Mycoplasmal pneumonia of pigs

Mycoplasmal pneumonia of pigs

P WALLGREN

Introduction

Mycoplasmal pneumonia of swine (MPS) is an insidious, subacute to chronic pneumonia characterized clinically by a non-productive cough, loss of condition, growth retardation and low mortality. It is caused by Mycoplasma hyopneumoniae. 39 The causative agent was defined in the USA32 and Europe17 during 1965. Mycoplasmal pneumonia of swine is globally spread and causes the pig industry great losses due to decreased growth of fatteners, either due to the disease itself or in combination with secondary infections.36, 52 It was first noticed in South Africa during 1956.29 During 1991, 28 per cent of the South African pigs were reported to manifest macroscopic lung lesions similar to those caused by MPS at slaughter.38

Aetiology

Mycoplasma hyopneumoniae is a small, extremely fastidious, slow-growing organism that can only be propagated on special media.13, 40 Overgrowth by other faster growing mycoplasmas jeopardizes the true result of a cultivation. Further, to avoid cross-reactions, M. hyopneumonia has to be separated from other closely related mycoplasmas, such as M. flocculare.13, 39, 40

Despite the difficulties in cultivating M. hyopneumoniae, the organism may survive in the environment for up to one month under humid conditions.16 However, as it is sensitive to heat, sunlight and dehydration it is, in general, rapidly inactivated in the environment.16

Epidemiology

The disease is typically introduced into uninfected herds by subclinically infected pigs which are often adult breeding animals. Herds situated a short distance from an infected herd(s)may also be infected by airborne transmission of M. hyopneumoniae, with cold outdoor temperature and high air humidity enhancing such transmission.21 Infections with M. hyopneumoniae have never been reported in minimal- disease herds located two miles from the closest conventional herd and whose owners deny the entrance of any person who has been in contact with conventional pigs during the previous 48 hours.16

Within herds, MPS is transmitted either in aerosols45 or by direct contact between pigs, and is often spread from older to younger pigs. Mycoplasma hyopneumoniae infections are most commonly observed among fatteners, i.e. those in units housing high densities of comparably young pigs. The influence of the disease is increased by poor hygienic conditions, which also pave the way for secondary invaders. Among the latter, Pasteurella multocida appears globally to be the most common, and it can frequently be isolated from fattening pigs with pneumonia.7, 11

Piglets are susceptible to M. hyopneumoniae infection because of their poor antibody response.55 Nevertheless, piglets of conventional dams (especially older sows 15) generally possess some immunity towards MPS due to protective, colostrum-derived antibodies.22, 53 In contrast, poorly immunized dams or those that possess no immunity (especially gilts) may even infect their offspring with MPS.8

Pathogenesis

After inhalation M. hyopneumoniae colonizes the lungs extracellulary in the vicinity of ciliated epithelial cells and provokes an inflammatory reaction that may develop as soon as one week post-infection.22, 27 The extent of the reaction correlates with the severity of infection. Circulating antibodies to M. hyopneumoniae have been demonstrated 14 days post-infection.12, 28, 47 It has been suggested that a cellmediated immune response towards M. hyopneumoniae plays a role in the pneumonic process.34, 49 This contention is supported by the fact that an increased ability of mononuclear cells to produce antibodies in vitro over time after exposure to M. hyopneumoniae has been demonstrated.54 Once an ability to produce antibodies to M. hyopneumoniae has been established, it appears to be effective in protecting the animal to a re-challenge of the micro-organism, even when pigs are exposed to stressors that affect the general immune response in a negative way.60

Clinical signs

Mycoplasmal pneumonia of pigs is a chronic, often insidious, disease with a high morbidity and a low mortality, at least when uncomplicated. The incubation period is one to three weeks43 and the clinical signs comprise a dry, unproductive cough that increases in intensity on physical exertion, unthriftiness and a decrease in appetite. If established early during the fattening period in a group of pigs, MPS will affect a great majority of them before they reach market weight.42, 44, 52, 62 However, the disease may go unnoticed since pigs with active infections decrease their physical activities and show no overt evidence that they are infected. Their inappetence is generally hidden from the farmer since pen mates of the affected pigs will consume the pooled feed ration. As individual pigs in a herd become diseased at different times, this ‘feed stealing’ can, at least to some extent, explain the phenomenon often referred to as ‘compensatory growth’.58

In uncomplicated cases of MPS, the clinical signs gradually decrease and disappear within 30 to 40 days after onset of the disease,43 and the lesions in the lungs heal completely within approximately 10 to 12 weeks.53 However, if the lung lesions are secondarily infected by bacteria, the course of the disease can be considerably prolonged. The most important secondary invader within the respiratory disease complex of pigs appears to be Pasteurella...

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