Coccidiosis

Coccidiosis

The family Eimeriidae consists of hundreds of sporozoan organisms that are parasitic to a variety of vertebrates. Coccidiosis is a common name for the diseases due to these sporozoan parasites. Coccidiosis has been reported in chickens, geese, ducks, turkeys, cattle, sheep, rabbits, pigs, fish, reptiles, horses, dogs, and cats, including man.

Eimeria is characterised by the presence of four sporocysts in each spore. The sporocysts occur primarily in the intestinal cells of the host.

Life cycle

Infection is due to ingestion of oocysts by birds. The oocysts are extremely resistant to adverse conditions and will remain viable in the soil for a long time, sometimes even up to more than 15 months. They are minute in size, measuring about 10 to 30 microns. Oocysts are so light that they can be dispersed by wind.

Following ingestion by a bird, the oocysts rupture in the digestive tract and liberate sporocysts that contain sporozoites. The sporozoites invade the cells of the host’s intestinal epithelium where they divide by schizogony to form merozoites.

The infected epithelial cells rupture, releasing more merozoites, which in turn invade more epithelial cells. The process continues until the intestinal wall is severely eroded by the infection.

Some of the merozoites become sex cells. Fusion of these sex cells or gametocytes leads to the formation of oocysts that are passed out in the faeces.

Pathogenesis

E. tenella is a parasite of the caecum of mainly young birds that are under four weeks old. Older birds are increasingly resistant to infection. The severity of the infection depends on the initial dose of the oocysts ingested by the bird. The greater the dose the more serious is the infection. A dose of 200000 oocysts is enough to kill 1 to 2 week old chicks. Death is usually due to severe haemorrhage of the caecal epithelium evident by the fifth day of the infection.

Infection in the young nonimmune chicks is rapid and serious. Spots of blood appear in the faeces around day 5 after infection. By day 7, the faeces are watery and contain oocysts. The birds show signs of loss of appetite and energy, looking very drowsy. About 90% of them die during the first 7 days of infection. E. necatrix causes a milder, chronic infection than E. tenella.

Bovine coccidiosis is not a serious disease but it can kill calves if the infection is quite heavy. The effects of infection in cattle are weight loss, slow growth and a reduction in milk production.

Control

Clean, uncrowded, dry living quarters are of great importance in controlling the infection in chickens and livestock.

Isospora

Isospora belli and Isospora natalensis are two eimerian parasites of man. They invade the small intestine and shed sporocysts that are detected in the faeces of an infected person. Symptoms are usually not serious but include abdominal pain, diarrhoea, flatulence, abdominal cramps, nausea, loss of appetite, lassitude, loss of weight and fever.