Nervous and Locomotor Systems

Mycoplasma hyorhinis

Clinical importance

Mycoplasmas are among the top concerns of the swine industry; different species cause different clinical signs. Mycoplasma hyorhinis (Mhr) infection is a ubiquitous pathogen that causes systemic disease in nursery-age pigs. The bacteria is part of the normal flora of the upper respiratory system of young pigs. Therefore, the majority of infected pigs is asymptomatic.

Etiology and transmission

Mycoplasma hyorhinis is a small bacteria without a cell wall, that compared to most Mycoplasmas grows quickly and is often a cell culture contaminant. M.hyorhinis infects epithelial cells in the upper respiratory system and as a result is spread through the oro-nasal route. Piglets can get infected by the sow or by their penmates, asymptomatic carriers. M.hyorhinis can also contaminate drinkers. Currently, the mechanism that allows the bacteria to migrate from the respiratory system and cause systemic disease is unknown.

Associated symptoms

The main symptom associated with M.hyorhinis is polyserositis in pigs from 3 to 10 weeks of age. Affected animals look gaunt and apathetic with rough, coarse hair coat. They have difficulty breathing, moving and therefore going to the feeders to eat. Lameness and swollen joints are also apparent. A slight fever may develop. Pigs may die suddenly or, if they survive, develop chronic arthritis and lameness that persists for several months. Clinical cough is rare and usually mild but is indistinguishable from M.hyopneumoniae symptoms when present. More recently, cases of conjunctivitis due to M.hyorhinis have been reported.

Which population is more likely to be affected by M.hyorhinis?

  1. Nursery pigs
  2. Growing-finishing pigs
  3. Adult sows

Associated lesions

Polyserositis with fibrous adhesions is the main lesion associated with M.hyorhinis. When opening up affected joints at necropsy, the synovial membranes are swollen and a sero-hemorrhagic liquid can be seen in the joint. In pneumonia cases, the lung lesions are similar to the ones seen in case of enzootic pneumonia.

The most common clinical sign associated with M.hyorhinis infection is:

  1. Sudden death
  2. Lameness
  3. Dry hacking cough


The best samples to take are swabs of fibrin from the thorax or abdomen, or from affected joints of non-treated animals. Culture or PCR tests are both valid in detecting the bacteria in the lesion. Samples from the upper respiratory tract are not appropriate as M.hyorhinis is a commensal and isolation of the bacteria in that region does not prove involvement in the disease.

Differential diagnosis

H. parasuis and S. suis should be considered in a differential diagnosis as they both cause lameness and polyserositis.

Treatment, Prevention and Control

Like other Mycoplasmas, M.hyorhinis is not susceptible to beta-lactams, but a number of other antibiotic treatments work effectively if the treatment is given early. For chronically affected animals, antibiotics cannot reduce the amount of adhesions or the inflammation causing the lameness or the pain associated with movement. The best prevention method is reducing stress on the animal to lessen the chance of systemic disease developing. No commercial vaccines are available but autogenous vaccines can be developed.

A nasal swab taking from a pig showing signs of lameness came back positive for a Mycoplasma hyorhinis PCR. This result is enough to make a conclusive diagnosis of M.hyorhinis induced lameness. True or False?



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Swine Diseases Copyright © by Perle Zhitnitskiy, DVM, MSpVM is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.