Zoonotic diseases such as swine brucellosis and leptospirosis are infections of near global distribution with serious health implications for humans and animals. Swine brucellosis is caused by the gram negative aerobic coccobacilli Brucella suis biovar 1, 2 and 3 bacterium (Cvetnic et al. 2004). Biovar 1 and 3 are highly pathogenic to humans. B. suis exposure in humans follows occupational and recreational activities such as feral pig hunting or laboratory work as well as food borne infections from inadequately cooked feral pig meat (Dahouk et al. 2005). Human infection with B. suis often results in serious disease. Clinical signs can be non-specific and influenza-like: intermittent fever, sweating, lethargy, loss of appetite, headaches, myalgia, malaise, diaphoresis and arthralgia. Due to its non-specific symptoms B. suis is often underreported (Donch et al. 2009). Infection is fatal in about 2% of people who are not treated. Cases are sometimes persistently recurrent in spite of antimicrobial therapy and women can experience spontaneous abortion.
Globally, feral pigs have been identified as a major reservoir of B. suis with the bacterium having been detected in wild boars in many parts of Europe, North America, South America, Asia and Africa (EFSA 2009). B. suis has also been reported in feral pigs in northern Australia particularly Queensland (Garner 1997; Mason and Fleming 1999). In 2008, approximately 200 feral pig blood samples were collected to survey for the presence of B. suis in north-west NSW. This survey did not detect any B. suis (Irwin et al. 2009). Since 2008 to 2012 there have been 16 human cases of B. suis in NSW. B. suis antibodies have also recently been found in pig-hunting dogs in NSW. In three separate cases between March and December 2012, five pig-hunting dogs tested serologically positive for B. suis. Three of these dogs presented to private veterinarians with orchitis. Only one of the dogs was clinically ill (Lee, 2012 per comms data). In light of these recent cases, the current study was undertaken to detect the current prevalence of B. suis in feral pigs in NSW.
In Australia, significant concern has also surfaced around the prevalence of Leptospira spp. in feral pigs. L. pomona sero-prevalence levels of ~10% (n=200) (Irwin et al. 2009), 13.9% (n=212) (Mason et al. 1998) and 20% (n=195) (BiosecurityAustralia 2000) have been detected in feral pigs of New South Wales. In southern Queensland, estimated true prevalence levels of 48% (n=83) have been detected (Pearson 2012) and in northern Queensland levels between 0-33% depending on the year (Heise-Pavlov and Heise-Pavlov 2003), have been recorded. Leptospira spp. have also been detected in domestic pigs in Victoria, South Australia and New South Wales (Chappel 1998) and cattle in New South Wales and Queensland (Keast et al. 1964; Black et al. 2001). In feral pigs leptospirosis has been found worldwide; Africa (Hunter 2004), Asia (Boqvist 2002), Europe (Vicente et al. 2002; Ebani et al. 2003; Jansen et al. 2007), North America (New et al. 1994; Saliki et al. 1998), New Zealand (Bolt and Marshall 1995) and also in domestic pigs of South America (Oliveira et al. 1980; Soldan et al. 1991; Ochoa et al. 2000).
Leptospirosis is a globally significant disease with human and animal health risks accompanied by economic implications. Incidence of this disease can be costly. It has been previously estimated that prevalence levels of 12% in Great Britain would result in the output loss or resource wastage of 22 million annually (Bennett et al. 1999).
Leptospirosis can be transmitted to humans and animals by contact with infected urine, blood and fluids from birth or abortion. It is also spread in contaminated water, food, pastures, soil and can take aerosol form from urine (Ellis 1994). Increases in herd sero-prevalence of Leptospira spp. has been linked to the release of free leptospires from the vertisol clays into the water tables following heavy rainfall. The leptospires can then infect susceptible cattle and/or maintenance hosts like feral pigs (Keast et al. 1964). As feral pigs are not restricted by fences there is a higher risk that they can be infected by Leptospira spp. and therefore pose significant biosecurity risks to healthy livestock. Contact between feral pigs and domestic livestock, cattle and pigs, has been documented in other studies (Wyckoff et al. 2009; Pearson 2012). Many animals infected with Leptospira spp. are asymptomatic and are the main source of disease transmission (Zelski 2007).
Exposure of cattle to Leptospira pomona can cause abortions, high fever, jaundice and reddish-brown discolouration of urine. Untreated infections in calves can be fatal. Abortion commonly occurs if the cows are more than 5 months pregnant. In dairy herds a sharp decrease in milk production can be experienced (Ellis 1994).
Human infection with Leptospira spp. causes fever, headaches, muscle pain and sometimes nausea and vomiting. If left untreated, complications can sometimes occur, which in rare situations leads to mortality (NSWHealth 2007). From 2008 to 2012 there have been 112 human cases of leptospirosis reported in NSW (NSWHealth 2012). Those most susceptible to leptospirosis include cattle farmers, particularly dairy farmers and abattoir workers that are exposed to aerosol when urine hits the floor of the respective workplace (Chappel et al. 1998), hunters and sewerage workers (Levesque et al. 1995).
The aim of this study was to determine the presence and prevalence of B.suis, L. pomona and L. hardjo in feral pig populations of New South Wales.
Serum collection. In 2012 and 2013, 195 serum samples were collected from feral pigs from northern NSW, Australia (Fig. 1). As in the case of many wildlife and feral-animal diseases it is impossible to construct and select from an authoritative sampling frame. Consequently, a convenience-based approach to sampling of animals was adopted in order to collect the required specimens. This involved recruiting the cooperation of feral pig hunters, rangers from Livestock Health and Pest Authorities, and rangers from the NSW National Parks and Wildlife Service already involved in shooting and culling of feral pigs. Obtaining specimens using this approach had the advantage of targeting the subset of the feral pig population presenting the greatest risk to humans, that is, the group most likely to be involved in exposure of humans to both of the zoonotic agents of interest. From each animal one blood sample was taken by making an incision under the front leg or jugular. Samples were kept at room temperature for three to four hours to let the serum separate followed by refrigeration and then submitted in chilled containers to Elizabeth Macarthur Agriculture Institute for analyses within 48 hours. The hunters involved with specimen collection were given information on how to reduce the risk of infection and were contacted again if the tests were positive.
Serological Tests. A serial-testing regime was adopted for evaluating sera for B. suis antibodies. All sera were initially screened for antibody using the Rose-Bengal Test (RBT) with only the positive sera assayed using the compliment fixation test (CFT). Sera were considered positive for RBT if a titre of one or more was detected and positive for CFT if a titre of eight or more was detected. Sera were only classified as positive for B. suis if both tests were positive. Two separate microscopic agglutination tests (MAT) were used to detect antibodies of Leptospira pomona and Leptospira hardjo. Sera were considered positive for Leptospira at MAT titres of 50 and above.
Culture and Polymerase Chain Reaction (PCR) Analysis. Blood samples that tested positive for both RBT and CFT were subjected to culture and PCR analysis for detection of B. suis. For PCR identification standard strains of B. ovis were used as a positive control.
Regions: 1 NE = New England; 2 CN = Central North; 3 NW = North West; 4 CW = Central West; 5 D = Darling; 6 W = Western.
Interim results as of 1 December 2012 are reported with the analysis of final results, including breakdown by regions, expected to be available by March 2013. With respect to L. pomona, the proportion of animals seropositive in the interim data set was 63% (119/195). Animals positive for L. pomona were distributed across the entire study site (Fig. 1).
Serologically positive B. suis samples were from region 1 NE (1/57 positive) and region 3 NW (6/36 positive). Titres for RBT were between one and three and titres for CFT were between 8 and >32. No B. suis organisms were cultured or detected by PCR analysis.
Serologically positive L. hardjo samples were from region 1 NE (5/57 positive), region 4 CW (2/10 positive) and region 5 D (2/63 positive). MAT titres were between 50 and 400.
The prevalence of L.pomona serological positives was high compared with levels previously recorded within Australia by Irwin et al (2009) who noted a prevalence of 13.9% in northwest NSW. High titres recorded from most of the samples suggested current or recent infection of L. pomona (Mason et al. 1998). High rainfall and flooding within the last few years could be responsible for the high level of L. pomona observed(Keast et al. 1964; Smith et al. 2012) considering that prevalence levels in 2008 were considerably lower (Irwin et al. 2009). Survival rates of L. pomona in the environment are enhanced under wet conditions thus increasing the opportunity for transmission to susceptible hosts. As well, the prolonged period of low transmission preceding this study may have increased the general susceptibility of the feral pig population to infection with Leptospira (BiosecurityAustralia 2000). Another factor accounting for the high seroprevalence of L. pomona in feral pigs could be the rodent plagues brought about by heavy rainfall and good seasons. Rodents have been identified as native hosts of various serovars of Leptospira (Vanasco et al. 2003) and they can efficiently spread infection by contaminating food and the environment with urine and faeces (Drummond 2001).
The negative culture and PCR results for B. suis indicate that the feral pigs were not bacteraemic at the time of sampling. Negative culture and PCR analysis may have resulted from the low sensitivity of the tests considering the animals are only bacteraemic for a limited time. The current results indicate that feral pigs within New South Wales have likely been exposed to B. suis at some point but have since recovered from the infection and that the disease may be present at low levels within northern NSW. Although the B. suis levels detected are quite low, when the severity of disease in humans is considered, the risk posed by this organism to pig hunters and others that come into contact with feral pigs is quite substantial.
Positive samples and titres recorded for L. hardjo were very low indicating that feral pigs may be incidental hosts of this serovar in New South Wales (Mason et al. 1998). This means that the threat of transmission from feral pigs to either humans or livestock is smaller than for L. pomona in the regions studied.
Exposure to live or recently-killed feral pigs poses a risk to human health. Minimising the opportunity for transfer of infection between animals and man is a priority. Strategies to reduce the risk of infection include wearing protective clothing such as gloves when handling animals, covering any cuts, avoiding contact with animal fluids (e.g. blood, urine) and avoiding cutting into reproductive tissues potentially harbouring the bacteria when gutting pigs and doing necropsies. When eviscerating feral pigs, face and eye protection should be considered, especially when the risk of exposure is high. Prevention of transfer of Leptospira infection from feral pigs to livestock also needs attention. Very effective vaccines are available for protecting cattle from L. pomona and L. hardjo (Bolin and Alt 2001). In grazing areas where feral pigs are prevalent use of such vaccines may benefit human health as well as the health and productivity of livestock.
This study was a collaborative project run by NSW Department of Primary Industries, Pfizer Animal Health Australia and Hunter New England Population Health. This study was performed under the approval of the Elizabeth Macarthur Agricultural Institute Animal Ethics Committee (AEC No.: M12/05). Many thanks are given to Livestock Health and Pest Authorities (New England, Darling, Western and North West), NSW National Parks and Wildlife Services (Narran Lakes and Glen Innes) and numerous recreational and occupational hunters that have assisted with sample collection.