Pigs came in the First Fleet from Cape of Good Hope, and 25 were unloaded from “Friendship”. The estimate is that there are approximately 20-25m feral pigs in Australia: these have derived from various breeds of ‘escaped’ domestic pigs (Sus scofra) and probably some other pig species (S.papuensis, S.celebensis).
Export of ‘wild boar’ meat from Australia exceeds 200,000 carcasses per year (mainly out from Qld) and is supplied by registered hunters. Genetic analysis of feral pigs from several regions of Australia shows that they are genetically diverse, and studies of southern Queensland confirmed that they have a high level of social contact forming a large open breeding population.1 It is assumed that the feral pigs in northern NSW form a more-or-less continuous population with those in southern Queensland: this has relevance with regards to epidemiology. Feral pigs are widely distributed in NSW as well as Qld and NT, with pockets in other states on the mainland.
Feral pigs require shelter from extreme temperatures and ready access to water, hence their occurrence near waterways and flood plains. They are omnivorous and will scavenge carrion as well as predate young, dead and dying lambs, and native species. Socially, they exist as matriarchal breeding groups of related sows, as bachelor groups, and as older males often operating alone or in pairs. They tend to have relatively well defined home ranges. Rapid increase in numbers occurs in good seasons and camping can damage crops and transport weeds; they compete with native species, and destroy habitats and waterways etc. Control is by targeted poisoning or baiting, aerial shooting etc. Hunting of feral pigs with dogs is legal in NSW and currently dogs are allowed to ‘hold’ feral pigs. Footage of feral pig hunting in NSW has been shown on public TV (Beyond the Divide).
The most significant zoonotic disease of feral pigs in NSW is brucellosis due to Brucella suis. The pathogen responsible for disease in feral pigs in Qld (and NSW) is B.suis Biotype1, this is significant human pathogen (zoonotic) and also readily infects other species such as dogs (ie it has a broad potential host range; an anthropozoonosis). The last case of B.suis in commercial pigs in NSW was in 1968, and Australia was declared free of bovine B.abortus in 1989. However, B.suis was identified in feral pigs in Qld in 1936, and has become widespread. Limited surveys of feral pigs failed to detect evidence of infection in NSW prior to 2008, even though there were human cases associated with hunting.2 A more recent survey in NSW detected a low incidence of seropositive animals (~3%) indicating active infection in a few localities.3
Infected pigs may present with abortion, infertility, metritis, orchitis, polyarthritis, or with minimal, or non-detectable, clinical signs. The organism is readily spread by ingestion, contamination of open wounds, inhalation etc, and infected pigs may be bacteraemic for months with or without clinical signs. Transmission occurs to pig dogs and hunters.2,4 Guidelines for limiting pig hunter risk have been distributed.
In recent years there have been a significant number of cases of B.suis in dogs in NSW: these were mostly, but not all, associated with pig-hunting. Clinical presentations have included epididymitis, disco-spondylitis, prostatitis, reproductive failure, and abscessations in various organs (liver, lung): infected dogs may be bacteraemic. A small number of cases have now been confirmed in dogs with no known association with pig-hunting, raising the concern about zoonotic implications of canine arthritis and synovitis, and pyothorax.
Human cases tend to present as undulating fever, malaise, anorexia, headaches, backpain, orchitis, valvular endocarditis and/or evidence of systemic infection: the resulting lymphadenopathy may mimic lymphoma.
There is no specific serological test available in Australia utilising B.suis as antigen, but the organism shares many antigens with B.abortus (they are both ‘smooth colony’ Brucella spp) and the Rose Bengal Test and complement fixation tests (CFT) are used as recommended by Office Internationale Epizootics. Neither of these test are highly specific nor sensitive (probably ~85%), therefore, interpretation requires consideration of clinical history, clinical signs and results of other ancillary testing if available etc.
In some cases, sera from a particular dog are repeatedly anti-complementary and cannot produce a CFT result. Culture of the organism is possible, but requires strict conditions to prevent handler infection.
Porcine and canine brucellosis are Notifiable Diseases on suspicion in NSW to DPI, and specific testing to confirm or exclude infection is undertaken at no charge to the submitter. Action with regards to any potentially infected dog will be as directed by appropriate authorities.5 Human brucellosis is notifiable to Department of Public Health. Thus far, there have been no instances of spread into commercial piggeries. A small number of pregnant cattle were experimentally infected with Qld B.suis Biovar 1 and developed transient low seropositivity using RBT, CFT and SAT, but did not suffer abortions.6 There have been no obvious cases yet of dog-to-human transmission: the possibility seems low, but the consequences would be significant. Br.canis (rough colony type) is exotic to Australia.
Other potential zoonotic diseases in pigs include leptospirosis. Limited serological surveys have focused on Leptospira species likely to be of concern with regards to commercial livestock and there is evidence of active widespread infection with L.pomona, a few with positive serology to L.hardjo, and individuals to other L.spp3,7; detection of serological response to Leptospira spp is dependent on recent active infection as antibody fades away within months, and epidemiology will be dependent upon seasonal, environmental and/or societal factors. The incidence of Q-fever (Coxiella brunetti) in feral pigs in NSW is at present unknown. There is a possibility that feral pigs could be infected with mycobacteriosis, meliodosis (in northern Australia), salmonellosis, sparganosis, hydatids etc, and some of the arboviruses including Murray Valley encephalitis. Feral pigs could pose a significant reservoir of infection for exotic diseases such as foot and mouth disease, swine vesicular disease, classical swine fever, trichinosis, Japanese B encephalitis etc.