GENERAL INTRODUCTION: GRAM-NEGATIVE AEROBIC OR CAPNOPHILIC RODS AND COCCI Brucella spp. infections

Introduction

Brucellosis in pigs is caused primarily by biovar 1, 2 or 3 of the five biovars of Brucella suis. Clinical signs in sows are abortion, birth of stillborn piglets, perinatal mortality, mastitis, arthritis and infertility. Orchitis and sterility may occur in infected boars. In endemic situations B. suis infections may be subclinical and farmers may be unaware that their herds are infected. Brucella suis biovars 1 and 3 are important zoonoses.

Porcine brucellosis was first described by Traum in Indiana, USA, in 1914.⁴⁴ The disease has a worldwide distribution , and despite the paucity of data its prevalence is considered to be low.⁴ Swine brucellosis appears to be endemic in most parts of South America and Southeast Asia, and is also believed to be widespread across sub-Saharan Africa.^{29, 31, 36} Although reported in Mozambique, it has not yet been diagnosed in South Africa.⁶ In the USA and Australia the infection (caused by B. suis biovar 1)  is mainly restricted to feral pigs,^{37, 45} while the wild boar (Sus scrofa) and the European hare (Lepus europaeus) are the wild reservoirs of swine brucellosis caused by B. suis biovar 2 in Continental Europe (except Finland, Norway, and Sweden).^{15, 23, 26}

Aetiology and epidemiology

Brucella suis organisms are small Gram negative coccobacillioccurring naturally in the smooth (S) form and whose colonies cannot be differentiated macroscopically from isolates of other S Brucella species.^1-3 There are five recognized biovars of B. suis (named from biovar 1 to biovar 5).  Biovars 1, 2 and 3 are main causes of brucellosis in swine.^{2, 7, 8}  Biovar 4 has been isolated from reindeer (Rangifer tarandus and its various subspecies) in the Arctic including Russia, Alaska and Canada where it is considered to be endemic in semi-domesticated reindeer. Brucella suis has also been isolated sporadically in moose (Alces alces), American bison (Bison bison), musk oxen (Ovibos moschatus), arctic foxes (Alopex lagopus) and wolves (Canis lupus) in the Arctic and constitutes a serious zoonotic problem.¹⁸ Brucella suis biovar 5 has been isolated exclusively from wild rodents in the former USSR.³⁶ Very rarely  brucellosis in pigs is caused by B. abortus and B. melitensis.³⁶

Brucella suis biovars 1, 2 and 3 can be isolated in suitable culture media and identified by simple bacteriological and molecular tests (see below).

Brucella suis biovars 1 and 3 are mainly pathogenic for pigs and humans but have also been reported to infect cattle sporadically.^{6, 16} 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 pigs¹⁹ and also in Australia among those involved in the killing and slaughter of feral pigs.³⁷ Human brucellosis caused by B. suis biovars 1 and 3 is thus almost entirely occupational^{1, 22} except 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 because of infection of the bovine udder.^{6, 29}

Brucella suis biovar 2 differs from B. suis biovars 1 and 3 in that it is restricted to Continental Europe and Northern Africa and infects wild boar and the European hare, which are the wild reservoirs.^{15, 23, 26} Otherwise, swine brucellosis 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 been reported twice as the cause of human brucellosis in immune-compromised patients.³⁰ Brucella suis biovar 2 can sporadically infect cattle.²⁰ However, B. suis biovar 2 is not likely to be transmitted between cattle because they are spillover and not preferential hosts for B. suis biovar 2, and are thus not likely to sustain the infection.²⁰

In modern industrial pig farming, infection is generally introduced into a herd 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 sex organs, from which bacteria can be shed in the semen for months or years, may be present in boars even though they appear healthy. Hence the potential exists for spreading the disease through artificial insemination.^{5, 36, 41} Other routes of spread  are transplacental and the ingestion of aborted materials, infected milk or contaminated feedstuff.¹ The feeding of contaminated kitchen waste containing uncooked pork may also be important.³⁶ Transmission of B. suis biovar 2 from wild boar to pigs in open air breeding systems or backyard farms is thought to be mainly through the venereal route. Infected hares can be involved in B. suis biovar 2 outbreaks in domestic pigs²⁶ through swill feeding with offal from...