Growth performance and whole-body composition of pigs experimentally infected with Mycoplasma hyopneumoniae

Mycoplasma hyopneumoniae (Mh) is the primary infectious pathogen responsible for enzootic pneumonia in pigs. Although Mh is thought to impair growth performance, whole-body composition, and fat and protein accretion in pigs with pneumonia have not been reported and the mechanism through which Mh reduces growth is unknown. The objectives of this study were to evaluate the effects of Mh on growth performance, whole-body composition, and protein and fat accretion in nursery pigs and to determine whether Mh infection increases the expression of interleukin (IL)-1beta, IL-6, and tumor necrosis factor-alpha (TNF-alpha). Sixty-four 2-wk-old Mh-free pigs were used (two trials) in a randomized complete block design. In each trial, two pigs were housed in each of 16 disease-containment chambers. At 4 wk of age, pigs were inoculated intratracheally with 3 mL of Mh broth (P5722-3, 10(7) cfu/mL) or sterile Friis culture medium. Clinical signs of disease and feed intake were monitored daily and body weight was determined weekly for 4 wk. Whole-body composition was determined from pigs killed 0, 14, and 28 d after inoculation, and the comparative slaughter technique was used to estimate protein and fat accretion. At death, gross lung lesions were quantified, and lung tissue was collected to verify the presence or absence of Mh, and to determine cytokine mRNA levels. Control pigs displayed no overt signs of infection and were Mh-negative and free of pulmonary lesions. Pigs inoculated with Mh showed pneumonic coughing (P < 0.005), were Mh-positive, and had pulmonary lesions that affected 4.5% (P < 0.01) and 14.1% (P < 0.001) of total lung surface area at 14 and 28 d, respectively, after inoculation. Ribonuclease protection assays revealed increased IL-1beta (P < 0.04) and TNF-alpha (P < 0.06) mRNA in lung tissue collected from a lesion site compared with tissue collected 10 cm from a lesion site or from control pigs. Interestingly, Mh did not depress weight gain or feed efficiency during any week of the 28-d study (P > 0.10). Moreover, Mh did not affect whole-body fat or protein accretion (P > 0.10). Thus, in spite of inducing disease and expression of inflammatory cytokines, Mh alone did not affect growth performance and whole-body composition of nursery pigs during the 4-wk experiment. The ability of pigs to contend with Mh may have resulted from the absence of other pathogens that generally co-exist with Mh under commercial conditions.