By Dr. Maral Rahmani
PoultryMed Veterinary Services
July 2025
Introduction
Coccidiosis remains one of the most economically significant enteric diseases in poultry worldwide. Caused by protozoan parasites of the genus Eimeria, the disease affects both commercial and small-scale flocks. It compromises gut integrity, reduces performance, and predisposes birds to secondary infections.
Etiology and Transmission
Coccidiosis is caused by ingestion of sporulated oocysts shed in the feces of infected birds. Once ingested, the oocysts release sporozoites that invade the intestinal epithelium, triggering multiple stages of intracellular replication. Each Eimeria species targets specific sections of the gastrointestinal tract:
- E. acervulina – upper intestine
- E. maxima – mid-gut
- E. tenella – ceca- In older birds like broiler breeders
- E. necatrix – mid to lower intestine (highly pathogenic)
- E. brunetti – lower intestine
The prepatent period ranges from 4 to 7 days. After this, birds begin shedding large quantities of oocysts into the environment, where they sporulate under warm, moist conditions and become infective.
Clinical Presentation
Clinical signs vary depending on species, challenge level, and immune status. Common signs include:
- Decreased feed intake
- Diarrhea (watery, foamy, or bloody)
- Poor growth or weight gain
- Pale combs, huddling, and depression
- Increased mortality, especially in naive or young birds
Subclinical infections, though less obvious, can cause significant production losses through reduced feed efficiency and uneven flock performance.
Diagnosis
Diagnosis involves:
- History and clinical signs
- Gross postmortem lesions
- Microscopic identification of oocysts in feces or intestinal scrapings
- Lesion scoring (e.g., Johnson and Reid system for broilers)
- Molecular tools (e.g., PCR) for species confirmation in complex cases
Control Strategies
In Commercial Flocks (Broilers and Breeders): Control requires integrated programs combining hygiene, management, and prophylaxis:
- Coccidiostats: Widely used in broiler diets. Shuttle and rotation programs help delay resistance.
- Vaccination: Increasingly used in broilers raised without antibiotics and in all broiler breeder replacement programs to induce early immunity.
- Litter management: Keeping litter dry limits sporulation.
- Monitoring: Lesion scoring and fecal oocyst counts support program adjustments.
Broiler breeders are particularly sensitive to coccidial challenges during the rearing phase. Subclinical infections can reduce uniformity, delay sexual maturity, and compromise long-term performance. Vaccination is the primary control tool in breeders, often supported by controlled oocyst cycling and regular monitoring to ensure effective immunity development before lay.
In Small and Backyard Flocks: Small flocks often face unique risks:
- Mixed ages and limited biosecurity increase exposure.
- Wet litter or muddy outdoor pens support oocyst survival.
- Birds may go untreated due to late recognition of clinical signs.
Key prevention tips for small flocks:
- Keep coops dry and well-ventilated
- Clean feeders and waterers regularly
- Avoid overcrowding and mixing age groups
- Consult a vet before using over-the-counter anticoccidials
Backyard birds can develop severe disease quickly, especially if the owners are unfamiliar with early signs. Education and timely intervention are crucial in these settings.
Conclusion
Coccidiosis remains a persistent challenge in all types of poultry production. Whether in commercial broiler breeders or small backyard flocks, the key to control lies in early detection, preventive management, and strategic use of medication or vaccination. Tailoring programs to flock size, age, and risk factors ensures optimal bird health and performance.
Figure 1: Fecal material with fresh blood – Photo by Dr. Maral Rahmani
Figure 2: Oocytes under the microscope from a clinical case – Photo by Dr. Maral Rahmani
