Coccidiosis is an intestinal disease affecting poultry flocks around the world. It is caused by the protozoan parasite Eimeria. Birds contract the parasites by ingesting them in the droppings of infected birds.
Coccidiosis results in intestinal lesions, impaired growth, poor feed utilization, poor flock uniformity and increased mortality. Because it damages the intestinal lining, coccidiosis predisposes birds to the development of necrotic enteritis, another intestinal disease caused by the bacterium Clostridium perfringens.
For years, the primary method for controlling coccidiosis, especially in broilers, has been in-feed anticoccidials. With their prolonged use, however, Eimeria organisms have become resistant to anticoccidials, hindering coccidial control. In addition, consumer demand for poultry raised without antibiotics has increased.
Stages of coccidia in chickens appear both within the host as well as outside. The developmental stages in the chicken give rise to a microscopic egg (called an oocyst) that is passed out in the droppings.
Under proper conditions of temperature and moisture, the oocyst develops within one to two days to form a sporulated oocyst, which is capable of infecting other chickens. At this stage, the oocyst contains eight bodies (called sporozoites), each of which is capable of entering a cell in the chicken’s intestine after the oocyst is eaten.
When sporozoites enter the cells, they divide many times producing either a few or many offspring (merozoites). The numbers produced depend on the species of coccidia involved. Each merozoite, in turn, may enter another intestinal cell. This cycle may be repeated several times. Because of this cyclic multiplication, large numbers of intestinal cells are destroyed.
Eventually, the cycle stops and sex cells (male and female) are produced. The male fertilizes the female to produce an oocyst, which ruptures from the intestinal cell and passes in the droppings. Thousands of oocysts may be passed in the droppings of an infected chicken; therefore, poultry raised in crowded or unsanitary conditions are at great risk of becoming infected.
These trends have prompted more producers to use vaccination to control coccidiosis. With vaccination, a controlled, balanced dose of Eimeria antigens are administered once to day-old chicks, initiating the development of natural, lifetime immunity against Eimeria.
Vaccination eliminates the need for withdrawal times, which are required when anticoccidials are used in the feed, as well as concerns about possible drug residues in poultry meat. Vaccination is also used to restore sensitivity to anticoccidials by replacing resistant, in-house, wild strains of Eimeria with drug-sensitive Eimeria strains.