Host and Husbandry Factors Related to Coccidiosis

Factors related to the host

• Age: Coccidiosis is a disease of young fowl. On a chicken farm, maximum excretion of oocysts is often observed between 28 and 35 days. The number of oocysts in the litter is then at its maximum, afterwards it declines. Fowl aged 1 to 14 days are less susceptible to coccidiosis.
• Genetic constitution: Some breeds or strains of chickens are less susceptible to coccidiosis. However, selection trials based on criteria of resistance to coccidia have not been successful because they are not compatible with the selection criteria for zootechnical performance.
• Immunity potential: After a first infection, species-specific immunity is established which may sometimes allow weak parasite development. Depending on the degree of protection, immunity contributes to a reduction in the severity of the lesions, in the production of oocysts and to an improvement in performance. 
• Diet: Nutrition plays a primordial role in the chicken's state of health. The immune response is not only improved by vaccinations and/or chemoprophylaxis against infectious diseases and hygiene but also by an intake of adequate nutritional constituents. 
• Anticoccidial protection: For more than half a century coccidiosis has been controlled by drug prophylaxis (continuous use of anticoccidials in the diet or treatments in drinking water). The farmer today has the choice between the use of anticoccidials and a vaccination for the control of this disease.
• Intercurrent diseases: Intercurrent illnesses promote coccidiosis by reducing host resistance, consumption of food and consequently the ingestion of anticoccidials.

Husbandry conditions

Coccidia are very prolific and generally do not kill their host. This balance between the parasite and its host may be disrupted by disturbances in husbandry conditions, as in the outbreaks of Eimeria tenella.

There are many stresses, often due to management errors (temperature, relative humidity, lighting, ventilation, diet, water supply, low amounts of anticoccidial incorporated in the feed), which increase the chicken´s susceptibility and promote the development of coccidia and the appearance of coccidiosis.

Lighting, for example, plays an important role; an intermittent lighting programme increases the risk of coccidiosis as compared with a continuous programme. This would be explained, on the one hand, by greater litter scratching activity during the lighted periods, which promotes sporulation and survival of the oocysts, and, on the other hand, by the cessation of feed consumption and therefore of anticoccidial intake during the periods of darkness.

Intensive rearing, because of the high stocking densities, the limited areas of movement, and the exhaustive selection of chickens, promotes infection of fowl and the development of clinical or sub-clinical coccidiosis.

It must be emphasised that contamination is inevitable and permanent in poultry even on farms where prevention and hygiene measures are correctly applied. Coccidia appear with the first group of chickens in a new farm building. It may then have serious consequences because the disease affects older chickens, which no longer have time to recover before slaughter. This initial contamination of the farm buildings has not yet been understood.

With regard to the interaction of numerous factors in the host-parasite balance, it is important to distinguish between primary and secondary coccidiosis.

1. Primary coccidiosis can result in subclinical (not directly noticeable) or clinical illnesses in which only coccidia are causing problems. No bacteria or viruses are involved.

The main cause of this type of coccidiosis on farms is the progressive exhaustion of the anticoccidial agent used in prevention of the disease. With routine placement of flocks, the number of resistant oocysts increases up to a threshold beyond which coccidiosis appears. A change of anticoccidial is needed. The speed of the appearance and development of resistance depends on the anticoccidial used, e.g., ionophorous or chemical products. To prevent the appearance of primary coccidiosis, it is essential to detect resistance before the disease appears. One method, the anticoccidiogram or AST (Anticoccidial Sensitivity Test) allows detection and evaluation of the potential degree of resistance of the coccidia to several anticoccidials. Depending on the results and the history of the farm, a new product regime consisting of one or several more effective anticoccidials can be suggested.

2. Secondary coccidiosis is the consequence of the poor state of health of the chickens or poor management. It appears suddenly, during the development of a group. It can be treated, but finding its primary cause is essential.
Any factor which provokes a reduction in the host's natural resistance or which leads to a reduction in feed and water consumption and consequently of anticoccidial intake promotes the development of coccidiosis.

Among these factors may be mentioned:

• Those related to the host: day-old chickens of poor quality, intercurrent illnesses, etc.
• Those related to husbandry: problems of general hygiene, stress and errors in management;
• Those related to diet: errors in the concentration of anticoccidial incorporated into the feed,  i.e., when a anticoccocidial is underdosed it is not sufficiently efficacious to control the development of the parasites and the appearance of the disease.