Institute for Animal Health: Disease Facts- Avian coccidiosis

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The infective stage (the sporozoite) of the parasite Eimeria is seen burrowing into a cell in the intestine (top right). The sporozoite is packed with special proteins, including enzymes that allow it to penetrate the cell (right) and then begin its development. As the parasite develops through its various stages, large numbers of the parasite are produced, which move into uninfected cells and spread the infection. The result is that the cells become leaky, blood vessels are damaged. The loss of blood and fluid is often fatal.

The life cycle of Eimeria is complicated (see below) but it ends with the production of infectious egg-like structures called oocysts which are excreted by birds into the environment. The oocyst can remain infective on the ground for many weeks. Eating the oocyst starts a new infection. The time interval between eating an infective oocyst and the appearance of clinical signs is generally between 4 and 6 days.

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A single oocyst of Eimeria tenella will produce a million more

One gram of litter can contain between 100,000 and 200,000 oocysts

Basic stages in the life cycle of Eimeria

Oocyst > Sporozoite > Merozoite > Microgametes > Oocyst

Eimeria are present wherever poultry are found. Under natural conditions the birds in constant contact with small numbers of oocysts and quickly develop an immunity, often without any showing any signs of disease. However, modern farming methods mean that large numbers of birds are kept together in houses. This favours the parasite and the result is that large numbers of oocysts are present, which leads to a diseased flock. As a consequence, farmers need to use medication or vaccination to protect their birds.

Chemotherapy

Birds reared under intensive conditions on litter are treated routinely with anticoccidial drugs in their food to prevent disease. But there are two big problems with this approach. These are the emergence of drug resistance, and drug residues in chicken meat.

Drug resistance tends to develop over time, sometimes quite rapidly (months), and sometimes after a very long period (many years). This problem is usually avoided by changing the anticoccidial drug at frequently so that the parasites do not have time to develop resistance. This approach is termed the 'shuttle' system.

The potential problem of drug residues is solved by stopping treatment for a certain length of time before the birds are slaughtered and eaten. This allows the birds to metabolise and excrete any potentially harmful substances.

Vaccination

A better biological approach to the control of coccidiosis is the use of vaccines. However the development of an effective vaccine has been difficult. One reason is that vaccination against one species of Eimeria does not give protection against any of the other species. one solution was to develop a vaccine using small numbers of fully virulent oocysts from the most important species of Eimeria.

A vaccine, based on naturally attenuated forms of Eimeria is now available and used mostly for egg laying birds. Chicks are treated soon after hatching, and they have life-long immunity from a single treatment.

The host range in birds is determined by the species of Eimeria

Chickens are parasitised by seven species:	Eimeria acervulina Eimeria brunetti Eimeria maxima Eimeria mitis Eimeria necatrix Eimeria praecox and Eimeria tenella
Turkeys are parasitised by at least seven species of which the most important are:	Eimeria adenoeides Eimeria gallopavonis Eimeria meleagridis and Eimeria meleagrimitis
Geese are parasitised by two species:	Eimeria truncata (unusually this is found in the kidney) Eimeria anseris

Institute for Animal Health