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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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