Pathogenes Inc.

The protozoa of the class Sporozoea include sporocyst-forming protozoa, many of which cause disease in animals and people around the world.   Coccidiosis is an enteric disease of poultry, caused by protozoan parasites of the subclass Coccidia, genus Eimeria. These are obligate intracellular protozoan parasites that replicate primarily in the intestinal epithelium. These parasites have a monoxenous (one host) life cycle and exhibit a high degree of host-species and tissue specificity. Overall, the combination of losses due to coccidiosis and the costs of prophylactic medication results in significant annual economic losses to the poultry industry.

Avian coccidiosis is especially severe in the poultry industry where intensive housing of birds favors the spread of the disease.  Significant economic loss is due to stunting during production.   Several Eimeria species can infect a single host via the oral route and/or by inhalation of the infectious particles from the environment. Once ingested, the parasite penetrates and damages the mucosal layer of the intestinal wall, causing acute morbidity, e.g, resulting in decreased growth and feed utilization in the infected birds.

Within a host animal's cells, sporozoites develop into multinucleate meronts, also called schizonts. Each meront nucleus develops into an infective body called a merozoite that enters new cells and repeats the process. After a variable number of asexual generations, merozoites develop into either microgametocytes or macrogametes. Microgametocytes develop into many microgametes which, in turn, fertilize the macrogametes. The resulting zygotes encyst  by forming a tough outer coat, and are then called oocysts, which are shed unsporulated in the feces. Under proper environmental conditions, the oocysts sporulate and become infective. Ingestion by susceptible animals then repeats the cycle.

Infection with Coccidia elicits a species-specific immunity. For example, there are seven known species of Coccidia which infect chickens, six of which are considered to be moderately to severely pathogenic.

Numerous methods have been developed to immunize poultry against Coccidia. All commercially successful methods are based on the administration of live encysted protozoa, often of an attenuated strain. The most common route of inoculation is oral, although other routes have been used.

The need for viable protozoa for vaccination is a common factor in all of the current vaccination methods. Non-viable protozoa, or antigens from protozoa, have been unsuccessful in conferring the high level of immunity needed to protect against clinical infection.   This is the area of our research.

Interestingly, limited passage of merozoites has been demonstrated.  For example sporozoites were infected into primary chick kidney cells (PCK) that are grown in culture as cell aggregates, using a modification of the method described in D. J. Doran, J. Parasit. 57: 891-900, (1971). However, once merozoites produced by this method are released, the culture is terminated. It is current dogma that Eimeria exhibits synchronized growth and therefore can not be induced to produce an immortal line of merozoites, ie., a line of merozoites that does not terminate, after a relatively short period of time, in the release of oocysts.