The findings with an infectious agent isolated from cases of tendovaginitis or arthritis of chickens were as follows:—

Cultures, stains, and darkfield studies of material containing this agent for spirochetes, bacteria, and pleuropneumonia-like organisms were negative.

The yolk sac was the preferred route of inoculation, all of the embryos dying in 4 to 12 days.

The agent passed through a bacteria-retaining filter, but with a drop in titer.

Antibiotic sensitivity tests revealed that the agent was most sensitive to the tetracycline antibiotics, less sensitive to chloromycetin, dihydrostreptomycin, and resistant to penicillin.

From electron micrographs it was concluded that the agent is a rigid, dense, coccobacillus from 0.2 µ to 0.5 µ, in size.

The agent produced a cytopathogenic effect in tissue cultures of chick heart fibroblasts.

The agent had no gross effect on suckling or newly weaned mice, pigs, or guinea pigs.

Reasons for suggesting that the agent is most similar to a rickettsia or possibly a large virus are discussed.

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