Toxoplasmosis

Toxoplasmosis, caused by a coccidian parasite known as Toxoplasma gondii, is an intracellular human and animal disease with a global distribution, especially in warm, moist areas. In human infection, T. gondii exists as active, proliferative trophozoites during the active phase of the infection and as a cyst in chronic infections. The trophozoite is crescent-shaped and measures 4 to 8 µm in length and by 2 to 3 µm in width, with one end more pointed than the other. In an acute infection, the trophozoites are present in the various tissues and tissue fluids but in the chronic stage, the cysts are mainly found in the central nervous system, muscle and tissues.

Toxoplasmosis has been reported from man, cats, pigs, cattle, sheep, dogs, rodents, birds and many other carnivorous animals. It is a common cause of abortion in cattle in England and New Zealand. The main source of human infection is the domestic cat. Other members of the family Felidae develop the infection and are responsible for maintaining it in areas where cats are absent.

Life cycle

T. gondii primarily exists in three forms: oocysts, tachyzoites, and bradyzoites. Oocysts are only produced in the definitive host, members of the family Felidae. When passed in feces and then ingested, the oocysts can infect humans and other intermediate hosts. They develop into tachyzoites, which are the rapidly multiplying trophozoite form of T. gondii. They divide rapidly in cells, causing tissue destruction and spreading the infection. Tachyzoites in pregnant women are capable of infecting the fetus. Eventually tachyzoites localize to muscle tissues and the CNS where they convert to tissue cysts, or bradyzoites. This is thought to be a response to the host immune reaction. Ingestion of cysts in contaminated meat is also a source of infection, as bradyzoites transform back into tachyzoites upon entering a new host.

For a long time, the life cycle of T. gondii remained unsolved. It is now known that the parasite is actually a coccidian of cats and other animals. Oocysts released by infected cats and other animals are very resistant to environment factors and usually remain viable in the soil and in the dust for more than a year. In warm, moist shaded areas, the cysts can survive for a year. Oocysts are ingested by reservoir hosts when they take water or food contaminated with cat faeces. The reservoir hosts may include carnivores and herbivores. Man acquires infection by eating infected raw or undercooked meat from pig or sheep and by accidental ingestion of infected faeces of cats or dogs.

Once ingested, the oocysts form sporozoites that penetrate the intestinal mucosa either by phagocytosis or by active penetration using enzyme-like factor. Within the intestinal mucosa, the sporozoites divide by schizogony to produce merozoites, including gametocytes. Fusion of the gametocytes produces oocysts, which are expelled in the faeces. The incubation period ofT. gondii in the cat is about 21 to 24 days.

Pathogenesis

The large number of humans and animals that have antibodies against T. gondii antigens seems to suggest that the infections are common but with apparently no accompanying symptoms. The newborn babies are the most affected because they are usually infected transplacentally, usually during the second or third trimester of pregnancy, often from asymptomatic mothers.

Toxoplasma gondii occurs chiefly in the cells of the reticulo-endothelial system. The parasites can be found practically in every organ but are particularly numerous in the brain. In children, the infection affects mainly the central nervous system causing meningitis, encephalitis and encephalomyelitis. In adults, the infection is characterized by the enlargement of the lymph nodes. In congenitally acquired infections, the lesions are normally dominant in the brain, spinal cord and the retina or choroid. Hydrocephalus or microcephalus, mental retardation including blindness in the newborn may indicate toxoplasmosis.

Epidemiology

Animals serve as reservoirs for human infection. Being pets, human contact with cats is usually very close and for this reason, cats are the main source of human infection. Farm animals are also important source of infection. Herbivores become infected by eating food and drinking water contaminated with infected faeces of carnivores.

Human infection may be acquired by direct contact with infected animal tissues, by eating contaminated food or meat and through accidental ingestion of oocysts present in water and faeces of domestic and wild animals. Oocysts can survive in moist, shaded soil for over a year. Invertebrates like flies, cockroaches, earthworms, stable fly, Stomoxys and fleas can transmit the parasite mechanically.

Reservoir hosts play a very important role in the maintenance and transmission of T. gondii in the wild. Rabbits, mice, rats and birds serve as reservoirs of T. gondii and frequently transmit infection directly to the predators by predation or by ingestion of oocysts.

Congenital transmission occurs through the placenta but this probably affects only a very small number of people, although it may be responsible for the majority of human fatal cases. In addition, the parasite can be transmitted via blood transfusion and cell transfer. Infection may be transmitted airborne by inhalation of the organisms discharged from the lungs of coughing infected animals or individuals.

Diagnosis

It is always difficult to find the parasites in the body fluids. The infection can be confirmed by demonstrating specific antibodies or the presence of the toxoplasmas in the cerebral spinal fluid when the brain is involved. Mice are very susceptible to infection with T. gondii and are usually used for serological tests.

Control

Although freezing may destroy most of the cysts, proper cooking of the meat is more reliable in ensuring its safety. The main source of human toxoplasmosis, however, is the ingestion of oocysts present in the faeces of domestic cats present in water and food. Therefore, boiling drinking water and maintaining proper hygienic standards help prevent infection. Acquired toxoplasmosis is treatable with sulphonamides