Eimeria necatrix in chickens
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2020;24(3):12.
Over the past few months, we have identified a number of cases of E. necatrix by histopathologic examination. The tissue damage caused by this organism is impressive and warrants a review of this infection. Chickens infected by Eimeria necatrix can have bloody droppings that contain fluid and mucus. The birds will appear depressed, lose weight, the flock can become uneven, and mortality can be greater than 25 percent. Egg layers can have decreased egg laying potential and can lose pigmentation.
On postmortem examination, the small intestines are markedly dilated, thickened, congested and white to yellow foci or plaques and petechial hemorrhages may be present over the serosal surface. In dead birds, these serosal lesions can appear white and black, leading to the description of a ‘salt and pepper’ appearance. When the intestines are opened, the lumen is often filled with fluid, blood and necrotic debris. The mucosa may be markedly thickened, irregular and dark brown due to severe confluent necrosis. Eimeria necatrix is found primarily in the jejunum, but can extend into the duodenum and ileum in severe cases.
Eimeria necatrix does not easily reproduce so it does not compete well with other coccidia. This low ability to reproduce results in this organism taking a long time to propagate. Consequently, older birds such as breeder or layer pullets 9-14 weeks of age are most commonly affected. In the life cycle of Eimeria necatrix, the asexual cycles occur in the small intestines and the sexual cycles occur in the ceca. As a result, the oocysts can only be found in the ceca. Since oocyst production is also poor, birds often die before oocysts are present in the feces.
The reason for Eimeria necatrix causing extensive tissue destruction can be observed histologically as the schizonts penetrate deep into the mucosa and submucosa and destroy blood vessels and smooth muscle (Fig. 1). This makes epithelial regeneration difficult and if birds survive, areas of scar tissue can develop in the intestine.
Figure 1. Large schizonts of E. necatrix (circle) in deep lamina propria of jejunum. Tunica muscularis (star). (20x)
Eimeria necatrix is considered one of the most pathogenic Eimeria species as it only requires 104 - 105 sporulated oocysts for infection. In older birds with intestinal signs and lesions, this infection should be on the differential diagnosis list. AHL
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2. Fitz-Coy SH. Parasitic Diseases. In: Avian Disease Manual, 7th ed. AAAP, 2013:153-178.