Coccidiosis can be one of the most economically important diseases in many livestock species. It can be especially devastating to recently weaned kids and, occasionally, cause losses in other age groups. Coccidia are everywhere; it is nearly impossible to find a goat without some coccidia. However, the presence of coccidia in the intestines of an individual does not mean the animal is actually suffering from coccidiosis. Coccidia only cause disease when their numbers become so great that pathological damage is done to the host. Usually poor management is the reason why coccidia numbers increase excessively; thus, coccidiosis may be considered a man-made disease. This also suggests that coccidiosis can be adequately managed. Unfortunately, two forms of the disease are frequently present, one in which the clinical signs are obvious, the other in which no immediately obvious signs are evident. It was thought for many years that the coccidia of sheep and goats were interchangeable between those species, but now it appears that most, if not all, coccidia species are unique to their host. Coccidia from chickens will not infect goats.
The objective of this article is to give goat producers sufficient information to adequately understand the organisms and the disease they can cause. Such understanding is necessary to control and prevent coccidiosis and to reduce or prevent acute and chronic losses. The typical life cycle of coccidia will be described in order to understand where damage is done and why changes in management may be useful. The forms of the disease and how to recognize and diagnose them will be discussed along with treatment and control measures, and a list of available drugs for the control and treatment of coccidiosis will be provided.
Coccidia Life Cycles
It should be clearly recognized that among the internal parasites of goats coccidia are only one of the many parasites of economic importance. Others include round worms (nematodes), flukes (trematodes), tape-worms (cestodes) and a myrad of bacteria, viruses and other organisms. Each type of parasite has a different life cycle, affects the host in different ways and is controlled with different drugs and management schemes.
Basic knowledge of the life cycle of coccidia is necessary to understand the damage done to the host (goat) and to demonstrate why control is so difficult. While several species of coccidia may inhabit the intestines of goats, two species are important pathogens and can cause serious damage: Eimeria ninakohlyakimovae and E. arlongi. Coccidia are intracellular parasites. They live and grow within cells lining the gastrointestinal tracts of their hosts. The oocyst, an egg-like structure, is passed in the feces of infected hosts. When first passed, the oocyst is not infective; it must first undergo a period of development (sporulation) in the environment which takes 2-3 days.
Oxygen and moisture are required for sporulation (the formation of sporozoites within the oocyst). The time required for development is temperature-dependent. In general, the warmer the weather, the faster the development, unless the temperature is high enough to kill the organism.
After sporulation, the oocysts are very resistant to environmental conditions and ordinary disinfectants will not kill them. Extreme desiccation or direct sunlight are about the only environmental factors that are detrimental to sporulated oocysts, and a sporulated occyst may survive for a year or longer if protected from direct sunlight. Therefore, areas under feed bunks and around water troughs may harbor infective oocysts for prolonged period of time.
When a susceptible goat ingests sporulated oocysts, sporozoites are released and enter cells lining its intestine. Within these cells, the organism becomes a schizont or meront, which grows to many times its original size.
Sporocysts escape from the oocyst, invade cells lining the intestinal tract. Asexually produced meroziotes will reinvade more host cells. Some of the merozoites act like eggs and sperm, fuse and the resulting oocyles are shed in the feces. These become "spores" and become infective in 2 to 3 days. The entire life cycle takes about 14 days. Clinical signs of disease depends on how may intestinal cells are damaged by invading organisms (modified after Wright, 1989).