Mycoplasmal arthritis of pigs

Previous authors: P WALLGREN

Current authors:
P WALLGREN - Professor, State Veterinarian, Dipl ECPHM, Department of Animal Health and Antimicrobial Strategies, National Veterinary Institute, Uppsala, 75195, Sweden

GENERAL INTRODUCTION: MOLLICUTES

Introduction

Mycoplasmal arthritis of pigs caused by Mycoplasma hyosynoviae generally affects pigs during the fattening period. A sudden stiffness or lameness of pigs aged 12 to 26 weeks may indicate an outbreak of the disease which is generally not fatal. One or several joints may be affected. The disease is frequently observed in countries that employ modern pig husbandry methods and monitor for the disease. Mycoplasma hyosynoviae was first identified in the USA¹⁴ and Denmark.¹ The disease was subsequently diagnosed in several countries such as the UK¹² and Germany,¹⁶ but it is rarely diagnosed in South Africa.⁸ Severe outbreaks of bursitis associated with M. hyosynoviae have occurred in Denmark.¹¹

Aetiology

Mycoplasma hyosynoviae  grows best in a mycoplasmal medium containing mucin.¹⁴ The isolation is facilitated by use of a medium that selectively suppresses the growth of M. hyorhinis.² In fluid media it produces granules in suspension, a granular deposit and a waxy pellicle. It requires sterol, does not hydrolyse urea, and utilizes arginine. Its growth is enhanced in an atmosphere of reduced oxygen tension during incubation. Colonies of M. hyosynoviae that measure 0,5 to 1mm develop on agar medium within two to four days of cultivation.

For the general characteristics of the mollicutes the General Introduction: Mollicutes should be consulted

Epidemiology

The tonsils serve as a reservoir for M. hyosynoviae,^{2, 7} and the microbe is common in the upper respiratory tract of sows¹⁵ and fatteners.² Acutely affected animals excrete large numbers of organisms in their mucosal secretions.¹⁵

Sows transfer protective antibodies to their piglets via colostrum and transmission of the infection from sow to offspring is therefore rare but does occur.^{5, 6, 15} Infection occurs most commonly in pigs more than six weeks of age. The disease may affect individual pigs or become epidemic. In the latter case, 10 to 50 per cent of the pigs may be affected before reaching market weight.¹³

Pathogenesis and clinical signs

A haematogenic spread of M. hyosynoviae is initiated either by release of organisms present in the tonsils or respiratory systems (presumably induced by stress such as cold, transportation, or change in feed) or by intranasal exposure to aerosols from other pigs that shed the organism. The septicaemia persists for eight to ten days,^{4, 13} and one or more joints may be infected during this phase, with the development of an acute arthritis. In experimentally induced cases the organism has been re-isolated 24 days post-inoculation from such joints.¹³

A sudden onset of lameness in one or more limbs of fattening pigs (generally aged 12 to 26 weeks) is typical of the disease. Soft, swellings of the tibio-tarsal joints have been observed among most of the pigs affected by M. hyosynoviae.⁸ Although an affected animal moves  with  difficulty, the diagnosis may be obscured by the fact that affected joints are not necessarily obviously swollen. The body temperature is generally normal but affected pigs may lose weight due to inappetence during the acute stage of the disease. The acute stage of the disease persists for three to ten days, whereafter the lameness gradually decreases.⁴ Persistent signs of lameness may occur if an infected pig is also affected by osteochondosis which may be common in the age categories of pigs that are likely to suffer from mycoplasmal arthritis.¹³

Pathology

The synovial membranes of affected joints are swollen, oedematous and hyperaemic and the volume of synovial fluid is markedly increased during the acute stage of the disease. The fluid is serofibrinous to serosanguinous in nature. Irregular erosions in the joint cartilage may be seen. Such lesions are, however, probably caused by osteochondrosis rather than induced by M. hyosynoviae.¹³

The lesions following infection with M. hyosynoviae are often mild and may therefore not be noticed during routine inspections in abattoirs. For example, in one farrow to finish herd in which arthritis was diagnosed in 1 per cent of examinations performed at slaughter, the actual number of pigs in the herd treated ante-mortally for acute lameness caused by M. hyosynoviae during the fattening period was 25 per cent.¹⁷

Diagnosis

A sudden onset of lameness during the fattening period may indicate an outbreak of M. hyosynoviae. The clinical diagnosis ought to be confirmed by isolation of the agent. Since M. hyosynoviae has been isolated from blood during the septicaemic phase of the infection, attempts to cultivate it from blood may be performed⁷ but the probability of isolating it from joint fluid is considerably greater.¹⁰

Antibodies to M. hyosynoviae have been demonstrated in synovial fluid from affected pigs by enzyme-linked immunosorbent assay (ELISA).³ Antibodies have also been demonstrated in serum, but this diagnostic tool requires further evaluation due to the high prevalence of pigs that will have experienced contact with the micro-organism during their lifetime.⁹ Paired serum samples should be examined. The development of arthritis is probably required to induce an active serological response to M. hyosynoviae, as...