coccidiosis is one of the most common and economically important diseases of chickens worldwide.
It is caused by a parasitic organism that damages the host intestinal system, causing loss of production, morbidity, and death
This disease has a major economic impact on the global poultry industry.
Coccidial parasites are protozoa belonging to the phylum Apicomplexa. Chicken coccidiosis is caused by seven species, all from the genus Eimeria: E. acervulina, E. Brunetti, E, maxima, E. mitis, E. necatrix, E. praecox, and/or E. tenella. The life cycles of these species are direct. Chickens ingest sporulated oocysts (the parasite egg) from contaminated litter, and these pass into the intestinal tract, where the parasites invade the cells of the intestinal wall. Several cycles of replication occur which lead to the formation of new oocysts which are shed in the feces. Depending on environmental conditions (including temperature and humidity), the oocysts sporulate and become infective. The entire cycle takes 4 to 6 days.
It is the replicative phases of the parasite which lead to damage to the intestinal tissues. Individual birds may show no clinical signs or may suffer a mild loss of appetite, weight loss or decreased weight gain, diarrhea (which can be bloody), dehydration, and death. Resistance develops rapidly and infections can be self-limiting, but naïve birds which consume large numbers of oocysts can be severely affected and die. Immunity is strictly species-specific which means that birds exposed to one Eimeria remain susceptible to infection from all other species.
The degree of injury caused by the seven species varies, with some developing deep in the intestinal mucosa, causing widespread damage and distinct gross lesions (e.g., E. tenella). Other species are less destructive but may still have a significant impact on production. All species are potentially important economically.
Methods of Control
Eimeria is present worldwide and is ubiquitous under intensive farming methods. Up to six species have been shown to occur simultaneously on one farm. The omnipresent nature of Eimeria precludes eradication as a practical option for control. Since species-specific immunity develops rapidly, the management of coccidiosis aims to achieve a balance between allowing natural immunity to build up and preventing high oocyst exposure to naïve birds. Hygiene, anticoccidial drugs, and vaccines all play major roles.
As species of Eimeria have direct life cycles, mechanical transmission is the primary means of spreading between farms and between sheds on a farm. Oocysts are resistant to the environment, both to climatic extremes and disinfectants, and can survive for weeks in soil. However, they only last for days in litter due to heat caused by fermentation and the presence of ammonia. Good hygiene, such as cleaning boots and exchanging clothes between sheds, and the eradication of rodents, assists in minimizing the transmission of oocysts. Effective farm management, such as well-maintained, drip-free water lines, minimizes the level of infective oocysts in the litter, as desiccation significantly reduces sporulation.
Chemotherapy has been the main approach for controlling coccidiosis in chickens. Anti-coccidial drugs are usually used preventatively and if a farmer were to wait for overt signs of disease before treating the flock, morbidity and mortality would be high and the economic damage was already done.
Almost all commercial
intensively farmed flocks are administered anti-coccidial drugs prophylactically. When given at the correct low preventative doses, Eimeria species can complete their life cycles without large numbers of infective oocysts building up in the environment. Such subclinical infections result in the development of strong, specific natural immunity without the overt disease.