Typically, Haemophilus parasuis has been implicated as the main cause of polyserositis in pigs. However, in recent years, Mycoplasma hyorhinis has been identified as the causative agent in many cases.

At the University of Minnesota Veterinary Diagnostic Laboratory, 55% of polyserositis and 12% of arthritis cases test positive for Mycoplasma hyorhinis by polymerase chain reaction (PCR).

Numerous experimental inoculations have reproduced polyserositis and arthritis lesions. However, very little research has been conducted regarding this pathogen, first described in 1955, as to the ecology and epidemiology of this organism needed to design effective control and prevention protocols.

A team at the University of Minnesota initiated a cross-sectional study to determine the prevalence of Mycoplasma hyorhinis in pigs of different age groups.

Three, 6,000-sow, farrow-to-wean herds, designated herds A, B and C, and their nurseries were selected. All three had a history of M. hyorhinis, but only herds A and B were experiencing high nursery mortality rates due to polyserositis at the time of sampling.

In sampling each herd, nasal swabs were collected from 60 sows and 60 pigs in each group at 1, 7, 14 and 21 days of age and in 30 nursery pigs at 28, 35, 42, 49, 56, 63, 70 and 77 days of age.

Because M. hyorhinis can be detected in the oropharyngeal surface, oral fluid samples were collected from one pig per age group throughout the nursery.

To confirm the presence of M. hyorhinis in polyserositis cases, tissue samples were collected from 10 clinically affected and 10 clinically healthy pigs necropsied at the age of peak mortality in the nursery from each herd. Samples were tested for disease by a quantitative PCR developed in the laboratory.

M. hyorhinis was detected in the nasal cavity in five of 60 sows in herd A, three of 60 sows in herd B, and none in herd C. In herds A and B, where M. hyorhinis was clinically present, prevalence in suckling piglets was low (8%) and high in postweaning pigs (98%).

In contrast, in herd C, with no clinical cases of disease, colonization in pigs was very low until the last week in the nursery.

Seven of the eight oral fluid samples collected from postweaning pigs tested positive for M. hyorhinis in herds A and B, while only one of eight tested positive in herd C.

Polyserositis was not observed in any healthy pigs from herds A and B, nor in any sick pigs in herd C. However, in herds A and B, polyserositis was detected in nine out of 10 and four out of 10 diseased pigs, respectively.

M. hyorhinis was also detected by PCR in the pericardium of eight of 10 diseased pigs in herd A and three of 10 diseased pigs in herd B. The bacterium was only isolated from the pericardium in herds A and B. In herd C, M. hyorhinis was not detected by PCR in any of the necropsied pigs.

These findings further support the existing evidence that M. hyorhinis causes polyserositis and arthritis in postweaning pigs. Thus, it is important to include it as a differential in polyserositis and arthritis cases. Infection may occur as early as 1 day of age, but most pigs get infected in the nursery.

Detection of this pathogen in the nasal cavity of a 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.

Researchers: Maria Clavijo, DVM; and Albert Rovira, DVM; University of Minnesota; and Deborah Murray, DVM, New Fashion Pork, Jackson, MN. For more information, contact Clavijo by phone (612) 868-4396 or e-mail clavi005@umn.edu.