The response of chickens to experimental infection with strains of M. gallisept1cumof different virulence andM. gallinarum

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Abstract
Three strains of M. gallisepticum comprising S6 of low passage (virulent), S6 of high passage and A514 of high passage in artificial media and one M. gallinarum strain of high passage, were inoculated into 18‐day‐old chick embryos. Four groups of infected chicks hatched, one corresponding to each mycoplasma strain; these were investigated during the first 28 days of life and again for a period in lay. During the first 28 days of life, the virulent S6 strain was recovered more frequently from a wider variety of tissues for a longer time than were the other two strains of M. gallisepticum; this strain also produced more severe clinical disease including nervous signs, respiratory lesions and swollen hocks. Although nervous signs were confined to birds infected with the virulent S6, both S6 strains were isolated from brains and hocks, suggesting this strain's proclivity for these sites. One A514 infected chick developed unilateral eye enlargement from which mycoplasma were isolated. All strains of mycoplasma were isolated from respiratory tissues, indicating a proclivity common to all. Recoveries of mycoplasma were often associated with signs and/or lesions but not always, and recoveries also occurred in the absence of signs and lesions. M. gallinarum caused no clinical signs or lesions in young chicks and was rarely isolated. In mature birds, mycoplasma were isolated from the respiratory tissues of some infected with either of the two S6 strains or with M. gallinarum, but not from A514 infected chickens. In both younger and mature chickens, mycoplasma were most likely to be recovered from respiratory tissues and the infra‐orbital sinus. Most chicks infected with the virulent S6, showed positive haemagglutination inhibition (HAI) reactions from 4 days of age which preceded the rapid serum agglutination (RSA) reactions. In contrast, chicks infected with other strains of M. gallisepticum either showed no positive HAI reactions, or developed them after 21 days. In either case, positive RSA preceded HAI reactions. The numbers of mature birds examined were small, but, comparing isolation of M. gallisepticum with serological results, it appears that false positives occur with the RSA and both false positives and negatives with the HAI test. No egg transmission of mycoplasma was observed.