Weaned pigs can also develop arthritis due to infections of Streptococcus suis (Type 2), but as pigs grow, Mycoplasma hyosynoviae is the most important pathogen causing arthritis in fattening pigs, finishers and adults (gilts, sows and boars).
Arthritis arises when the joints, which are sterile, are invaded by bacteria from the blood stream during disease challenges. In the joints, the bacteria multiply also causing inflammation. The join fluid increases in volume which causes swelling and pain leading to lameness (Figures 1, 2 and 3).
There is often a large variation in the occurrence of arthritis and lameness between pig herds. Although some cases of arthritis do spontaneously resolve themselves, most untreated cases of arthritis lead to chronic lameness.
Pathogenesis of mycoplasmal arthritis
Mycoplasmal arthritis of pigs is caused by M. hyorhinis and M. hyosynoviae. M. hyorhinis generally affects pigs during the growing period (3 - 10 weeks of age) and also causes polyserositis. M. hyosynoviae causes arthritis in pigs weighing 35 - 115 kg and is a frequent cause of lameness in adults. Both pathogens are present on most pig farms, in some cases without the symptoms of clinical disease being present.
The tonsils serve as a resevoir for M. hyosynoviae and M. hyorhinis and play a critical role in pathogen transmission. Acutely affected pigs excrete large numbers of mycoplasmal organisms in their mucosal secretions.
Both mycoplasmal pathogens can be detected in the tonsils, nasal cavaties and in the lungs. Triggers for systemic spreading can be:
- Stress due to transfer or movement of animals
- Poorly adjusted ventilation
- Concurrent infections with other bacteria (A. pleuropneumoniae, P. multocida)
- Viruses (PRRSv) causing respiratory problems
M. hyorhinis and M. hyosynoviae can be latently present in the joints without causing infections. In severe outbreaks, morbidity rates of 10-50% can be seen.
Joint swelling and lameness are the most obvious and persistent clinical signs of infectious mycoplasmal arthritis. Usually more than one limb is affected, meaning that the pig will be reluctant to rise and walk. Piglets can be infected with both mycoplasma pathogens either directly or from the sow, via pen mates, and/or via the environment.
When there are few other clinical signs apparent, there is no alteration in the gait of affected pigs. In more severe incidents of the disease, acute lameness in one or more limbs may be seen, progressing from single affected joints to a complete inability to stand.
Infectious agent - relevance
M. hyorhinis and M. hyosynoviae are widespread in pigs with reports from around the world including the USA, Asia, Europe and Latin-America.
Mycoplasmal arthritis must be suspected when lame animals are seen. In most cases, heat and swelling in a joint is sufficient to suggest mycoplasmal arthritis. During post-mortem examinations when arthritic joints are cut, synovial fluid in the joint cavity is yellowish-brown in colour (Figure 4) and may contain flakes of fibrin. The change in colour from its normal clear and transparent form and consistency indicates an arthritis infection. Arthritis presence in affected pigs is confirmed by further analysing the synovial fluid. Final proof of M. hyorhinis or M. hyosynoviae infection is based on culture and PCR testing in corresponding laboratories.
Mycoplasmal arthritis diagnosis options include:
- Bacterial culture from tonsil tissue and synovial fluids
- Oral fluid and nasal swab (Figure 5) testing by real-time PCR
- Serum testing by ELISA (Figure 6)
Treatment and control of mycoplasmal arthritis
There is no vaccine currently available for mycoplasmal arthritis. Antibiotic administration is the only available treatment for mycoplasmal arthritis in pigs. Vetmulin® injectable is registered for the treatment of mycoplasmal arthritis at 1.5 ml/20 kg body weight for 3 days.
Joint-associated infectious arthritis is a problem often caused by M. hyorhinis and/or M. hyosynoviae. The disease is present globally and affects piglets (suckling and nursery), pigs from 35 - 115 kg, and adult animals. Mycoplasmal arthritis management is based on accurate diagnosis which is required to support treatment decisions for the prudent use of antibiotics via parenteral application. Vetmulin® injectable is registered for the treatment of mycoplasmal arthritis.