Abstract
Little is known about the importance of mycoplasmas in equine respiratory disease, and this project was undertaken to investigate the occurrence of mycoplasma antibody in horses. Two mycoplasmas, M. equirhinis and an unnamed species, N3, were selected from 13 strains recently isolated from the nasopharynx of horses. Immunization of rabbits and horses resulted in the development of specific antibody. Complement-fixation (CF) was found to be more sensitive than metabolic-inhibition and growth-inhibition for the detection of antibody in equine hyperimmune sera, and this method was used for the survey of antibody in other horses. Six ponies were chosen which were apparently free of mycoplasmas, without CF antibody to M. equirhinis and with low levels to N3. Two were inoculated intranasally with M. equirhinis, two with N3 and two remained as controls. N3 was not recovered from any pony, but within five days M. equirhinis was recovered from every animal, indicating that it had spread naturally from the inoculated ponies to the others. After six weeks it became difficult to isolate the organisms, and levels of CF antibody rose first in the nasal secretions and later in the sera. After several weeks antibody declined to indetectable levels. No clinical signs of infection were seen. In a survey of 817 sera taken over one year from 85 racehorses in seven stables, every horse was found to have CF antibody to M. equirhinis and N3 at some time during the sampling period. In five stables most horses had no M. equirhinis antibody when they entered the stable, but within two months they invariably developed significant titres, implying that they had been infected. When compared with a survey of 256 sera from 81 horses completed two years previously, it appeared that antibody to both mycoplasmas was more prevalent in the recent survey.