SEROEPIDEMIOLOGIC STUDY OF NATURAL TRANSMISSION OF MYCOPLASMA-HYOPNEUMONIAE IN A SWINE HERD

A cohort of 57 pigs in a farrow-to-finish swine herd with mild clinica l mycoplasmal disease was followed to determine patterns of seroconver sion to Mycoplasma hyopneumoniae (MH), detected with an enzyme-linked immunosorbent assay (ELISA). Survival analysis was used to evaluate th e relationship between time to seroconversion and possible risk factor s for MH infection (or enzootic pneumonia). Pigs were housed in outdoo r pens at approximately 9 weeks of age, when passively acquired MH ant ibodies had decayed. From 9 to 11 weeks of age and during a 5 week per iod, pigs were exposed by direct (nose-to-nose) or indirect contact to older seropositive gilts. Blood samples were collected from each pig at 3 week intervals until market age, when they were either slaughtere d or selected for breeding. Antibody concentration was measured as the ratio of optical densities of the serum sample to the positive contro l (S/P). Based on the sample distribution of S/P ratios from pigs in a n MH-free herd, pigs were considered positive when S/P ratios were gre ater than 0.34. At the beginning of the study, all pigs were seronegat ive to MH. Seroconversion was first detected after 21 days, and was mo st frequent about 11 weeks after exposure to older seropositive gilts. By the end of the study, 11 pigs (19%) had seroconverted, with S/P ra tios ranging from 0.40 to 1.11. The presence of gross lung lesions sho wed a moderate to good agreement with ELISA results (kappa = 0.62). Hi stologic lesions were evident in virtually all slaughtered pigs, rangi ng from mild, non MH-specific lesions to severe lesions typical of MH infection. No secondary respiratory pathogens were isolated. Clinical signs were mild and there was no significant difference (P > 0.4) in w eight gain between seropositive and seronegative pigs, or between pigs with and without lung lesions. A Cox regression model was fitted to t he seroconversion data, and opportunity of contact (direct or indirect ) was the only significant variable. After adjustment for breed and an tibody S/P ratio prior to exposure, pigs in direct contact with seropo sitive gilts were seven times more likely to seroconvert than those in only indirect contact.