Brucella suis infection

Brucella suis infection

J GODFROID, C O THOEN AND R D ANGUS

Introduction

Brucellosis in pigs is caused by one of the first three of the five biovars of Brucella suis. Clinical signs in sows are abortion, birth of stillborn piglets, mastitis and sterility, and mortalities in piglets. Orchitis and sterility may occur in infected boars. Infection with B. suis in pigs may be symptomless with the result that farmers may be unaware that their herds are infected. It is a zoonosis.

Porcine brucellosis was first described by Traum in Indiana, USA, in 1914.30 The disease is widely distributed in the world, but its prevalence is low with the exception of South America and Southeast Asia where the prevalence is higher.4 It is mainly restricted to feral pigs in the USA and Australia, and wild boars (Sus scrofa) in Europe.16, 24, 31 Brucellosis in pigs occurs sporadically in several African countries.7 Although it has been reported in Mozambique in southern Africa, it has not yet been diagnosed in South Africa.8

Aetiology and epidemiology

Brucella suis is a small Gram-negative coccobacillus that requires the addition of blood or serum to the growth medium but not atmospheric carbon dioxide for primary isolation.1, 2, 3 The organism occurs invariably in its smooth phase. It has five recognized biovars which are typed on the basis of differences in biochemical tests, serological reactivity and phage susceptibility.2, 6, 7

Brucella suis biovars 1 and 3 are mainly pathogenic for pigs and humans but have also been reported to infect cattle.8, 10 These two biovars have a worldwide distribution. Epidemics of brucellosis in humans have been reported in the USA among packing-house workers where the usual source was infected pigs13 and also in Australia among those involved in the killing and slaughter of feral pigs.24 Human brucellosis caused by B. suis biovars 1 and 3 is thus almost entirely occupational.1, 14

An important exception to the foregoing occurs in countries such as Brazil and Colombia in South America, where B. suis has become established in cattle, and brucellosis in humans caused by biovars 1 and 3 is emerging as an increasingly serious public health problem as a result of the consumption of unpasteurized milk as they are capable of colonizing the bovine udder.8

Brucella suis biovar 2 differs from B. suis biovars 1 and 3 in that it is restricted to Europe and, besides pigs and wildboars, infects the European hare (Lepus capensis) which can act as a reservoir. Otherwise, brucellosis in pigs and wild boars due to B. suis biovar 2 closely resembles the disease caused by B. suis biovars 1 and 3.1 Remarkably, B. suis biovar 2 is very rarely a human pathogen and has only once been reported as the cause of human brucellosis.4

Rangifer brucellosis, i.e. brucellosis in reindeer (Rangifer tarandus tarandus) and caribou (Rangifer tarandus groenlandicus) is caused by B. suis biovar 4 throughout the Arctic region, Siberia, Canada and Alaska and constitutes a serious zoonosis. When clinical signs are present in these animals, abortions and metritis are seen in females and orchitis occurs in males. In both sexes, abscess-formation in joints, bursitis and lameness are observed in some cases. Transmission to humans may be by direct contact, or through the consumption of raw milk or other inadequately heated products from infected animals.12 In the former USSR, B. suis biovar 5 is typified by strains isolated from rodents.2

Infection is generally introduced into a herd of pigs by the purchase of infected animals. Boars may be the main source of infection and transmission occurs chiefly via the semen at mating. Indeed, persistent granulomatous lesions in the testes and accessory male sex organs, from where bacteria can be shed in the semen for months or years, may be present in boars even though they appear healthy (hence the potential for spreading the disease through artificial insemination).5, 27 Other routes of spread within a pig herd are transplacental and the ingestion of aborted material, infected milk or contaminated feedstuff.1 The feeding of contaminated kitchen waste containing uncooked pork may also be important.22

The course of the clinical disease is usually acute when B. suis is first introduced into a non-infected herd of pigs. Spread of the infection within the herd may be rapid and manifests mainly as a syndrome associated with a high rate of abortions in pregnant animals. If no control measures are taken, the course of the infection then generally changes in the herd: some infected individuals will either show a chronic form of the disease (with or without excretion of B. suis) or free themselves of the infection (self-cure) while others may remain noninfected. The latter are at risk and may subsequently become infected because of the presence of B. suis in the herd. This sequence of events explains why a herd may be infected for years but for long periods may be free of clinical signs; this phenomenon is seen particularly in fattening herds.

Pathogenesis, clinical signs and pathology

Unlike the pathogenesis of ruminant brucellosis, after being exposed to B. suis pigs develop a bacteraemia that may persist for up to 90 days or more.1 During and subsequent to the bacteraemia, localization of B. suis occurs in various tissues including the male and...

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