Parasites

Parasites encompasses protozoa, including amoebae and flagellates, as well as multicellular parasites such as helminths (roundworms) and platyhelminthes (flatworms).

Protozoa

Amoebae are unicellular microorganisms which are trophozoites in that they have 2 stages; an active motile feeding stage that is non infectious and a cyst stage that is infectious. Important amoebae include the following:

• entamoeabae histolytica: have a highest infection rate in tropical and sub-tropical (which includes Florida and Southern California) area.

• naegleria are found in contaminated water (includes Florida) and cause a fatal meningoencephalitis.

• acanthamoebae are a potential pathogen in ocular keratitis. Improper contact lense maintenance can result in infection. Incubation time is longer than naegleria.

• Giardia lambia is a flagellate. Symptoms can be mild to severe (particularly for those with immunocompetency like AIDS patients).

• Balantidium coil is the only member of the ciliate group which is pathogenic for humans, particularly for AIDS patients.

• cryptosporidium typically caues watery diarrhea with abdominal pain that lasts for 1-4 weeks or more. Children, the elderly and immunosuppresed are particularly susceptible to severe or protracted disease. It can be spread when the feces of an infected animal is deposited in the water supply. One of the largest breakouts occurred in Milwaukee in 1992. Crytosporidium can be particularly severe in AIDS patients. Crytosporidium belongs to the group coccidia.

• isospora belli also belong to the coccidia. It reproduces in the intestinal epithelium and results in tissue damage.

• enterocytozoan is a species of the microsporidia and causes chronic diarrhea in AIDS patients.

• pneumocystis carinii is an important cause of pneumonia in immunocompromised people and is transmitted by inhalation (but not from person to person). Untreated P. carinii mortality is almost 100%.

Plasmodium are blood and tissue protozoa. These parasites require two hosts; (1) a mosquito for sexual reproduction and (2) animals for asexual reproduction. Infection is by bite of the anopheles mosquito. Human pathogens include the following:

• P. vivax (the most common human plasmodia) which invades immature erythrocytes and is characterized by schuffners dots which are little red dots in the parasites. P. vivax has a malaria paroxysm of (1) a cold phase (chills), (2) hot phase (fever) (3) and a wet phase (sweating) which occurs ever 3 days (48 hours).
•  P. ovale which also invades immature erythrocytes and is also characterized by schuffners dots. Paraoxysm lasts 3 days as with P. vivax.
• P. falciparum has no erythrocyte selectivity and is the most likely malaria to cause severe disease and death. Chrloroquine is the drug of choice for treatment. Paroxysm lasts 3 days (48 hrs) as with P. vivax and P. ovale.
• P. malariae infects mature erythroctyes. Unlike with the above 3 agents, its paroxysm is every 4 days (72 hours).

Babesia is common in the Northeast (like Nantukket) and is transmitted by Ixodid ticks.

Toxoplasma gondii Toxoplasma is a globally distributed pathogen of humans and animals. Between 30-80% of the human population carries latent infeciton with this opportunisitc parasite. T. Gondii develops in the intestinal cells of cats. Humans can become infected by ingestion of infective ooxycts from cat fecal contamination or by ingestion of improperly cooked meat from an intermediate host. Infection can be serious in immunocompromised people. This is a risk in AIDS patietns, where about 25% of those carring latent T. gondii infection undergo reactivating toxoplasmosis in the central nervous system.

Leishmania are flagellated protozoan affecting tissue and blood. Three disease producing species are L. donovani (which is the major species and causes visceral leishmaniasis as compared to the other cutaneous producing species), L. tropica and L. braziliensis.

Trypanosomes include include the following:

• African trypanosomiasis which cause sleeping sickness
• Trypanosoma cruzi which is transitted by the reduuvid bug (kissing bug). After the bug bites on the face it deposits its feces which transmits the infection. It causes chagas disease and occurs mainly in children under 5.