encephalitozoon spp., along

with elimination of spore shedding. Albendazole is not effective for Enterocytozoon infections, although

clinical improvement occurs in some patients. Oral purified fumagillin appears to eradicate Enterocytozoon

bieneusi in many patients, but serious adverse events and parasitic relapse have been seen. Antiretroviral

combination therapy results in complete clinical response with elimination of intestinal microsporidia.

Prevention:

 The presence of infective spores in human clinical specimens suggests that taking precautions when handling

body fluids and following personal hygiene measures,such as hand washing, may be important in preventing

primary infections in the health care setting.

Blood and Tissue Protozoa:

Sporozoa, Flagellates (Blood, Tissue):

Sporozoa (Malaria and Babesiosis):

Plasmodium vivax

Plasmodium ovale

Plasmodium malariae

Plasmodium falciparum

Plasmodium knowlesi

Babesia spp.

Flagellates (Leishmaniae, Trypanosomes):

Leishmania tropica complex

Leishmania mexicana complex

Leishmania braziliensis complex

Leishmania donovani complex

Leishmania peruviana

Trypanosoma brucei gambiense

Trypanosoma brucei rhodesiense

Trypanosoma cruzi

Trypanosoma rangeli

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Plasmodium spp.:

 Malaria has been well documented as an ancient disease in Egyptian and Chinese writing beginning in 2700

bc. By 200 bc, malaria was identified in Rome, spread throughout Europe during the twelfth century, and

arrived in England by the fourteenth century. By the early 1800s malaria was found worldwide.

Malaria has played a tremendous role in world history, influencing the outcome of wars, the movement of

populations, and the development and decline of various nations. Before the American Civil War, malaria was

found as far north as southern Canada; but it was no longer endemic within the United States by the 1950s. It is

estimated that more than 500 million individuals worldwide are infected with Plasmodium spp., and as many as

2.7 million people a year, most of whom are children, die from the infection. Malaria is endemic in more than

90 countries with a population of 2400 million people, representing 40% of the world’s population. At least

90% of deaths caused by malaria occur in Africa.

 The vector for malaria is the female anopheline mosquito. When the vector takes a blood meal, sporozoites

contained in the salivary glands of the mosquito are discharged into the puncture wound Within an hour, these

infective sporozoites are carried via the blood to the liver, where they penetrate hepatocytes and begin to grow,

initiating the preerythrocytic or primary exoerythrocytic cycle. The sporozoites become round or oval and

begin dividin repeatedly. Schizogony results in large numbers of exoerythrocytic merozoites. Once these

merozoites leave the liver, they invade the red blood cells (RBCs), initiating the erythrocytic cycle. A dormant

schizogony may occur in P. vivax and P. ovale organisms,which remain quiescent in the liver. These resting

stages have been termed hypnozoites and lead to a true relapse, often within 1 year or up to more than 5 years

later.

Delayed schizogony does not occur in P. falciparum, P. malariae, or P. knowlesi. Once the RBCs and

reticulocytes have been invaded, the parasites grow and feed on hemoglobin. Within the RBC, the merozoite (or

young trophozoite) is vacuolated, ring shaped, more or less ameboid, and uninucleate. The excess protein and

hematin present from the metabolism of hemoglobin combine to form malarial pigment. Once the nucleus begins

to divide, the trophozoite is called a developing schizont. The mature schizont contains merozoites (whose

number depends on the species), which are released into the bloodstream. Many of the merozoites are destroyed

by the immune system, but others invade RBCs and initiate a new cycle of erythrocytic schizogony. After

several erythrocytic generations, some of the merozoites begin to undergo development into the male and female

gametocytes. Although malaria is often associated with travelers to endemic areas, other situations resulting in

infection include blood transfusions, use of contaminated hypodermic needles, bone marrow transplantation,

congenital infection, and transmission within the United States by indigenous mosquitoes that acquired the

parasites from imported infections.

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PLASMODIUM VIVAX (BENIGN TERTIAN MALARIA):

General Characteristics:

 P. vivax infects only the reticulocytes; thus, the parasitemia is limited to approximately 2% to 5% of the available RBCs (

Figures 21).

Figure 21 The morphology of malaria parasites. Plasmodium vivax: 1, Early trophozoite (ring form). 2, Late trophozoite with Schuffner’s dots

(note enlarged red blood cell). 3, Late trophozoite with ameboid cytoplasm (very typical of P. vivax). 4, Late trophozoite with ameboid cytoplasm.

5, Mature schizont with merozoites (18) and clumped pigment. 6, Microgametocyte with dispersed chromatin. 7, Macrogametocyte with compact

chromatin. Plasmodium malariae: 1, Early trophozoite (ring form). 2, Early trophozoite with thick cytoplasm. 3, Early trophozoite (band form). 4,

Late trophozoite (band form) with heavy pigment. 5, Mature schizont with merozoites (9) arranged in rosette. 6, Microgametocyte with dispersed

chromatin. 7, Macrogametocyte with compact chromatin. Plasmodium ovale: 1, Early trophozoite (ring form) with Schuffner’s dots. 2, Early

trophozoite (note enlarged red blood cell). 3, Late trophozoite in red blood cell with fimbriated edges. 4, Developing schizont with irregularly

shaped red blood cell. 5, Mature schizont with merozoites (8) arranged irregularly. 6, Microgametocyte with dispersed chromatin. 7,

Macrogametocyte with compact chromatin. Plasmodium falciparum: 1, Early trophozoite (accole or applique form). 2, Early trophozoite (one ring

is in headphone configuration/double chromatin dots). 3, Early trophozoite with Maurer’s dots. 4, Late trophozoite with larger ring and Maurer’s

dots. 5, Mature schizont with merozoites (24). 6, Microgametocyte with dispersed chromatin. 7, Macrogametocyte with compact chromatin. Note:

Without the appliqué form, Schuffner’s dots, multiple rings/cell, and other developing stages, differentiation among the species can be difficult. It

is obvious that the early rings of all four species can mimic one another very easily. Remember: One set of negative blood films cannot rule out a

malarial infection.

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Splenomegaly occurs during the first few weeks of infection, and the spleen will progress from being soft and

palpable to hard, with continued enlargement during a chronic infection. If the infection is treated during the

early phases, the spleen will return to its normal size. A secondary or dormant schizogony occurs in P. vivax

and P. ovale, which remain quiescent in the liver. These resting stages have been termed hypnozoites. After a

few days of irregular periodicity, a regular 48-hour cycle is established. An untreated primary attack may last

from 3 weeks to 2 months or longer. Over time, the paroxysms (symptomatic period) become less severe and

more irregular in frequency and then cease altogether. In approximately 50% of patients infected with P. vivax,

relapses occur after weeks, months, or even after 5 years or more. The RBCs tend to be enlarged (young

RBCs), there may be Schüffner’s dots (exclusively found in P. vivax and P. ovale) after 8 to 10 hours, the

developing rings are ameboid, and the mature schizont contains 12 to 24 merozoites .

Pathogenesis and Spectrum of Disease:

 In patients who have never been exposed to malaria, symptoms such as headache, photophobia, muscle aches,

anorexia, nausea, and sometimes vomiting may occur before organisms can be detected in the bloodstream. In

other patients with prior exposure to the malaria, the parasites can be found in the bloodstream several days

before symptoms appear. Severe complications are uncommon in P. vivax infections, although coma and

sudden death or other symptoms of cerebral involvement have been reported, particularly in patients with

varying degrees of primaquine resistance. These patients can exhibit cerebral malaria, renal failure, circulatory

collapse, severe anemia, hemoglobinuria, abnormal bleeding, acute respiratory distress syndrome, and jaundice.

Acute cerebral malaria involves changes in mental status and if untreated may result in fatality within 3 days.

Plasmodium ovale:

General Characteristics

Although P. ovale and P. vivax infections are clinically similar, P. ovale malaria is usually less severe, tends to

relapse less frequently, and usually ends with spontaneous recovery, often after no more than 6 to 10 paroxysms

, Like P. vivax, P. ovale infects only the reticulocytes, so that the parasitemia is limited to approximately 2% to

5% of the available RBCs. For many years the literature has stated that as with P. vivax, a secondary or dormant

schizogony occurs in P. ovale, which remain quiescent in the liver.

 However, newer findings indicate that hypnozoites have never been demonstrated by biologic experiments.

After a few days of irregular periodicity, a regular 48-hour cycle is established. Over time, the paroxysms

become less severe and more irregular in frequency and then stop altogether. In some patients infected with P.

ovale, relapses occur after weeks, months, or up to 1 year or more. The RBCs tend to be enlarged (young

RBCs), Schüffner’s dots (also known as James stippling) are present from the beginning of the cycle, the

developing rings are less ameboid than those of P. vivax, and the mature schizont contains an average of eight

merozoites.

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Pathogenesis and Spectrum of Disease:

The incubation period is similar to that for P. vivax malaria, but the frequency and severity of the symptoms are

much less, with a lower fever and a lack of typical rigors. The geographic range is usually described as being

limited to tropical Africa, the Middle East, Papu New Guinea, and Irian Jaya in Indonesia. However, P. ovale

infections in Southeast Asia may cause benign and relapsing malaria in this area. In both Southeast Asia and

Africa, two different types of P. ovale circulate in humans. Human infections with variant-type P. ovale are

associated with a higher level of parasitemia.

Plasmodium malariae (QUARTAN MALARIA):

General Characteristics

 P. malariae invades primarily the older RBCs, limiting the number of infected cells ( Figures 21). The

incubation period between infection and symptoms may be much longer than that for P. vivax or P. ovale

malaria, ranging from about 27 to 40 days. A regular periodicity is seen from the beginning, with a more severe

paroxysm, including a longer cold stage and more severe symptoms during the hot stage. Collapse during the

sweating phase is not uncommon. A regular periodicity of 72 hours is seen from the beginning of the

erythrocytic cycle. The infection may end with spontaneous recovery, or there may be a recrudescence or series

of recrudescence (recurrence of symptoms) over many years. These patients are left with a latent infection and

persisting low-grade parasitemia for many, many years. The RBCs tend to be normal to small (old RBCs), there

is no true stippling, the RBCs may have fimbriated edges, the developing rings tend to demonstrate “band”

forms, and the mature schizont contains an average of 6 to 12 merozoites.

Pathogenesis and Spectrum of Disease:

 Proteinuria is common in P. malariae infections and may be associated with clinical signs of nephrotic

syndrome. With a chronic infection, kidney problems result from deposition within the glomeruli of circulating

antigenantibody complexes. A membrane proliferative type of glomerulonephritis is the most common lesion

seen in quartan malaria. Because chronic glomerular disease associated with P. malariae infections is usually

not reversible with therapy, genetic and environmental factors may play a role in the disease, as well. The

patient may have a spontaneous recovery, or there may be a recrudescence or series of recrudescence over

many years (>50 years).

In these cases, patients are left with a latent infection and persisting low-grade parasitemia.

Plasmodium falciparum (MALIGNANT TERTIAN MALARIA):

General Characteristics

 Plasmodium falciparum invades all ages of RBCs, and the number of infected cells may exceed 50% ,

Schizogony occurs in the spleen, liver, and bone marrow rather than in the circulating blood. Ischemia caused

by the obstruction of vessels within these organs by parasitized RBCs will produce various symptoms,

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depending on the organ involved. A decrease in the ability of the RBCs to change shape when passing through

capillaries or the splenic filter may lead to plugging of the vessels Also, only P. falciparum causes

cytoadherence, a feature that is associated with severe malaria. The asexual and sexual forms circulate in the

bloodstream during infections by four of the Plasmodium species. However, in P. falciparum infections, as the

parasite grows, the RBC membrane becomes sticky and the cells adhere to the endothelial lining of the

capillaries of the internal organs. Thus, only the ring forms and the gametocytes (occasionally mature

schizonts) normally appear in the peripheral blood. Periodicity of the cycle will not be established during the

early stages, and the presumptive diagnosis may be totally unrelated to a possible malaria infection. If the fever

does develop a synchronous cycle, it is usually a cycle of 36 to 48 hours. Because P. falciparum infects young

and old RBCs, very heavy parasitemia can occur. The RBCs are all sizes; there is no true stippling, but

Maurer’s dots (coarse granulation in the cytoplasm of RBCs) are sometimes present; often there are multiple

rings per RBC and the rings are delicate and often have two dots of chromatin, appliqué or accolé forms (ring

forms identified within the marginal regions of the erythrocytes); and the gametocytes are crescent-shaped.

Pathogenesis and Spectrum of Disease:

 The onset of a P. falciparum malaria attack occurs 8 to 12 days after infection and is characterized by 3 to 4

days of vague symptoms such as aches, pains, headache, fatigue,

anorexia, or nausea. This stage is followed by fever, a more severe headache, and nausea and vomiting, with

occasional severe epigastric pain. At the onset of fever, there may be a feeling of chilliness. As with the other

Plasmodium spp., periodicity of the cycle will not be established during the early stages.