Mycoplasmal pneumonia of pigs

Previous authors: P WALLGREN

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 pneumonia of swine (MPS), caused by Mycoplasma hyopneumoniae,³⁹ is an insidious, subacute to chronic pneumonia characterized clinically by a non-productive cough, loss of condition, growth retardation and low mortality. The causative agent was first isolated in 1965 in the USA³⁰ and Europe.¹⁸ Mycoplasmal pneumonia of swine occurs worldwide causing great losses to pig producers due to decreased growth of fatteners, either as a result of  the disease itself or in combination with secondary infections.^{36, 51}

Aetiology

Mycoplasma hyopneumoniae is a small, fastidious, slow-growing organism that can only be propagated on special media (see General Introduction: Mollicutes).^{12, 35} Overgrowth by other faster growing mycoplasmas may jeopardize cultivation. Mycoplasma hyopneumonia should be differentiated from other closely related mycoplasmas (see Diagnosis), such as M. flocculare.^{14, 39, 40}

It may survive in the environment for up to one month under humid conditions.¹⁷ but as it is sensitive to heat, sunlight and dehydration it is, in general, rapidly inactivated in the environment.¹⁵

Epidemiology

The disease is typically introduced into uninfected herds by subclinically infected pigs which are usually adult breeding animals. Herds  a short distance from an infected herd(s) may become infected by airborne transmission of M. hyopneumoniae, particularly during conditions of cold outdoor temperature and high  humidity.²² Infections with M. hyopneumoniae have not been reported in minimal- disease herds located two miles from the closest infected herd.¹⁷

Within herds, M. hyopneumoniae 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 common among fatteners, i.e. those in units housing high numbers of  young pigs.  The disease is exacerbated by poor hygienic conditions, which also pave the way for secondary infections  such as Pasteurella multocida  is frequently isolated from fattening pigs with pneumonia.^{8, 12, 56}

Piglets are susceptible to M. hyopneumoniae infection because of their poor antibody response.⁵⁴ Nevertheless, piglets of conventional dams (especially older sows¹⁶) generally possess some immunity to  M. hyopneumoniae due to protective, colostrum-derived antibodies.^{23, 54} In contrast, poorly immunized dams or those with no immunity (especially gilts) may  infect their offspring.⁸

Multiple strains of M. hyopneumoniae are found in  pig populations, but certain strains appear to be responsible for  outbreaks.³² Limited genetic variation was found  among M. hyopneumoniae isolates in a in a herd with commingled pigs from different sources.³⁷ Nevertheless,  infections with multiple strains may occur within herds  associated with increased severity of  pneumonic lesions at slaughter, suggesting that reducing the number of different strains may lead to fewer lung lesions at slaughter and better respiratory health of  pigs.³⁵

Pathogenesis

After inhalation, M. hyopneumoniae colonizes the lungs extracellularly in the vicinity of ciliated epithelial cells and provokes an inflammatory reaction that may develop as early as one week post-infection.^{23, 27} The extent of the reaction correlates with the severity of infection. Circulating antibodies to M. hyopneumoniae have been demonstrated 14 days post-infection.^{13, 28, 47} It has been suggested that  cell-mediated immune responses may play a role in the pneumonic process.^{33, 48} This contention is supported by the fact that an increased ability of mononuclear cells to produce antibodies in vitro after exposure to M. hyopneumoniae has been demonstrated.⁵³ Antibodies provide protection  to animals following challenge, even when pigs are exposed to stressors that affect the general immune response negatively.⁵⁹

Clinical signs

Mycoplasmal pneumonia of pigs is a chronic, often insidious, disease with high morbidity and 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, 51, 61} 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.⁵² However, if the lung lesions are secondarily infected by bacteria, the course of the disease can be considerably prolonged. The most important...