Bacterial diseases create a significant economic impact in today’s swine industry by causing an increase in mortality rates and a reduction in feed efficiency and growth. Polyserositis is one of the main causes of mortality in nursery pigs. The bacterium Haemophilus parasuis is typically considered the main cause of polyserositis. However, during the last years we have identified another bacterium, Mycoplasma hyorhinis, as the main cause of polyserositis in many cases. In fact, 55% of polyserositis and 12% of arthritis cases received at the Minnesota Veterinary Diagnostic Laboratory test positive for this pathogen by PCR. Many of these pigs are actually coinfected with H. parasuis and M. hyorhinis. Although this pathogen was first described in 1955, very little research has been generated regarding the ecology and epidemiology of this organism, which is needed to design effective control and prevention protocols.

In order to begin to generate this vital information, our group at the University of Minnesota initiated a cross sectional study with the objective of estimating the colonization prevalence of M. hyorhinis in pigs of different age groups. Three 6000 farrow-to-wean herds, designated as herds A, B and C, and their nurseries were selected. Although all three herds had history of M. hyorhinis disease, only herds A and B were experiencing high nursery mortality due to polyserositis at the time of sampling. The sampling of each herd included the collection of nasal swabs from 60 sows, 60 piglets in each group of 1, 7, 14 and 21 days of age as well as 30 pigs in each group of 28, 35, 42, 49, 56, 63, 70 and 77 days of age. Additionally, since M. hyorhinis can be detected in the oropharyngeal surface, oral fluid samples were collected from one pen per age throughout the nursery. In order to investigate the role of M. hyorhinis in polyserositis cases tissue samples were collected from ten clinically affected and ten clinically healthy pigs necropsied at the age of the peak of mortality in the nursery. Samples were tested for M. hyorhinis by a quantitative PCR developed in our laboratory.

M. hyorhinis was detected in the nasal cavity of 5/60 sows in herd A, 3/60 in herd B and none in herd C. In herd A and B, where clinical cases of M. hyorhinis were present, the colonization prevalence in suckling piglets was low (avg=8%) and high in post-weaning pigs (avg=98%). In contrast, in herd C, where M. hyorhinis clinical cases were absent, colonization in pigs was very low until the last week in the nursery. A total of 7/8 oral fluids collected from post-weaning pigs tested M. hyorhinis positive in herd A and B, while 1/8 tested positive in herd C. Polyserositis was not observed in any of the healthy animals from all three herds or in the diseased pigs from herd three. However, in herds A and B polyserositis was observed in 9/10 and 4/10 diseased pigs respectively (Figure 2). M. hyorhinis was detected by PCR in the pericardium of 8/10 diseased pigs in herd A and 3/10 in herd B. Isolation of M. hyorhinis from the pericardium was achieved only in herds A and B. In herd three M. hyorhinis was not detected by PCR in any of the necropsied pigs.

In summary, M. hyorhinis is an important cause of polyserositis and arthritis in post-weaning pigs. It can be detected by PCR in nasal swabs, tonsil swabs and oral fluids. Detection of this pathogen in the nasal cavity of an individual pig does not imply disease; however, testing nasal swabs of a group of pigs may be useful to determine the time of exposure in a herd. Colonization may occur in pigs as early as one day of age, but most of the pigs become colonized sometime in the nursery. High prevalence of M. hyorhinis nasal colonization in weaned pigs appears to be correlated to the presence of M. hyorhinis in polyserositis cases.