Mycoplasma hyorhinis - easy to isolate but hard to treat

Wednesday, August 10, 2011

This bacterium, which causes fever, lameness and mobility problems in young pigs, doesn't respond well to antibiotics and is best prevented by controlling other respiratory diseases that may trigger it

by S. ERNEST SANFORD

Mycoplasma hyorhinis is a bacterium that causes polyserositis and arthritis virtually identical to those caused by Haemophilus parasuis (the Glässer's disease organism), and Streptococcus (Strep) suis.

M. hyorhinis primarily affects pigs three to 10 weeks old, causing fever, dsypnea (heavy breathing), swollen joints, lameness and various degrees of difficulty in walking and moving. Major lesions seen at necropsy of clinically affected pigs include pleuritis, pericarditis, peritonitis (these lesions combine to be called polyserositis) and arthritis.

M. hyorhinis was the first mycoplasma isolated from pigs. Unlike the closely related M. hyopneumoniae, the causative organism of enzootic pneumonia, it's relatively easy to isolate. It is commonly found in nasal secretions and the respiratory tract of growing pigs. For years, it had been considered a secondary invader that proliferates during periods of stress or superimposed on other respiratory diseases. Its role as a primary pathogen is still not fully accepted, but it has become very prominent as the cause of pneumonia and polyserositis in growing pigs in recent years.

Currently, M. hyorhinis is identified in 50 per cent of all pleuritis cases submitted to the University of Minnesota's Veterinary Diagnostic Laboratory and 40 per cent of all polyserositis cases are mixed H. parasuis and M. hyorhinis co-infections.

There are at least two other mycoplasmas in swine that should be mentioned to avoid confusion. They are Mycoplasma hyosynoviae and Mycoplasma suis. M. hyosynoviae is the cause of synovitis/arthritis in pigs 12-24 weeks of age. M. suis is the cause of eperythrozoonosis and was for decades known as Eperythrozoon suis. The name was changed to M. suis in 2002 based on nucleic acid analysis. M. suis infection is not restricted to pigs.

In a recent study, researchers at the University of Minnesota found that M. hyorhinis can be detected in pigs as early as one day of age, but most pigs become infected in the nursery after weaning and co-mingling with other weaned pigs.

Epidemiology. M. hyorhinis is transmitted from sows to suckling pigs in the farrowing room or from older to younger pigs in the nursery after weaning, or later in the grow-finishing barn. It can be found in nasal or sinus secretions of about 10 per cent of sows and from nasal secretions of 30-40 per cent of weaned pigs. It is also commonly isolated from pneumonic lungs of slaughter hogs.

Some strains of the organisms produce pneumonia in gnotobiotic (germ-free) pigs, but it's still not clear if it is a primary cause of pneumonia in normal pigs in commercial herds. Once one pig in a group is infected, the infection spreads rapidly through that population. Infection does not lead automatically to clinical disease. Many infected pigs may never exhibit signs of clinical disease.

Clinical signs. Outbreaks of polyserositis caused by M. hyorhinis generally occur in pigs three to 10 weeks of age, but may occasionally occur in young adults also. Lameness and arthritis with M. hyorhinis tend to occur in finisher pigs.

Clinical signs are first seen usually three to 10 days after exposure to the organism and often after a precipitating stress factor. Acute signs consist of roughened hair coat, moderate fever, listlessness, being off feed, reluctance to move, laboured breathing, tucked-up appearance, abdominal tenderness, lameness and swollen joints.

These acute phase clinical signs may begin to wane 10 to 14 days after the initial onset. Lameness and swollen joints tend to persist as sub-acute and chronic manifestations of the disease for two to three months and may be prolonged for as long as six months in some pigs.

Diagnosis. Diagnosis of M. hyorhinis disease is made on gross findings of polyserositis and, to a lesser extent, arthritis, at postmortem backed up by isolation or PCR confirmation of M. hyorhinis from the lesions. Acute gross polyserositis lesions consist of serofibrinous and fibrinopurulent pericarditis, pleuritis and to a lesser extent, peritonitis. The fibrin from these lesions is the best sample for identification of M. hyorhinis by isolation or PCR.

Joints are swollen and joint membranes are red and contain increased amounts of blood-tinged joint fluid. Nursery pigs three to 10 weeks old are likely to have polyserositis, whereas lameness and arthritis are more common in finishing pigs.

Sub-acute and chronic lesions consist of more organized fibrous adhesions and much thickened, cloudy membranes rather than the serofibrinous exudates seen in the acute phase lesion. In sub-acute and chronic cases, joint fluids can be bloody and greatly increased in volume.

Treatment and control. Treatment of M. hyorhinis cases has not usually been very rewarding. It is sensitive to tylosin and lincomycin in laboratory tests, but the reality is that treatment in field cases with these same antibiotics is often less than satisfactory.

The great inflammatory response accompanying M. hyorhinis infection seems to either prevent antibiotic penetration to the infecting organisms or the infection itself is self-perpetuating. Nevertheless, when outbreaks occur in the field, treatment with tylosin or lincomycin is recommended and might be beneficial.

There are no vaccines currently available for the control of M. hyorhinis disease and there are no prospects of development of a commercial vaccine in the foreseeable future.

Prevention. Control programs are based on emphasizing control of other ongoing respiratory diseases that may precipitate outbreaks of M. hyorhinis disease. BP

S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington. Email: ernest.sanford@boehringer-ingelheim.com