CASE NOTES
Swine brucellosis in a small domestic piggery
Megan Davies, District Veterinarian and Emergency Management Coordinator, Narrabri
Posted Flock and Herd November 2024
Introduction
This article describes the management of a Brucella suis outbreak on a property in north-western NSW in July 2023. The case involved infection in domestic dogs and domestic pigs following contact with feral pigs. Being a zoonotic disease, management of the case involved risks to the veterinarians, Local Land Services staff and owners of the infected animals - all of whom were involved in testing at-risk animals and the subsequent depopulation of the property. The process of depopulating, disposal and decontamination of the property also presented practical challenges to the stakeholders involved.
The ongoing challenge for veterinarians and government departments will be working out how to manage the animal welfare impacts and zoonotic risks of this disease, without providing a disincentive to producers reporting clinical signs of the disease in pigs.
CASE REPORT / SERIES
History
In May 2023, a 3-year-old Bull Arab bitch presented to the private veterinary hospital in Narrabri with symptoms of lethargy, pale mucous membranes and spinal pain. The dog had a history of contact with feral pigs on the property of residence, as well as being fed pig meat from both feral pigs and domestic pigs. It was not a 'pig hunting' dog, rather a domestic pet with intermittent contact with feral pigs. The private veterinarian collected blood samples (EDTA and clotted) and submitted them to the State Veterinary Diagnostic Laboratory, requesting testing for brucellosis. The laboratory results returned strong positive results on both the Brucella Rose Bengal test (RBT) and the Brucella Complement Fixation Test (CFT).
The private veterinarian, in consultation with NSW Department of Primary Industries (DPI), arranged for testing of in-contact dogs. Fourteen in-contact dogs were tested, with two dogs returning positive results. The ongoing surveillance and management of the infected dogs was conducted by the private veterinarian. When collecting further details from the owner, the private veterinarian was made aware of a small domestic piggery present on the dog owner's property. The dogs had contact with these domestic pigs as they accompanied the owner when he was caring for the pigs. The private veterinarian contacted the local district veterinarian, to discuss the case and the possibility of the domestic pigs being the source of the infection in these dogs.
Further investigation into the brucellosis status of the pigs in the piggery was considered warranted, given the known contact between the pigs and the infected dogs.
District veterinarians attended the property and collected blood samples from 15 pigs. Due to the risk of zoonosis, personal protective equipment (PPE) was worn, including overalls, boots, gloves and masks. The piggery was inspected and found to have insecure fences which allowed smaller pigs to escape the pens and roam freely around the property. The owner also reported regular contact between the domestic pigs and feral pigs – describing how he often had to chase feral boars out of the outdoor sow pens. The owner also reported that several domestic pigs had escaped, and now lived free-range on the property, coming into the vicinity of the piggery once daily for feeding.
Laboratory Results
The State Veterinary Diagnostic Laboratory protocol for testing for Brucella suis involves parallel testing using the Rose Bengal Test (RBT) and the Complement Fixation Test (CFT). These serological tests look for Brucella antibodies, and thus have the potential for false negatives in early stages of infection. When conducted in parallel, these tests are reported to have a specificity of 99% and a sensitivity of 94% (R Powell, pers. comm. 2023).
In accordance with the NSW DPI Procedure for Managing Porcine Brucellosis in Domestic Pigs, animals are considered positive if they score 1+, 2+ or 3+ on the RBT and have any CFT titre result above 0. The laboratory results for the 15 pigs sampled are listed in Table 1. Two of these pigs are considered 'infected'.
Table 1. Laboratory test results for pigs
| Pig ID number | Rose Bengal (RBT) Results | Complement fixation (CFT) titres |
|---|---|---|
| 1 | Negative | <4 |
| 2 | Negative | <4 |
| 3 | Negative | 128 (positive) |
| 4 | Negative | <4 |
| 5 | Negative | 8 (positive) |
| 6 | 1+ (weak positive) | 32 (positive) |
| 7 | Negative | <4 |
| 8 | Negative | <4 |
| 9 | 2+ (medium positive) | 64 (positive) |
| 10 | Negative | <4 |
| 11 | Negative | <4 |
| 12 | Negative | <4 |
| 13 | Negative | 64 (positive) |
| 14 | Negative | <4 |
| 15 | Negative | <4 |
Regulatory actions
The NSW Department of Primary Industries Procedure for Managing Porcine Brucellosis in domestic pigs was used to determine the course of action following the diagnosis of Brucella suis in this herd. In consultation with the owner, it was determined that depopulation of the pig herd was the best option for minimising the zoonotic risk and containing the spread of the disease.
Destruction of all pigs on site—including as many of the free-roaming pigs as could be captured—was conducted by Authorised Officers under the NSW Biosecurity Act 2015, using firearms. Carcasses were deep-buried on farm in accordance with Environmental Protection Authority (EPA) guidelines. All hard surfaces within the piggery were grossly decontaminated with a high-pressure water cleaner, and then disinfected with a 2.5% bleach solution. Full PPE was worn by all persons involved in the destruction, disposal and decontamination process and all equipment was either disinfected or disposed of at the conclusion of the operation.
A biosecurity direction was issued to the owner, directing that the piggery must not be restocked for a minimum of 12 months, and directing that improved biosecurity measures, including improved fencing must be implemented prior to restocking.
Discussion
Swine brucellosis is widespread throughout the feral pig population in southern QLD and is now considered endemic in feral pigs in northern NSW (Department of Primary Industries, 2023). The extent of spread into other regions of NSW is unknown. B. suis is a notifiable disease in NSW, but is not considered an emergency animal disease. The most likely method of entry for B. suis into a domestic pig herd is through contact with feral pigs (DPI, 2023). A 2014 study indicated that 3% of 239 feral pigs tested in northern NSW were positive for brucella antibodies (Riddout et al. 2014).
Brucellosis can also affect dogs and humans. Recent studies looking at the seroprevalence in dogs showed that B. suis is common in dogs that have contact with feral pigs, with one in 10 testing seropositive (Kneipp et al. 2021). It is unknown in this case whether the domestic pigs and the dogs were infected independently, through separate incidences of contact with feral pigs, or whether the dogs were infected by the domestic pigs, subsequent to their infection by feral pigs.
Given that the disease is endemic in NSW, the risk to domestic pigs is ongoing. Pigs housed in outdoor pens, insecure yards, and with known contact with feral pigs are at high risk. In contact dogs and people are also susceptible to infection with the bacteria.
Domestic pig owners need to be aware of this risk, and the associated risk to their own health, and that of their dogs. The risk can be minimised by preventing contact between domestic pigs and feral pigs, and between pigs and dogs. This requires adequate pig-proof fencing around all piggeries, containment of dogs, and management of feral pigs on properties. Pig-proof fencing is an interesting concept, as those who manage fences will attest - pigs are adept at breaking through fences, even those that are buried underground and / or electrified.
It is possible that for many owners of small piggeries, and those who raise pigs for their own consumption, the costs involved in making their piggeries 'feral-pig proof' will prove too high. These piggeries may remain at risk of infection in their pigs, with the consequential zoonotic risk, or may choose to cease housing pigs. Larger operators who can house their pigs indoors, or manage feral pig access to their sites, may be able to minimise the risk of exposure to B. suis more economically, and have a greater economic incentive to do so.
Education of pig owners, particularly those in areas with high feral pig numbers, is critical to ensure the health and safety of domestic pigs, their owners, and dogs who may have contact with pigs. In the author's experience, many pig owners are unaware of the risks posed by B. suis carried within the feral pig population.
Management of pigs diagnosed with B. suis is challenging. The current procedure recommends depopulation of the herd as the safest and most effective way of containing the disease. Owners who wish to keep pigs of high value may apply for the approval of the Chief Veterinary Officer to undertake a test-and-cull program. In the experience of the author, these programs are labour intensive, expensive, and can still result in an eventual depopulation of the entire herd, if cases continue to be detected.
Unfortunately, because B. suis is not an emergency animal disease, there is no arrangement for compensation payments for euthanased livestock, nor any financial assistance available to cover the cost of the depopulation, disposal and decontamination process. It is unlikely that the average owner of a small piggery would have the required equipment, skills and knowledge to safely and effectively euthanase and dispose of their herd, without exposing themselves to greater risk of infection, nor is it reasonable to expect them to have to do so. Management of this aspect of containing outbreaks and minimising zoonotic risk requires further consideration.
Depopulation of small commercial herds in rural areas presents its own challenges. In this case the piggery buildings were small and low roofed, preventing the access of machinery to manage carcasses. This resulted in a lot of manual handling during the disposal process. This situation is relatively common across small piggeries in north-western NSW. The yards of the piggery were dirt-floored, making cleaning and disinfection a challenge. The process of depopulation also results in further contamination of the site with blood etc. - and it is challenging to reduce this contamination in dirt-floored yards. Data on the viability of the bacteria in the Australian environment are scarce, so restrictions on restocking times are based on extrapolations of how long sites are likely to remain contaminated based on persistence of the bacteria and overseas data. Additionally, the fencing at this site was not secure, so while the piggery was not allowed to be restocked for 12 months, there was nothing to stop feral pigs from accessing the site and being exposed to any remaining contamination.
Feral pig control is also critical to limiting the spread of the disease. This is an ongoing challenge for all landholders, and requires an ongoing concerted effort, including all neighbours within a geographical area. Without the ability to substantially decrease feral pig numbers, B. suis will remain a threat to our producers and communities, and veterinarians need to remain vigilant for cases in both dogs and domestic pigs.
Acknowledgements
The author acknowledges the substantial practical assistance provided by North West Local Land Services staff in the management of this case. The author also recognises the technical expertise and moral support provided by DPI, and the ongoing efforts to manage the infected dogs by Western Namoi Veterinary Services veterinary team. The author thanks the owner of the pigs for their co-operation and assistance with resolving this case.
References
- Department of Primary Industries (2023) DPI Primefact Brucellosis (Brucella suis) in pigs Fifth Edition www.dpi.nsw.gov.au Accessed 11/01/2023
- Kneipp CC, Sawford K, Wingett K, Malik R, Stevenson MA et al. (2021) Brucella suis Seroprevalence and Associated Risk Factors in Dogs in Eastern Australia, 2016 to 2019 Frontiers in Veterinary Science 1033
- Mason R and Fleming P (1999) Serological survey for Brucella antibodies in feral pigs from eastern Australia Australian Veterinary Journal 77:331-332
- Ridoutt C, Lee A, Moloney B, Massey P, Charman N et al. (2014) Detection of brucellosis and leptospirosis in feral pigs in New South Wales Australian Veterinary Journal 92:343-7