There are four species of blood flukes that are primarily associated with disease in humans (known as

schistosomiasis, bilharziasis, or snail fever), all belonging to the genu Schistosoma. These four species are

Schistosoma haematobium, S. japonicum (Oriental blood fluke), S. mekongi, and S. mansoni. A fifth species, S.

intercalatum, is a pathogen primarily in animals but has been associated with human disease. The blood flukes

differ

in morphology and life cycle characteristics from the other trematodes, but because they all belong to the same

genus, they are very similar, and may be difficult to distinguish from each other. They do, however, require a

freshwater snail as the only intermediate host.

Figure 58: Paragonimus westermani egg.

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General characteristics

 Unlike the other trematodes, adult schistosomes are not flattened, but are rather long, thin, and rounded in

shape. There is an oral sucker surrounding the mouth and a ventral sucker located just slightly below the oral

sucker. The adult male averages 1.5 cm in length and is wider than the female, having a ventral fold that wraps

around the female when they mate (Figure 59). The adult female averages 2 cm in length and is very thin.

The eggs of each species are distinct, and can be distinguished by size, spine morphology, and sometimes

specimen type (Figure 60). The size range for eggs of S. haematobium is 110 to 170 μm long by 40 to 70 μm

wide and they have a sharply pointed terminal spine. They are fully embryonated without an operculum. The size

range for the eggs of S. japonicum is 70 to 100 μm long by 50 to 65 μm wide, and they have a small lateral spine

that is sometimes difficult to detect .

 S. mekongi eggs are smaller than those of S. japonicum, ranging in size from 50 to 65 μm long by 30 to 55 μm

wide. They are fully embryonated without an operculum and have a small lateral spine. The size range for eggs of

S. mansoni is 115 to 180 μm long by 40 to 75 μm wide, and they have a large lateral spine. S. mansoni eggs are

unoperculate, immature when released, and take up to 8 to 10 days to develop a miracidium. S. intercalatum eggs

are fully embryonated without an operculum, have a terminal spine, and range in size from 140 to 240 μm long by

50 to 85 μm wide. S. intercalatum eggs resemble those of S. haematobium and can be differentiated by ZiehlNeelsen acid-fast positivity. In addition, S. intercalatum eggs are only found in feces, not in urine specimens

One of the main differences in the schistosomes from other trematodes is that instead of being hermaphroditic,

there are separate male and female adult worms. In human infection, the adult worms live in either the veins that

supply the intestine (S. japonicum and S. mansoni) or the veins that supply the urinary bladder (S. haematobium).

 The eggs are passed from the body in either the feces or the urine. To reach the inside of the intestine or bladder,

the eggs must penetrate the tissue from the veins. This is accomplished via a spine that is distinctive among the

major species. The embryonated egg will release the miracidium (Figure 62) once it reaches freshwater, and will

enter the snail host, where it will develop into the infectious cercaria. The freeswimming cercariae are capable of

penetrating through the human skin directly and do not encyst on aquatic vegetation or other aquatic wildlife

(Figure 63). The cercariae penetrate the host tissue until they reach a vein; then they travel to capillaries near the

lungs and then to the portal vein of the liver, where they mature. When they are mature, the adult males will pair

with the females and then travel to the veins of either the intestine or thebladder, where the eggs are produced.

Pathology and spectrum of disease

 Infection with only a small number of worms may be asymptomatic. Quite often, penetration of the skin by

the cercariae causes localized swelling and itching. The migration of the larvae through the body may cause

transient symptoms of fever, malaise, cough (when they migrate in the lungs), or hepatitis (when in the liver).

The adults are able to acquire some host antigens on their outer surface, and so may not elicit an immune

response, although the eosinophil count may be high.

Severe tissue damage, with associated pain, fever, and chills, may occur when the eggs travel through the

tissue to reach the intestine or bladder. There may also be bloody diarrhea or blood in the urine (hematuria).

Necrosis, lesions, and granulomas may develop, as well as obstruction of the bowel or ureters. Penetration of

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human skin by the cercariae of blood flukes that commonly infect other mammals or aquatic birds may cause a

schistosomal dermatitis known as “swimmer’s itch.” Erythema, edema, and intense itching may develop that

usually disappear within 1 week. The cercariae of these species are not able to complete the life cycle by

entering the human bloodstream, and are destroyed by the host immune system.

Figure 59: Mating of Schistosoma mansoni male and femaleWorms.

Figure 60, Schistosoma mansoni egg. B, Schistosoma japonicum egg. C, Schistosoma haematobium egg.

Figure 61 :S. japonicum egg. Figure 62 :S. mansoni miracidium.

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Laboratory diagnosis

 The standard method of diagnosis is by the detection of characteristic eggs in feces or rectal biopsy, for S.

japonicum, S. mekongi, S. mansoni, and S. intercalatum (and perhaps S. haematobium if these worms have

migrated to a bladder vein that is close to the intestine); and in urine (usually concentrated before examination)

or bladder tissue biopsy for S. haematobium.

A wet mount with/without iodine from a sedimentation or concentration method can be examined for eggs.

Figure provides images of three different schistosome eggs. To optimize recovery of S. haematobium in urine,

the specimen should be collected between noon and 2 pm. There are some antibody-based assays that are

available for diagnosis of schistosomal IgG antibody (enzyme immunoassay [EIA], enzyme-linked

immunosorbent assay [ELISA], and immunoblot), but these methods cannot distinguish between current and

previous infections. This type of assay may, however, be useful for travelers who have returned from endemic

areas.

Several nucleic acid-based testing methods have been developed that demonstrate high sensitivity and

specificity using genomic or mitochondrial sequences. In addition, schistosome DNA has been identified in

patients’ plasma using real-time polymerase chain reaction (PCR).

Therapy

 The drug of choice for treatment of schistosome infections is praziquantel, given in two or three doses in a

day. Infection with S. mansoni may require a larger dose than that for the other species.

Prevention

 Because human infection is by direct penetration of the cercariae, prevention of schistosome infection is more

difficult to achieve. Educational programs are required to help people in endemic areas understand how to help

prevent infection.

 Sanitary conditions need to be improved with proper disposal not only of human wastes but also that of

domestic animals (in areas with S. japonicum and S. mekongi). A safe water supply for bathing and washing

Figure 63: S. mansoni cercaria.

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clothes is also necessary. Various snail control methods have been tried, but these methods are very costly and

would need to be repeated on a regular basis to have the desired effect.

Table Body Sites and Parasite Recovery (Trophozoites, Cysts, Oocysts, Spores, Adults, Larvae, Eggs,

Amastigotes,Trypomastigotes)

Site Parasites

Blood

Red cells

Plasmodium spp.

Babesia spp.

White cells Leishmania spp.

Toxoplasma gondii

Whole

blood/plasma

Trypanosoma spp.

Microfilariae

Bone marrow Leishmania spp.

Trypanosoma cruzi

Plasmodium spp.

Central

Nervous

System

Cutaneous

ulcers

Taenia solium (cysticerci)

Echinococcus spp.

Naegleria fowleri

Acanthamoeba spp.

Balamuthia mandrillaris

Sappinia diploidea

Toxoplasma gondii

Microsporidia

Trypanosoma spp

Intestinal tract Leishmania spp.

Acanthamoeba spp.

Entamoeba histolytica

Entamoeba dispar

Entamoeba coli

Entamoeba hartmanni

Endolimax nana

Iodamoeba bütschlii

Blastocystis hominis

Giardia lamblia

Chilomastix mesnili

Dientamoeba fragilis

Pentatrichomonas hominis

Balantidium coli

Cryptosporidium spp.

Cyclospora cayetanensis

Isospora belli

Microsporidia

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Ascaris lumbricoides

Enterobius vermicularis

Hookworm

Strongyloides stercoralis

Trichuris trichiura

Hymenolepis nana

Hymenolepis diminuta

Taenia saginata

Taenia solium

Diphyllobothrium latum

Clonorchis sinensis (Opisthorchis)

Paragonimus spp.

Schistosoma spp.

Fasciolopsis buski

Fasciola hepatica

Metagonimus yokogawai

Heterophyes heterophyes

Liver, spleen Echinococcus spp.

Entamoeba histolytica

Leishmania donovani

Microsporidia

Lung Cryptosporidium spp.*

Echinococcus spp.

Paragonimus spp.

Microsporidia

Muscle Taenia solium (cysticerci)

Trichinella spp.

Onchocerca volvulus (nodules)

Trypanosoma cruzi

Microsporidia

Skin Leishmania spp.

Onchocerca volvulus

Microfilariae

Urogenital

system

Trichomonas vaginalis

Schistosoma spp.

Microsporidia

Microfilariae

Eye Acanthamoeba spp.

Toxoplasma gondii

Loa loa

Microsporidia

Note: This table does not include every possible parasite that can be found in a particular body site; the most

likely organisms have been listed.

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*Disseminated in severely immunosuppressed individuals

Table Epidemiology of the More Common Groups of Human Parasites

Parasite

Group

Habitat (Reservoir Mode of Transmission Prevention

Protozoa,

Intestinal

Amebae

Single-celled organisms

generally found

in humans. Although certain

animals

harbor some of these

organisms,

they are not considered

important

reservoir hosts.

Humans acquire infections by

ingesting

food and water contaminated

with fecal

material containing the

resistant, infective cyst stage

of the protozoa.

Various sexual practices have

also been documented in

transmission.

Preventive measures

include

increased attention to

personal

hygiene and sanitation

measures; elimination of

sexual

activities that may

involve

fecal-oral contact.

Flagellates The flagellates are generally

found in

humans. Although certain

animals

harbor some of these

organisms,

they are not considered

important

reservoir hosts; one exception

may

be animals, such as the

beaver, that

harbor Giardia lamblia.

Contaminated

water supplies are also a

source.

Humans acquire infections by

ingesting

food and water contaminated

with fecal

material containing the

resistant,

infective cyst stage of the

protozoa; in

some cases (Dientamoeba

fragilis), no

cyst stage has been identified;

the

trophozoite forms may be

transmitted

from person to person in

certain

helminth eggs.

Preventive measures

include

increased attention to

personal

hygiene and sanitation

measures; elimination of

sexual

activities that may

involve

fecal-oral contact;

adequate

water treatment

(including

filtration) is required;

also

awareness of

environmental

sources of infection.

Ciliates Balantidium coli is generally

found in

humans, but it is also found in

pigs.

In some areas of the world,

Humans acquire infections by

ingesting

food and water contaminated

with fecal

material containing the

Preventive measures

include

increased attention to

personal

hygiene and sanitation

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pigs are

considered important reservoir

hosts.

resistant,

infective cyst stage of the

protozoa

measures, as well as

elimination of sexual

activities

that may involve fecaloral

contact.

Coccidia Coccidia are found in humans.

In some

cases (e.g., cryptosporidiosis)

animal

reservoirs (cattle) can serve as

important hosts. The muscle

of

various animals may contain

sarcocysts that are infective

for

humans through the

consumption of

raw or poorly cooked meat.

Numerous waterborne

outbreaks

with Cryptosporidium spp.

have been

reported throughout the world.

Coccidian oocysts are

extremely

resistant to environmental

conditions,

particularly if they are kept

moist

These protozoa are acquired

through

ingestion of various meats or

by

fecal-oral transmission through

contaminated food and/or

water. The

infective forms are called

oocysts

(Cryptosporidium spp.,

Isospora

(Cystoisospora) belli,

Cyclospora

cayetanensis) or sarcocysts

(Sarcocystis

spp.), which are contained in

infected

meat. Cryptosporidia have also

been

implicated in nosocomial

infections.

Preventive measures

include

increased attention to

personal

hygiene and sanitation

measures; elimination of

sexual

activities that may

involve

fecal-oral contact.

Adequate

water treatment

(including

filtration) is mandatory;

awareness of

environmental

sources of infection also

is

important.

Microsporidia Microsporidia can infect

every living

animal, some of which

probably

serve as reservoir hosts for

human

infection. However, host

specificity

Infection with microsporidial

spores usually

occurs through ingestion;

however,

inhalation of spores and direct

inoculation from the

environment almost

certainly occur.

Preventive measures

include

increased attention to

personal

hygiene and sanitation

measures; increased

awareness of

environmental

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has not been well defined to

date.

The spores are

environmentally

resistant and can survive years

if

kept moist.

exposure possibilities;

and

adequate water

treatment.

Protozoa,

Other

Sites

Amebae

Free-living amebae are

associated with

warm, freshwater

environments; they

are also found in soil.

Although

humans can harbor these

organisms,

person-to-person transfer is

thought

to be rare. Environmental

sources

are the primary link to human

infection. Contaminated eye

care

solutions have been linked to

organisms that cause keratitis.

Infection occurs through

contact with

contaminated water; organisms

enter

through the nasal mucosa and

may

travel via the olfactory nerve

to the

brain. Disease can be very

severe and

life-threatening; keratitis is

also caused

by these organisms, and

infection can

be linked to blindness or

severe corneal

damage. Eye infections can be

linked to

contaminated lens solutions or

direct,

accidental inoculation of the

eye from

environmental water and/or

soil sources.

Avoidance of

contaminated

environmental water and

soil

sources; adequate care of

contact lens systems

Flagellates Trichomonas vaginalis

infection is found

in a large percentage of

humans;

humans may present as

symptomatic or

asymptomatic.

T. vaginalis is found in the

genitourinary

system and is usually acquired

by

sexual transmission.

Awareness of sexual

transmission; treatment

of all

partners when infection

is

diagnosed in an

individual

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Person-to-person transfer is

very

common; reinfection is also

common, particularly if sexual

partners are not treated.

patient.

Protozoa,

Blood

and Tissue

Malaria,

Babesiosis

Humans harbor the five

species of

malaria (Plasmodium vivax,

P. ovale,

P. malariae, P. knowlesi, and

P.

falciparum). Other animals

can carry

Babesia spp., and animal

reservoir

hosts play a large role in

human

transmission.

These organisms are

arthropod-borne,

Plasmodium spp. by the

female

anopheline mosquito and

Babesia spp.

by one or more genera of ticks.

These

infections can also be

transmitted

transplacentally, via shared

needles,

through blood transfusions,

and from

organ transplants.

Vector control;

awareness of

transmission through

blood

transfusions, shared drug

needles, congenital

infections,

and organ transplants.

Careful

monitoring of the blood

supply.

Malaria prophylaxis if

traveling

to endemic areas.

Flagellates

(leishmaniae

Some strains of leishmaniae

have

reservoir hosts (e.g., dogs for

the

Mediterranean strain of

Leishmania

donovani and wild rodents for

the

African strains of L.

donovani.) L.

tropica also has been linked to

the

same two animal reservoirs

Transmission is through the

bite of infected

sandflies. Infection can also

occur from

person to person (cutaneous

lesions),

from blood transfusion, shared

needles,

and organ transplants.

Vector control; avoiding

environmental sources

(e.g.,

dogs, wild rodents);

careful

handling of all clinical

specimens from infected

patients.

Flagellates

(trypanosomes

Humans are the only known

hosts for

Trypanosoma brucei

gambiense

(West African

Transmission is through the

bite of the

infected tsetse fly and through

blood

transfusion, shared needles,

Vector control;

awareness of

potential

exposure/infection

from blood sources

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trypanosomiasis);

Trypanosoma brucei

rhodesiense

(East African

trypanosomiasis)

infections are found in a

number of

antelope and other ungulates

that

act as reservoir hosts. Rodents

and

some mammals are reservoir

hosts

for Trypanosoma cruzi

and organ

transplants.

Transmission of T. cruzi is

through the

infected feces of the triatomid

bug; the

bug takes a blood meal,

immediately

defecates, and the human host

scratches the infected feces

into the

bite site; bug saliva contains

an irritant

that stimulates scratching

(transfusions, shared

needles,

organ transplants).

Laboratory

accidents while handling

infected blood have been

reported.

Nematodes,

intestinal

These roundworms generally

do no

have animal reservoirs

relevant to

human infection. One

exception is

the pig ascarid; human

infections

have been reported. These

worms

are found worldwide; Ascaris

lumbricoides is probably the

most

common parasite of humans,

although some would argue

that

Enterobius vermicularis is

number

one. Strongyloides stercoralis

is

particularly important as the

causative agent of severe

disease in

the compromised host.

A. lumbricoides and Trichuris

trichiura eggs

must undergo development in

the soil

before they are infective; thus

children

who play in the dirt are a

particularly

high-risk group. Ingestion of

food and

water contaminated with

infective eggs

is the primary route of

infection.

Hookworm and S. stercoralis

infections are

initiated by larval penetration

of the skin

from contaminated soil.

Pinworm infection (E.

vermicularis) is

acquired through ingestion of

infective

eggs from the environment

Avoiding ingestion of

contaminated soil and/or

avoiding frequenting soil

contaminated with

hookworm

eggs (pets, soil, water,

warmth,

warm weather);

treatment for

pinworm is

recommended, but

reinfection is common

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(hand-tomouth).

Nematodes,

Tissue

Trichinella spp. have a

number of

animal reservoir hosts,

including

bears, walruses, pigs, rodents,

and

other animals. Dog and cat

hookworms cause cutaneous

larva

migrans (CLM), and the dog

and cat

ascarid, Toxocara spp., causes

visceral and ocular larva

migrans

(VLM, OLM). These

infections can be

serious and cause severe

disease if

not treated.

Trichinella organisms are

acquired by

ingestion of raw or poorly

cooked

infected meat.

CLM is caused by skin

penetration of

infective larvae from the soil;

children

should avoid sandboxes where

dogs and

cats are known to defecate.

Larval

migration is limited to the

skin.

VLM and OLM are caused by

accidental

ingestion of Toxocara spp.

eggs from

contaminated soil; larval

migration

occurs throughout the body,

including

the eyes.

Adequate cooking of

infected

meat; awareness of

possibility

of contaminated soils for

dog

and cat hookworms

and/or

ascarids; covering of all

sandboxes where pets

have

access to defecation and

children play.

Nematodes,

filarial

Wuchereria bancrofti, Loa

loa, and

Onchocerca volvulus have no

animal

reservoirs and are found only

in

humans, whereas Brugia spp.

can

also be found in cats and

monkeys.

Dracunculus medinensis can