Eimeria is a microscopic, single-celled parasite belonging to the Apicomplexa group, primarily internal parasites of animals. Widely distributed, Eimeria commonly inhabits the intestinal tracts of various animal species. Its widespread presence significantly impacts animal health.
Coccidiosis: The Disease Caused by Eimeria
Coccidiosis, caused by Eimeria parasites, predominantly affects the intestines of infected animals. Symptoms range from mild to severe, including diarrhea (sometimes bloody), dehydration, weight loss, and reduced growth rates.
The severity of coccidiosis is influenced by the specific Eimeria species, host age, and amount of infective parasites ingested. In poultry, Eimeria tenella, Eimeria acervulina, and Eimeria maxima are economically significant species.
Coccidiosis has substantial economic implications for livestock and poultry industries worldwide. It leads to losses such as animal mortality, reduced weight gain, decreased feed efficiency, and treatment expenses. Global economic losses in poultry alone are estimated to be billions of dollars annually.
How Eimeria Spreads
Eimeria parasites exhibit high host specificity, typically infecting only one or a few closely related animal hosts. For instance, Eimeria tenella infects chickens, Eimeria bovis targets cattle, and Eimeria stiedae is found in rabbits. This specificity often extends to particular intestinal locations, allowing multiple Eimeria species to coexist.
Transmission occurs through the fecal-oral route. The cycle begins when an infected host sheds non-infectious unsporulated oocysts into the environment via feces. These oocysts are protected by a tough cell wall, making them resistant to environmental pressures.
In the external environment, under suitable conditions of oxygen, moisture, and temperature, these unsporulated oocysts undergo sporulation. This transforms them into an infectious form containing four sporocysts, each with two sporozoites. Sporulated oocysts can remain viable in the environment for months to years.
A new host becomes infected by ingesting sporulated oocysts, often through contaminated feed, water, or bedding. Inside the host’s digestive tract, oocysts release sporozoites, which invade intestinal cells. This initiates asexual reproduction (schizogony), causing significant damage to the intestinal lining as parasites multiply. The life cycle continues with sexual reproduction (gametogony), forming new unsporulated oocysts that are then excreted, restarting the cycle.
Controlling and Preventing Eimeria Infections
Controlling and preventing Eimeria infections involves strategies to reduce exposure and enhance host resistance. Good hygiene and sanitation practices are foundational, including thorough cleaning and disinfection of animal pens, regular removal of wet bedding, and effective pest control. Proper biosecurity, such as controlling farm entry, also minimizes oocyst introduction and spread.
Reducing stocking density in animal housing also aids prevention, as overcrowding increases contact with contaminated litter. Anticoccidial drugs, often called coccidiostats, have been a primary control method, typically administered in animal feed or water.
Vaccination programs are gaining importance. Live attenuated vaccines establish immunity and may help reduce drug resistance in field populations. However, drug resistance due to continuous medication use is a significant challenge. This necessitates integrated control approaches, such as rotating different drugs or combining vaccination with medication programs, to maintain effectiveness and sustainability.
Eimeria and Human Health
Eimeria species that infect livestock and poultry are highly host-specific and not transmissible to humans. They do not cause disease in people.
Human coccidiosis is caused by different, related Apicomplexan parasites such as Cyclospora, Cystoisospora belli, and Cryptosporidium. While these parasites share some life cycle similarities and can cause gastrointestinal symptoms like diarrhea, they are distinct species with different host specificities. Therefore, Eimeria species from farm animals do not pose a direct health risk to humans.