Often, the declaration from your veterinarian, “these pigs have Mycoplasma,” is immediately followed with a question: “What does that mean?”
The confusion is understandable when we recognize that there are several quite different Mycoplasma species that very commonly infect pigs. Additionally, Mycoplasma infections do not always cause clinical disease.
Mycoplasma are bacteria-like organisms that have a long history causing disease in swine. The most well-known and most economically important is Mycoplasma hyopneumoniae, a very common cause of pneumonia.
Three other species cause disease in swine: M. hyosynoviae (Mhs) causes arthritis (more below); M. hyorhinis can cause disease in young pigs; and, M. suis (formally known as Eperythrozoon suis or “Epy”) can cause anemia in pigs (Table 1). Importantly, none of these Mycoplasma are obligatory pathogens; pigs (herds) can be infected with no discernible clinical disease. In addition, there are at least six more species, including M. flocculare, that can be isolated from swine and sometimes can contribute to erroneous interpretation of certain diagnostic tests.
Grow-Finish Arthritis, Lameness
The remainder of this discussion focuses on M. hyosynoviae (Mh). Concerns have increased in the last several years about arthritis and lameness in the grow-finish stage. There may be an actual increase in prevalence of lameness, or perhaps an increased awareness due to control of other major diseases (such as porcine circovirus type 2), increased recognition of lost economic opportunity of lameness or welfare concerns.
Because lameness has several primary causes and a host of risk factors (Table 2), it is difficult to point to one single factor as causal. Astute clinical examination, response to treatment, and confirmation of diagnosis by laboratory testing should align before pronouncing Mycoplasma hyosynoviae as the sole cause of lameness.
Mycoplasma hyosynoviae infects the upper respiratory tract and can lead to an indefinite tonsil carrier state. The infection can be transmitted from carrier pigs to non-infected pigs by contact, usually after 4-8 weeks of age. The rate of spread of infection likely is influenced by herd immunity, environmental factors and stocking density, whereas the outcome of infection (disease or no disease) is often related to concurrent joint stressors, such as trauma, body conformation (genetics), nutritional imbalances, subclinical osteochondrosis and variation in pigs’ resistance or immunity.
Once infected, Mh can penetrate mucosal barriers and be found in the blood for 4-10 days after infection. In the more susceptible animals, the organism may localize in synovium of joints (connective-tissue membrane that lines the cavity of a joint and produces synovial fluid) and cause inflammation. Clinical lameness typically occurs in 3- to 5-month-old pigs. Of the 50 most recently confirmed cases of Mh arthritis at the Iowa State University Veterinary Diagnostic Laboratory, all were pigs between 10 and 28 weeks of age. Half of the cases were in pigs 17 to 20 weeks old.
Clinically, the onset of lameness is abrupt with no fever. The hock and stifle joints of hind legs are most commonly affected; “dog-sitting” and reluctance to move are often the first signs observed. Because larger, heavily muscled joints, such as the stifle and shoulder, are often involved, obvious joint enlargement may not be apparent. Joints may be “puffy,” but no fibrin or pus is expected within joints with Mh. The most remarkable change with Mh-associated arthritis may simply be increased amounts of joint fluid. Generally, less than 1 milliliter (ml) of joint fluid can be aspirated from the stifle of a normal 200-lb. pig. If 3-5 ml can be collected, and especially if the fluid is amber to blood-tinged in color, Mycoplasma arthritis should be highly suspected (Figure 1).
The diagnosis of Mh arthritis can be a challenge since affected pigs may be lame, down, without a fever and may not have obvious joint enlargement. Live, acutely affected, untreated pigs are preferred for diagnostic workup. Prior to opening the joints, one should attempt to aspirate joint fluid from the hock and stifle joints. If more than 1 ml (often 2-5 ml) of clear to amber-colored fluid can be aspirated, M. hyosynoviae arthritis should be the primary differential. Joint fluid and formalin-fixed synovium should be submitted for laboratory confirmation and to rule out other infectious processes. The joint surfaces should also be evaluated for osteochondrosis as a primary diagnosis or as a risk factor by gross and histopathologic examination.
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Kent Schwartz, DVM
Iowa State University Veterinary Diagnostic Laboratory