Bacterial diseases have a significant economic impact in today’s swine industry by causing an increase in mortality rates and a reduction in feed efficiency and growth.

Systemic bacterial infections are a major cause of mortality in nursery pigs. Polyserositis, the fibrin coating on the surface of multiple organs, is the gross lesion observed frequently with systemic bacterial infections (Figure 1). The bacterium Haemophilus parasuis is typically considered the main cause of polyserositis.

However, during the last few 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 polymerase chain reaction or PCR (Figure 2). Many of these pigs are actually coinfected with H. parasuis and M. hyorhinis. Although M. hyorhinis was first described in 1955, since then very little research has been generated regarding the epidemiology of this organism, which is needed to design effective control and prevention protocols.

Field Epidemiology
Drs. Neto, Strait and Schwartz from Iowa State University provided information in the April edition of this column on the age distribution for M. hyorhinis and M. hyosynoviae detection in growing pigs (go to www.nationalhogfarmer.com, click on National Hog Farmer Weekly Preview and click on the April 4, 2011 edition).

They also pointed out the need for current information on the epidemiology and economic impact of these bacteria. In order to begin to generate this vital information, our group at the University of Minnesota conducted a cross-sectional field study with the objective of estimating the colonization prevalence of M. hyorhinis in pigs of different age groups. Three, 6,000-head, farrow-to-wean herds, designated as herds A, B and C, and their nurseries were selected. Although all three herds had a 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 peak mortality in the nursery. Samples were tested for M. hyorhinis by a quantitative PCR developed in our laboratory.

Study Findings
We detected M. hyorhinis in the nasal cavities of 5/60 sows in herd A, 3/60 in herd B and none in herd C. In herds A and B, where clinical cases of M. hyorhinis were present, the colonization prevalence was low in suckling piglets (avg=8%), but high in postweaning 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 postweaning pigs tested M. hyorhinis-positive in herds 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, nor in the diseased pigs from herd C. However, in herds A and B, polyserositis was observed in 9/10 and 4/10 diseased pigs, respectively. 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 C, M. hyorhinis was not detected by PCR in any of the necropsied pigs.

Conclusions
These findings can be summarized as follows:

• M. hyorhinis is an important cause of polyserositis and arthritis in postweaning pigs.

• M. hyorhinis can be detected by PCR in nasal swabs, tonsil swabs and oral fluids.

• Detection of M. hyorhinis in the nasal cavity of an individual pig does not imply disease; however, testing nasal swabs in 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 and was associated with increased nursery mortality in this study.

• More work is needed to determine the factors that lead to clinical disease associated with M. hyorhinis.

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Maria Jose Clavijo, DVM; Albert Rovira, DVM; Simone Oliveira, DVM; Jerry Torrison, DVM, all of the University of Minnesota Veterinary Diagnostic Laboratory, St. Paul, MN, and Deb Murray, DVM, New Fashion Pork, Jackson, MN.