Age affected: Piglets pre-weaning, weaners, growers / finishers, gilts, boar.
Causes: Mycoplasma hyosynoviae; Mycoplasma hyorhinis.
Effects: Arthritis, lameness, inappetence, stunting.
Mycoplasma arthritis is caused by the fragile bacterium Mycoplasma hyosynoviae which can be distinguished from the other two major pig mycoplasmas by its fermentation of arginine but not glucose or urea and by its antigens. It grows best in anaerobic conditions. It is more sensitive in vitro to tiamulin than other mycoplasmas. It is commonly found on the tonsils of healthy carrier sows and occurs in the pharyngeal region in 7-8 week old pigs. Infection is by the oronasal route and has taken place in many herds by 10-12 weeks of age. Septicaemia develops 2-4 days after infection and the organisms settles out in the joints to a variable extent to produce the characteristic arthritis and synovitis. It may also cause lesions in the lungs. The organism can be recovered from the joints from 4 days post infection, when the synovial villi (on the lining of the joint cavity) become mildly oedematous and hyperaemic. There is gradual enlargement of the villi, cell detachment, neutrophil (white blood cell) invasion and protein exudation into the synovial fluid by Day 10. Mycoplasmas can be seen as the bases of the villi and between the synovial cells using electron microscopy. Antibody develops as animals recover. Joint infections may follow infection by M. hyorhinis in some cases.
Infection is oronasal by pig to pig transmission or indirectly from contaminated drinkers or aerosols and has taken place in many herds by 10-12 weeks of age. The organism is delicate but may survive drying for up to 4 weeks, thus allowing some indirect infection from the environment. It is normally spread to uninfected farms by carrier pigs.
Few clinical signs are apparent in most infections, but it can cause arthritis in pigs weighing between 35 kg and 115 and is a frequent cause of lameness in recently purchased breeding stock. Fever is absent, and the only apparent effect is alteration in the gait of the affected pigs. More severe disease may occur as acute lameness in one or more limbs, progressing from a single affected joint to complete inability to stand within 24 hours. There may be a rise in rectal temperature (to 40°C) for1-2 days after exposure, pronounced lameness after bending tests, severe pain on movement and screaming when pushed by pen mates. As affected animals cannot compete for food and water, death may result from cannibalism. Affected animals usually weigh more than 35 kg. Bursitis (inflammation of pressure points) and soft, fluctuating swellings of the joints may develop but external signs may not be apparent. Affected pigs limp, shift weight from one leg to another or may be unable to rise for 3-10 days. Non-immune gilts or boars introduced into infected herds are chiefly affected with sudden lameness 7-21 days after introduction, and clinical signs may occur 22-24 days after mixing large finishers.
Acute lameness in older finishing pigs or recently introduced breeding stock with no fever or evidence of polyserositis suggests the presence of mycoplasma arthritis. The withdrawal of joint fluid from tarsal, elbow or knee joints (after sedation) may aid diagnosis. In M. hyosynoviae infection the fluid is clear, yellowish-brown in colour and may contain flakes of fibrin. In streptococcal arthritis, fluid is haemorrhagic and turbid with many neutrophils and in osteochondrosis, clear and yellowish. Isolation of the organism confirms. It may be recovered from blood for 7-11 days from 1-4 days post exposure, from joints from 5-21 days until at least day 64, tonsil from days 6-61, lymph nodes from 6-15 days and spleen from 6-15 days. Serum antibody may also be demonstrated, but maternal antibody persists until 8-10 weeks and active antibody only appears from 8-16 weeks.
Joint changes may be present at slaughter. The synovial fluid resembles that taken in life and the articular surface is usually normal. Swelling and hyperaemia (reddening) is seen in the synovial membrane of affected large limb joints. Mycoplasma-associated bursitis may be found on the lateroplantar aspect of the tarsus and on the laterodistal aspect of the elbow.
Treatment of animals with early clinical signs by injection with tiamulin at 15 mg/kg on 2-3 successive days has been found useful. Isolation of the organism from the joints of such treated animals is difficult. Tylosin or lincomycin may also be used for a similar period, and enrofloxacin may be given at 2.5 mg/kg for 3 days. Animals in contact should be treated. Where the introduction of breeding stock to infected herd results in lameness (usually after 14-21 days), a course of treatment such as lincomycin in drinking water should be given at least 2-3 days before the expected appearance of the clinical signs and the treatment repeated 10 days later. There is no vaccine.