Seven species of Eimeria are known to infect chickens. These are Eimeria acervulina, E. brunetti, E. maxima, E. mitis, E. necatrix, E. praecox, and E. tenella. Although any of these species may be found in a poultry operation, E. acervulina, E. maxima, and E. tenella appear to be most prevalent in the U.S. This protozoan parasite exists as a highly resistant oocyst form in litter. The oocysts vary in size, with E. maxima being the largest (about 20 x 30 microns) and E. mitis the smallest (about 14 x 16 microns). The oocysts are discernable only by use of a compound microscope. Due to the wide range of oocyst sizes, it is nearly impossible to distinguish between different species of coccidia. Molecular techniques have been developed that uses DNA extracted from the parasite to speciate the oocysts (see Detection section).
Eimeria have a complex life cycle that begins after oocysts that are in the litter are ingested by chickens. The grinding action of the gizzard coupled to the enzymes in the gut of chickens leads to release of the sporozoite stage. The sporozoites search out particular regions of the gut and invade the epithelial cells lining the intestine. This invasion may occur within 1 to 6 hours after the oocysts are ingested. After invasion, the sporozoites undergo replication, which leads to a rapid increase in another stage of the parasite called merozoites. This developmental stage breaks out of the gut cells and invades more cells of the gut, multiplying once again. The effects of coccidiosis are generally associated with the lysing of host epithelial cells by merozoites. As many as four generations of merozoites may develop in the gut during an infection. The number of generations is dependent on the particular species of Eimeria. Some, as yet unidentified, signal tells the merozoites to develop into the sexual stages called micro- (male) and macro-(female) gametocytes. These develop into micro- and macrogametes which fuse to form a zygote. The zygote develops into an oocyst stage that are eventually released in the feces. These oocysts are covered by a hard shell, but first must undergo further development (sporulation) in litter to become infectious for chickens. The whole process between oocyst ingestion and release may take between 4-6 days to complete.
The effects of coccidiosis are due to a number of factors. All of the observed effects are related to disruption of the epithelial cells lining the intestine by the release of parasite stages. While infection with high doses of some Eimeria species (E. tenella, E. necatrix) may cause death to chickens, usually the effects are insidious and are not apparent to the poultry farmer until the chickens are sent to market. The main effects that cause economic losses are a decreased weight gain due in part to the malabsorption of nutrients through the gut wall. This effect causes an increased feed conversion ratio, which is the amount of feed converted into body weight, because feed that is consumed is used inefficiently. Chickens that are infected with high levels of coccidia display symptoms such as droopiness and emaciation and may never achieve weight gain equal to their uninfected counterparts.
The success of the U.S. and worldwide broiler industry is due in part to the development of anti-coccidial drugs. These compounds are added to poultry feed and prevent the intracellular development of Eimeria stages inside the chicken gut. The drugs are removed from feed about 1 week prior to the chickens being sent to market as a way of preventing drug residues in the meat product. While anti-coccidial drugs continue to be the primary means of preventing avian coccidiosis, there is an effort, due to the ability of Eimeria parasites to become resistant to drugs, to develop alternative control measures.
One such approach is the administration of a mixture of low doses of virulent or attenuated Eimeria species oocysts. These live vaccines have been administered in a number of ways including spraying on newly hatched chicks or the incorporation into a gel matrix that are ingested by chicks soon after hatch. Because chickens develop immunity to Eimeria, there has been substantial effort to develop "subunit" vaccines against coccidiosis. These vaccines utilize genetic engineering technology to produce protein components of Eimeria parasites. The rationale behind this approach is that harmless, laboratory strains of bacteria can be utilized to produce "recombinant" proteins that may be used to immunize chickens either in ovo (in the egg) or at hatch. If successful, chickens will be resistant to a subsequent Eimeria infection because they have been immunized with a protein on the surface of the parasite. As yet, there are no commercial subunit vaccines available to prevent avian coccidiosis.
Chickens that become infected with one species of Eimeria develop immunity to challenge infection. However, the immunity appears to be very specific, such that immunity against one Eimeria species will not protect against another species. In fact, this is why commercial live or attenuated coccidiosis vaccines are successful but must contain multiple species of Eimeria. There are reports that immunity against certain strains of E. maxima does not protect against other strains of this species. Why this phenomenon occurs in E. maxima is a topic a much research and discussion. The induction of solid immunity to coccidiosis depends on a number of factors including the primary infectious dose and number of doses, the immune and nutritional status of chickens ingesting oocysts (e.g. some viruses are known to depress immunity), as well as the genetic makeup of the chickens (e.g. some chicken strains are more susceptible to coccidiosis than others).
The immune response to avian coccidia appears to involve the production of antibodies as well as immune cells and their products (i.e cytokines). The latter cellular response appears to be necessary and sufficient to protect against subsequent Eimeria infection. A great deal of research is directed at identifying which cytokines are important to immunity and if these immune-stimulators can be used in conjunction with vaccines to improve protective efficacy.
In addition to observing clinical signs, traditional methods for detecting Eimeria infection in poultry is a gross examination of intestinal tissue during necropsy for coccidial lesions. Most species of avian coccidia will invade a specific area of the gut. For instance, E. acervulina tends to infect the upper intestine and in particular the duodenum, whereas E. tenella infects the caecum. Other species are generally found in specific regions (e.g. E. maxima- middle intestine), but can, during heavy infections, spread to other areas of the gut. Confirmation of a coccidial infection is by examination of gut scrapings using a compound microscope at 250 - 400 X magnification. An excellent description of techniques for diagnosing coccidiosis can be found in "A Guide for Diagnosis of Coccidiosis in Chickens", Long, P.L. and Reid, W.M., Research Report 404, The University of Georgia, College of Agriculture Experiment Stations, August 1982. While examination of fecal material in litter for Eimeria oocysts is helpful in diagnosing coccidial infection, it is nearly impossible to differentiate species of Eimeria based on microscopic observation of oocysts because morphology of the oocysts are almost identical. Molecular techniques have been developed utilizing polymerase chain reaction (PCR) that can differentiate between species of Eimeria and have been used by a number of groups to estimate species distribution in commercial poultry houses.
Coccidia have been found wherever poultry are raised. The spread of this parasitic disease is enhanced by poor biosecurity and management practices as well and by the very fact oocysts are so resistant to destruction. While there are seven species of Eimeria that are known to infect chickens, typically a poultry facility will contain only 1-3 species at a time. In the U.S., the species E. acervulina, E. maxima, and E. tenella are found most often. However, reports of increased incidence of E. mitis and E. praecox have been surfacing. There is evidence that protection against the major species of coccidia will allow for the emergence of minor species in a poultry operation. Thus, vaccines, be they oocyst-based or subunit, must provide protection against those seven species that are pathogenic for chickens.