Acanthamoeba spp.

General characteristics

Unlike N. fowleri, Acanthamoeba spp. do not have a flagellate stage in the life cycle, only the trophozoite and cyst.

Several species of Acanthamoeba cause granulomatous amebic encephalitis (GAE), primarily in immunosuppressed,

 Acanthamoeba spp. also cause amebic keratitis Motile organisms have spine-like pseudopods; there is a wide organism

size range (25 to 40 μm), with the average diameter of the trophozoites being 30 μm. The nucleus has the typical large

karyosome, similar to that found in N. fowleri. This morphologic characteristic can be seen on a wet preparation.

The cysts are usually round with a single nucleus, also having the large karyosome as in the trophozoite nucleus. The

double wall is usually visible, with the slightly wrinkled outer cyst wall and what has been described as a polyhedral inner

cyst wall. This cyst morphology is identifiable in organisms cultured on agar plates.

Pathogenesis and spectrum of disease :

GAE (granulomatous amebic encephalitis):

Meningoencephalitis caused by Acanthamoeba spp. May present as an acute suppurative inflammation of the brain and

meninges similar to N. fowleri infection. The incubation period of GAE is unknown; several weeks or months are

probably necessary to establish disease. The clinical course tends to be subacute or chronic and is usually associated with

trauma or underlying disease, not as a result of swimming. GAE may present with symptoms of confusion, dizziness,

drowsiness, nausea, vomiting, headache, lethargy, stiff neck, seizures, and sometimes hemiparesis. Unlike PAM caused by

N. fowleri, both trophozoites and cysts are found throughout the tissue. Also, dissemination to other tissues such as the

liver, kidneys, trachea, and adrenals can occur in immunocompromised individuals; or additional unusual sites also

include the ear and necrotic bone from a bone graft of the mandible. Some patients, especially those with AIDS, can

develop erythematous nodules, chronic ulcerative skin lesions, or abscesses. Keratitis Acanthamoeba spp. also cause

keratitis and corneal ulceration. Clinicians need to consider acanthamoebic infection in the differential diagnosis of eye

infections that are not responding to bacterial, fungal, or viral therapy.

These infections are often due to direct exposure of the eyes to contaminated materials or solutions. Use of

contact lenses is the leading risk factor for keratitis. Conditions that are linked with disease include the use of home-made

saline solutions, poor contact lens hygiene, and corneal abrasions. A contact lens can act as a mechanical vector for

Figure 29 Naegleria fowleri in brain tissue. Hematoxylin and eosin stain.

431

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

transport of amebae present in the storage case onto the cornea. Subsequent multiplication and invasion of the tissue may

occur.

Laboratory diagnosis :

 Routine Methods:

Tissue stains are effective, and cysts isolated from cultures can be stained with Gomori’s silver methenamine, periodic

acid-Schiff, and calcofluor white. Identification of Acanthamoebae in ocular samples and other tissues can be difficult,

even for trained laboratory professionals; in histologic preparations, the organisms appear similar to keratoplasts, as well

as neutrophils and monocytes. It has been estimated that up to 70% of clinical Acanthamoeba keratitis cases are

misdiagnosed as viral keratitis. Also, the average time to diagnosis of keratitis attributable to Acanthamoeba infection can

average 2.5 weeks longer for non–contact lens wearers than for contact lens users.

 Other Methods :

CSF or bronchoalveolar lavage fluid cytospin preparations can be used to look for amebae in patients with

GAE or respiratory symptoms.

Organisms have also been found in the adrenal gland, brain, eyes, kidneys, liver, pancreas, skin,

spleen, thyroid gland, and uterus.

In infections caused by Acanthamoeba spp., periodic acid-Schiff stains the cyst wall red and methenamine silver stains the

cyst black.

Therapy :

Disseminated Infections

Trophozoites and cysts of Acanthamoeba isolates vary in their sensitivity to antimicrobial agents. They are sensitive in

vitro to ketoconazole, pentamidine, hydroxystilbamidine, paromomycin, 5-fluorocytosine, polymyxin, sulfadiazine,

trimethoprim-sulfamethoxazole, azithromycin, and extracts of medicinal plants, especially, to combinations of these

drugs. In vitro testing confirms strain and species differences in sensitivity.

Balamuthia mandrillaris

Pathogenesis and spectrum of disease

The disease is very similar to GAE caused by Acanthamoeba spp. The clinical presentation is subacute or chronic and is

usually not associated with swimming in freshwater. No characteristic clinical symptoms, laboratory findings, or

radiologic indicators have been found to be diagnostic for GAE. Whether single or multiple, the lesions in the brain

involve mainly the cerebral cortex and subcortical white matter. Symptoms include headache, nausea, vomiting, fever,

visual disturbances, dysphagia, seizures, and hemiparesis. The clinical course ranges from a few days to several months.

In immunocompetent hosts, an inflammatory response occurs; however, with rare exceptions, these patients also tend to

die with severe CNS disease.

Laboratory diagnosis :

432

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

these organisms can be cultured in mammalian cell cultures using monkey kidney cells ,Although serum antibodies have

been identified in infections, laboratory testing is notroutinely available.

Therapy :

In vitro studies indicate that B. mandrillaris is susceptible to pentamidine isethiocyanate and that patients may

benefit from this treatment.

Trichomonas vaginalis:

General characteristics:

 Infection is acquired primarily through sexual intercourse, hence the need to diagnose and treat asymptomatic

males. The organism is capable of survival for extended periods in a moist environment such as damp towels and

underclothes; however, this mode of transmission is thought to be very rare. Trichomoniasis is the primary nonviral sexually transmitted disease worldwide. Infection with T. vaginalis has major health consequences for

women, including complications in pregnancy, association with cervical cancer, and predisposition to HIV

infection.

The life cycle of T. vaginalis has a single trophozoite stage, and is very similar in morphology to other

trichomonads. The trophozoite is 7 to 23 μm long and 5 to 15 μm wide. The axostyle is usually obvious and

protrudes through the bottom of the organism, whereas the undulating membrane ends halfway down the side

ofthe trophozoite. There are a large number of granules evident along the axostyle.

Pathogenesis and spectrum of disease:

 Growth of the organism results in inflammation and large numbers of trophozoites in the tissues and the

secretions. As the acute infection becomes more chronic, the purulent discharge diminishes, with a decrease in the

number of organisms. Symptoms such as vaginal or vulval pruritus and discharge are often sudden and occur

during or after menstruation as a result of the increased vaginal acidity. Symptoms include vaginal discharge

(42%), odor (50%), and edema or erythema (22% to 37%). Complaints also include dysuria and lower abdominal

pain.

Figure 30 : Trichomonas, Giemsa stain

433

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

From 25% to 50% of infected women may be asymptomatic and have a normal vaginal pH of 3.8 to 4.2 and normal

vaginal flora. Even in the carrier form, about 50% of women will become symptomatic during the following 6

months.

Although vaginitis is the most common finding in women with trichomoniasis, other complications include

distention of a fallopian tube with pus, endometritis, infertility, low birth weight, and cervical erosion. There is also

an increased association with HI transmission and cervical dysplasia.

Dysuria, often the earliest symptom, occurs in about 20% of women with vaginal trichomoniasis. Infected males

may be asymptomatic, or the infection may be selflimited, persistent, or result in recurring urethritis. In nonspecific

urethritis, T. vaginalis has been detected in 10% to 20% of subjects and in 20% to 30% of those whose sexual

partners had vaginitis. Once established, the infection persists for an extended period in females but only for about

10 days or less in males. T. vaginalis is the cause of 11% of all cases of non–gonococcal urethritis in males.

Respiratory distress has been reported in a full-term, normal male infant with T. vaginalis with severe respiratory

problems following delivery. A wet preparation of thick, white sputum demonstrated few leukocytes and motile

flagellates, which were identified as T. vaginalis.

This study supports previous data confirming that the organism may cause neonatal pneumonia.

Laboratory diagnosis:

 Humans are the only natural host for T. vaginalis, and organisms reside in the vagina and prostate; they usually

do not survive outside the urogenital tract. The parasites feed on the mucosal surface of the vagina, where bacteria

and leukocytes are abundant. The preferred pH for good

parasitic growth in females is slightly alkaline or acidic (6.0 to 6.3 optimal), not the normal pH (3.8 to 4.2) of the

healthy vagina. The organisms can also be recovered in urine, in urethral discharge, or after prostatic massage.

Often, the organisms are recovered in centrifuged urine sediment from both male and female patients.

Wet mounts:

 The identification of T. vaginalis is often based on the examination of wet preparations of vaginal and urethral

discharges, urine, and prostatic secretions. This examination must be performed within 10 to 20 minutes after

sample collection; if not, organisms lose motility and may not be identified. Several specimens may need to be

examined for detection of the organisms. The sensitivity associated with wet mount examinations varies between

40% and more than 80%. Often, the percent detection from this procedure is quite low with limited sensitivity and

specificity.

Stained Smears:

 Giemsa or Papanicolaou stain can be used. However, atypical cellular changes can be misinterpreted, particularly

on the Papanicolaou smear. The organisms are routinely missed on Gram stains. The number of false-positive and

false-negative results reported on the basis of stained smears strongly suggests that confirmation should be

accomplished by observation of motile organisms either from the direct wet mount or from appropriate culture

media.

434

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Culture:

 A convenient plastic envelope method has been developed, which allows immediate examination and culture in

one self-contained system. This system is serves as the specimen transport container, the growth chamber during

incubation, and the “slide” durin microscopy. Once it is inoculated, it requires no opening for examination, and

positive growth will occur within 5 days. The sensitivity of this system is reported to be superior to those of othe

available culture methods.

Antigen Detection:

 Several diagnostic tests have been developed, including the XenoStrip-Tv and the OSOM Trichomonas Rapid

Test both of which are more sensitive than the wet mount.

Molecular Diagnostics:

 The use of polymerase chain reaction (PCR) methods has led to improvements in T. vaginalis detection;

nonviable organisms and cells and target sequences can also be detected.

Therapy:

 Metronidazole is recommended for the treatment of urogenital trichomoniasis. Tinidazole has also been used for

therapy.

Pentatrichomonas hominis:

 Pentatrichomonas hominis derives its name on the basis of the morphologic structure of the trophozoite. The

organism has five anterior flagella and a parabasal body. The organism is recovered worldwide and is considered to

be nonpathogenic although it has been isolated from patients with diarrhea.

General characteristics:

 There is no known cyst stage. The trophozoite resides in the large intestine where it feeds on bacteria. The

organism resembles Trichomonas vaginalis, measuring 5 to 15 μm in length and 7 to 10 μm in width with an

undulating membrane and an axostyle. The organism’s undulating membrane extends the entire length of the body,

differentiating it from Trichomonas vaginalis.

Pathogenesis and spectrum of disease:

 Although the organism is considered nonpathogenic, it is the most commonly identified flagellate other than G.

lamblia and Dientamoeba fragilis. The organism may be associated with infections in warm climates.

Laboratory diagnosis:

 Similar to Trichomonas vaginalis, the wet preparation is commonly used to identify motile trophozoites.

However, permanent stained smears provide the greatest sensitivity and specificity.

Therapy:

435

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

 Because the organism is nonpathogenic, it is important to differentiate it from Trichmonas vaginalis. However, if

identified, no treatment is necessary.

TOXOPLASMA GONDII:

Toxoplasma gondii is a protozoan parasite that infects most species of warm-blooded animals, including humans.

Members of the cat family, Felidae, are the only known definitive hosts for the sexual stages of T. gondii and serve

as the main reservoirs of infection. Cats become infected with T. gondii through carnivorism or by ingestion of

oocysts. After tissue cysts or oocysts are ingested by the cat, organisms are released and invade epithelial cells of

the cat small intestine, where they undergo an asexual cycle followed by a sexual cycle with the formation of

oocysts, which are excreted in the feces. The uninfective oocyst takes 1 to 5 days after excretion to become

infective.

Cats shed oocysts for 1 to 2 weeks and large numbers may be shed, often more than 100,000 per gram of feces.

Oocysts survive in the environment for several months to more than 1 year and are resistant to disinfectants,

freezing,and drying. However, they are killed by heating to 70° C for 10 minutes. The life cycle in the cat takes

approximately 19 to 48 days after infection with the oocysts but only 3 to 10 days after the ingestion of meat

infected with cysts (e.g., a mouse) .

General characteristics:

There are three infectious stages of T. gondii: the tachyzoites(in groups or clones), the bradyzoites (in tissuecysts),

and the sporozoites (in oocysts from cat feces).

Tachyzoites rapidly multiply in any cell of the intermediate host and in epithelial cells of the definitive host (cats).

Bradyzoites are found within the tissue cysts and usually multiply very slowly; the cyst may contain few to

hundreds of organisms, and intramuscular cysts may reach 100 μm in size. The tissue cysts can be found in visceral

organs such as the lungs, liver, and kidneys; however, they are more prevalent in the brain, eyes, and skeletal and

cardiac muscle. Intact tissue cysts can persist for the life of the host and do not cause an inflammatory response.

Tachyzoites are crescent-shaped and are 2 to 3 μm wide by 4 to 8 μm long . One end tends to be more rounded than

the other. Giemsa is the stain of choice; the cytoplasm stains pale blue, and the nucleus stains red and is situated

toward the broad end of the organism.

Cysts are formed in chronic infections, and the bradyzoites within the cyst wall are strongly periodic acid-Schiff

positive. During the acute phase, there may be groups of tachyzoites that appear to be cysts; however, they are not

strongly periodic acid-Schiff positive and have been

termed pseudocysts.

Pathogenesis and spectrum of disease:

As the tachyzoites actively grow, increase in number, and eventually rupture from the cell, they invade adjacent

cells. This process creates additional lesions. Once the cysts are formed, the process becomes quiescent, with little

or no multiplication and spread. In the immunocompromised or immunodeficient patient, a cyst rupture Tachyzoites

or primary exposure to the organisms often leads to lesions. The organisms can be disseminated via the lymphatic

system and the bloodstream to other tissues.

436

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Toxoplasmosis can be categorized into four groups: (1) acquired in the immunocompetent patient; (2)acquired or

reactivated in the immunodeficient patient; (3) congenital; and (4) ocular.

Laboratory diagnosis:

The most common method of diagnosis for toxoplasmosis is serologic testing for T. gondii– specific

antibodies.Other procedures include PCR; examination of biopsy

specimens, buffy coat cells, or cerebrospinal fluid; orisolation of the organism in tissue culture or in laboratory

animals.The serologic diagnosis of toxoplasmosis a number of additional procedures include enzyme

immunoassays, enzymelinked immunosorbent assays (ELISAs), direct agglutination, an immunosorbent

agglutination assay, an indirect immunofluorescence assay (IFA), immunocapture, and immunoblot tests.

Therapy:

 Treatment is recommended for the following conditions: clinically active disease, diagnosed congenital

toxoplasmosis in newborns, pregnant women with infection during gestation, patients with chorioretinitis, and

disease in symptomatic compromised patients. Therapy is also recommended for preventive or suppressive

treatment in HIV-infected persons. The currently recommended drugs work primarily against the actively

dividing tachyzoite form of T. gondii and do not eradicate encysted organisms (bradyzoites).

The most common drug combination used to treat congenital toxoplasmosis consists of pyrimethamine and a

sulfonamide , plus folinic acid in the form of leucovorin calcium to protect the bone marrow from the toxic

effects of pyrimethamine. Spiramycin is recommended for pregnant women with acute toxoplasmosis when

fetal infection has not been confirmed in an attempt to prevent transmission of T. gondii from the mother to the

fetus.

In immunosuppressed persons with toxoplasmosis, the combination of pyrimethamine and sulfadiazine plus

leucovorin is the preferred treatment. Clindamycin, in combination with other antiparasitic medications, is

frequently used for the treatment .

Intestinal Nematodes (Roundworms):

Helminths Nematodes Intestinal (roundworms):

Ascaris lumbricoides

Enterobius vermicularis (pinworm)

Strongyloides stercoralis (threadworm)

Trichostrongylus spp.

Trichuris trichiura (whipworm)

Capillaria philippinensis (hookworms)

Ancylostoma duodenale (Old World)

Necator americanus (New World)

 There are more than 60 species of nematodes known to infect humans. Nematodes are nonsegmented,

elongate, cylindrical worms with a well-developed digestive tract and reproductive system. The adult worms

437

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

have separate sexes, with the male generally smaller than the female. Most nematodes are diagnosed by finding

the characteristic eggs in the stool. The infective stage of the nematodes varies with species; for example,

transmission may occur through the ingestion of eggs, whereas others burrow through the skin and migrate to

the intestine.

The nematodes have very diverse life cycles providing different routes of transmission as well as disease

symptoms.

ASCARIS LUMBRICOIDES:

General characteristics:

Ascaris lumbricoides is the most common and the largestroundworm. The parasite with higher prevalence in

the tropical regions. Eggs are ingested and hatch in the duodenum, penetrate the intestinal wall, and migrate to

the hepatic portal circulation.

The adult worms live and reproduce in the lumen of the small intestine. The ovum is a thick, oval mammillated

(outer protrusions) and embryonated egg. The eggs are passed in the feces and become infective 2 to 6 weeks

following deposition, depending on the environment.

Epidemiology:

Transmission is through the fecal-oral route, usually through the ingestion of eggs on contaminated material.

Ascaris eggs are capable of survival within harsh environmental conditions, including dry or freezing

temperatures.

Pathogenesis and spectrum of disease:

Many A. lumbricoides infections are asymptomatic. The presentation of symptoms correlates with the length of

infection, the number of worms present, and the overall health of the host. Intestinal symptoms range from mild

to severe intestinal obstruction. Some patients will develop pulmonary symptoms and present with immunemediate hypersensitivity pneumonia. The worms may cause an immune condition known as Lِ ffler’s syndrome

characterized by peripheral eosinophilia.

Figure 31 Bile-stained mammillated A. lumbricoides

ovum.

Figure 32 A. lumbricoides adult male worm. Note the

curvedposterior end

438

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Laboratory diagnosis:

 Female worms have an extremely high daily output of eggs, making diagnosis relatively easy through the

identification of eggs in feces. The large, broadly oval mammillated

ova are typically stained brown from bile (Figure 31). Some eggs will be decorticated, or lacking the

mammillated outer cover. Infertile eggs may be oval or irregular shaped with a thin shell and containing

internal granules. Adult worms may also be identified in feces.

The male is smaller (15 to 31 cm) with a curved posterior end (Figure 32) and contains three well-characterized

lips. Larvae may be found in sputum or gastric aspirates as a result of larval migration during development

within the human host.

Therapy:

Anthelmintic treatment is recommended for all infections.Preferred therapy includes oral albendazole,

mebendazole, or pyrantel pamoate.

Prevention: