Protozoa - Microbial Food Poisoning

Protozoa

Table 33.5 lists common protozoa responsible for variouskinds of illnesses that are transmitted through food and water. While many of these agents are more appropriately discussed in textbooks of microbiology or clinical medicine, a few merit special mention here.

Entamoeba histolytica

Source

Contaminated food and water. Raw vegetables which have not been washed well are an important source.

Human to human transmission is also quite common.

Mode of Action

Recent studies have demonstrated that E. histolytica can be differentiated into at least 18 zymodemes.* Pathogenic strains are all from particular zymodemes of which 7 have been iden-tified so far.

Entamoeba histolytica exists in two forms—vegetative(trophozoite) form and cystic form. Trophozoites invade the colon where they multiply and subsequently get encysted. The cysts are excreted in stools. Ingestion of cysts results in release of trophozoites which colonise the large intestine, some of them even invading the bowel wall causing ulcerations. 

Occasionally the trophozoites may get transported to other organs such as liver where they produce abscesses.

Incubation Period

About 2 to 4 weeks 

Clinical Features

Entamoeba histolytica produces a clinical syndrome referredto as amoebiasis, which has a worldwide distribution and is a major health problem in developing countries. It is estimated that 15% of the population in India may be affected by amoe- biasis.

Intestinal amoebiasis varies in severity from mild abdom- inal discomfort and diarrhoea to fulminating dysentery. Extra- intestinal amoebiasis may involve liver, lungs, brain, spleen, etc.      

Diagnosis

·              Stool analysis: Microscopy for detection of trophozoites, cysts, and pus cells.

·              Serological tests: Indirect haemagglutination test, counter immunoelectrophoresis, ELISA, etc.

Treatment

Symptomatic cases can be treated with metronidazole (30 mg/ kg/day for 8 to 10 days) or tinidazole. Abscesses must be treated surgically. Asymptomatic carriers can be treated (if they are food handlers) with diiodohydroxyquin or diloxanide furoate.

Cryptosporidium parvum

This protozoon causes severe diarrhoea in immunocompro- mised adult patients and immunocompetent children.

Source

·              Contaminated food and water, especially the latter. Decontamination of water can be done by filtration, distilla- tion, or reverse osmosis.

Clinical Features

1.Severe persistent diarrhoea.

2.Malnutrition (in children).

3.Pulmonary manifestations.

4.Toxic megacolon.

5.Cryptosporidiosis is often fatal in AIDS patients. 

Incubation Period

Biopsy of intestine (small or large) reveals Haematoxylin and eosin darkly stained structures 4 to 5 microns in diameter near the tips of microvilli of epithelial brush border.

Treatment

1.IV fluids and electrolytes.

2.Antidiarrhoeal drugs

Microsporidia

The microsporidian genera which cause human disease include Nosema, Pleistophora, Encephalitozoon, Enterocytozoon, and Septata.

Microsporidiosis generally occurs only in immunodeficient patients and can take the form of diarrhoea, keratoconjunc-tivitis, hepatitis, myositis, ascites, cholangitis, and renal or urogenital infections. It has been estimated that microsporidia account for 10 to 40% of AIDS-related diarrhoea.

Treatment involves the administration of albendazole (400 mg twice daily), which helps in relieving microsporidial diar-rhoea, but relapses are common.