Emergence of Brucella suis in dogs in New South Wales, Australia: clinical findings and implications for zoonotic transmission
Mor et al. BMC Veterinary Research
Emergence of Brucella suis in dogs in New South Wales, Australia: clinical findings and implications for zoonotic transmission
Siobhan M. Mor 0 1
Anke K. Wiethoelter 1
Amanda Lee 2
Barbara Moloney 4
Daniel R. James 3
Richard Malik 1
0 Tufts University School of Medicine , 145 Harrison Avenue, Boston 02111, MA , USA
1 Faculty of Veterinary Science, The University of Sydney , Sydney 2006, NSW , Australia
2 New South Wales Department of Primary Industries , Woodbridge Road, Menangle 2568, NSW , Australia
3 Small Animal Specialist Hospital , 1 Richardson Place, North Ryde 2113, NSW , Australia
4 New South Wales Department of Primary Industries , 161 Kite Street, Orange 2800, NSW , Australia
Background: Animal reservoirs of brucellosis constitute an ongoing threat to human health globally, with foodborne, occupational and recreational exposures creating opportunities for transmission. In Australia and the United States, hunting of feral pigs has been identified as the principal risk factor for human brucellosis due to Brucella suis. Following increased reports of canine B. suis infection, we undertook a review of case notification data and veterinary records to address knowledge gaps about transmission, clinical presentation, and zoonotic risks arising from infected dogs. Results: Between 2011 and 2015, there was a 17-fold increase in the number of cases identified (74 in total) in New South Wales, Australia. Spatial distribution of cases largely overlapped with high feral pig densities in the north of the state. Ninety per cent of dogs had participated directly in pig hunting; feeding of raw feral pig meat and cohabitation with cases in the same household were other putative modes of transmission. Dogs with confirmed brucellosis presented with reproductive tract signs (33 %), back pain (13 %) or lameness (10 %); sub-clinical infection was also common (40 %). Opportunities for dog-to-human transmission in household and occupational environments were identified, highlighting potential public health risks associated with canine B. suis infection. Conclusions: Brucellosis due to B. suis is an emerging disease of dogs in Australia. Veterinarians should consider this diagnosis in any dog that presents with reproductive tract signs, back pain or lameness, particularly if the dog has a history of feral pig exposure. Moreover, all people in close contact with these dogs such as hunters, household contacts and veterinary personnel should take precautions to prevent zoonotic transmission.
Brucellosis; Brucella suis; Dog; Emergence; Zoonosis; Australia
In a recent global review of diseases at the
wildlifelivestock interface, brucellosis ranked amongst the top
ten diseases [
]. Because the disease is readily
transmitted between wildlife, domestic animals and humans, new
and re-emerging foci represent an ongoing challenge
worldwide with foodborne and occupational exposures to
livestock and livestock products recognised as the main
traditional risk factors in humans [
recreational activities such as hunting of feral animals and
wildlife have emerged as an alternative risk factor [
]. Out of
the four terrestrial zoonotic Brucella species – B.
melitensis, B. abortus, B. suis, and B. canis – only B. abortus and
B. suis have been frequently found in wildlife [
particular, contact with bison (Bison bison), elk (Cervus
elaphus) or African buffalo (Syncerus caffer) as well as
reindeer (Rangifer tarandus) have been identified as
important risk factors for human brucellosis due to B.
3, 5, 6
] and B. suis biovar 4 [
Australia is currently free of many of the human
pathogenic Brucella species; B. melitensis and B. canis
are exotic and B. abortus was eradicated from cattle and
buffalo by 1989 [
]. However, B. suis biovar 1 is endemic
in feral pigs (Sus scrofa), and was thought to be limited
to east Queensland (QLD) [
] until recently when
seropositive feral pigs were identified in northern New
South Wales (NSW) . Hunting and dressing of
carcasses of feral pigs has been associated with human B.
suis biovar 1 infections in Australia [
] and the
United States (US) .
Feral pigs are one of the most successful invasive
species worldwide due to their adaptable, highly
reproductive and opportunistic omnivore nature [
]. Due to the
severely negative impacts on crop and livestock farming
as well as wildlife predation and habitat degradation,
they are regarded as a threat to biodiversity in Australia.
Consequently, land owners in the state of NSW are
required by law to institute control measures (e.g. hunting,
trapping or poisoning) on their properties [
is also permitted on public land and state forests [
An estimated 100–200,000 hunters kill up to 500,000
feral pigs per year in Australia [
] and dogs are widely
used to bail, locate and hold feral pigs [
Since 2011, a growing number of B. suis infections
have been reported in dogs in NSW. Prior to 2011, the
only published evidence for canine infection in Australia
was a single laboratory report citing isolation of B. suis
from a canine testis in QLD in 1968 [
sporadic case reports from other countries confirm that dogs
can be infected [
]. Since B. suis biovar 1 is second
only to B. melitensis in terms of pathogenicity for humans
, concerns about the potential for dog-to-human
transmission in NSW initially led to the recommendation
that affected dogs be euthanized [
knowledge of the natural history of infection, clinical
presentation and zoonotic implications of canine B. suis infection
is meagre and the policy is under review. There is a need
for better scientific evidence to underpin sound,
riskbased policy responses.
To address these gaps, we documented the
epidemiology and clinical findings of canine B. suis cases
diagnosed between 2011 and 2015. Whether infected dogs
pose an ongoing threat to their owners and household
contacts and/or other dogs is of principal interest to policy
makers. Thus, we examined exposure histories of affected
dogs with a view to expanding current understanding of
modes of acquisition in dogs. We also reviewed veterinary
records to identify opportunities for occupational and
Data on cases notified between 1 January 2011 and 31
December 2015 were obtained from NSW Department
of Primary Industries (DPI), which incorporates the State
Veterinary Diagnostic Laboratory (SVDL). SVDL is the
only veterinary laboratory that performs serological
testing for brucellosis in NSW. As a notifiable disease, other
laboratories which diagnose cases through other means
(e.g. culture) are required to report cases to DPI. To
ensure that no cases had been unreported, DPI staff
contacted private laboratories to request information on
cases they had diagnosed.
Suspect cases were initially screened at SVDL using
the sensitive Rose-Bengal agglutination test (RBT). Dogs
with RBT agglutination scores of 1+ (low positive) to 3+
(high positive) were subjected to confirmation using the
more specific complement fixation test (CFT). A positive
case was defined as a dog with positive culture, or
positive RBT and reciprocal CFT titre ≥16. Since neither of
these serological tests is perfect, dogs with positive RBT,
but anti-complimentary CFT or reciprocal CFT titre <16
and history (pig hunting, eating raw feral pig meat,
contact with a positive case) and/or indicative clinical signs
were considered inconclusive cases. Data accompanying
the laboratory records included: date of blood collection,
name and location of referring veterinarian, serology
results, signalment (gender, breed, age), and history as
provided in the laboratory submission form or obtained
during follow-up of cases by DPI. In addition, data on
the total number of dogs tested were obtained from DPI.
Dogs that underwent repeat testing were only counted
once. Following preliminary analysis, veterinarians
attending cases were contacted and invited to share the full
records of affected dogs. Owner details were removed from
records to ensure confidentiality. A unique identifier was
retained enabling dogs with the same owner to be linked.
To assess temporal trends, the number of dogs tested
and positive/inconclusive cases identified annually were
plotted in R (v3.1.3, R Foundation, Vienna, Austria)
taking into consideration repeat testing of individual
animals. Spatial distribution of cases aggregated by town
was mapped in ArcMAP (v10; ESRI, Redlands, CA).
Data on feral pig density was obtained from DPI surveys
conducted in 2009 [
]. To examine transmission
pathways at household level, network analysis was performed
using the igraph package in R [
]. A cluster was defined
as a household that included ≥1 dog that tested positive/
inconclusive for B. suis.
Demographic and clinical information were extracted
from veterinary records, tabulated in Excel and
summarized as counts and percentages. Fisher’s exact test was
used to investigate associations between clinical
presentation and signalment. As body temperature was not
reliably documented in the majority of cases, we elected
not to report on this measurement. We note, however,
that fever was not invariably present in dogs for which
temperature was recorded. Where absence of other
clinical signs was not specifically mentioned in the case
notes, these were presumed not to be present.
Subclinical cases were defined as cases that did not have any
clinical signs consistent with brucellosis but which were
reactive on RBT/CFT.
Between 2011 and 2015, 437 unique dogs were tested for
brucellosis at SVDL, of which 72 (16.5 %) were
seroreactive (46 positive, 26 inconclusive cases). One
additional case was notified to DPI but was excluded since
the dog resided in QLD. A further case was excluded
because it was identified as part of a research study and the
dog was not subjected to evaluation sufficient to
determine exposure history or clinical status. During follow-up
with private laboratories, a further two cases (both
positive) were identified. Thus, we present the clinical findings
from 74 dogs, diagnosed by either SVDL or private
laboratories. Veterinary records and/or additional information
were provided by the referring veterinarian for 50 of these
74 cases (67.6 %). Since information on possible
incontact animals was not available for the cases diagnosed
by private laboratories, assessment of household clustering
and potential exposure pathways was limited to the 72
cases diagnosed at SVDL.
Epidemiology and transmission
Figures 1 and 2 show the temporal trend and geographic
distribution of cases, respectively. The proportion of
positive/inconclusive dogs increased from around 9 % in
2012/13 to 17–22 % in 2014/15. Cases were largely
spatialised to northern NSW, where feral pig density is highest.
Forty one discrete clusters were reported to SVDL
(Fig. 3). Records revealed linkages across clusters. For
example, dogs from different households were
sometimes taken on the same hunting trips. Typically, index
cases presented with clinical findings consistent with
brucellosis or following injuries sustained during pig
hunting. Clinical suspicion led to testing of these index
cases, confirmation of which led to further testing of
incontact animals and detection of sub-clinical cases in
the same household. Only three index cases presented
sub-clinically; in all cases there was a history of pig
hunting and/or feeding of raw feral pig meat. The two
earliest clusters in NSW had links to QLD. The first
(October 2011) involved a dog that had been pig hunting
in QLD. The second (June 2012) involved a dog mated
to a female ‘pig dog’ from QLD 5 weeks earlier. The
latter dog was used for pig hunting but had not been
hunting in QLD for 4 years.
Potential exposure histories were available for 57/72
cases diagnosed by SVDL (38/41 household clusters). Pig
hunting was practiced by 36/38 households and was the
most plausible source of infection in 51/57 cases (89.4 %).
However, within pig hunting households, not all
seroreactive dogs had participated in this activity (Fig. 3: M1, fed
raw feral pig meat; O1, no known contact with feral pigs).
Serology was performed on offspring of seroreactive
dogs in two pig hunting households. In one household
the female (Fig. 3: P1) and male (P2) canine parents
tested seropositive and inconclusive, respectively, while
the 5-month old offspring tested seronegative. Review of
veterinary records revealed that 2 pups from the same
litter had been aborted prematurely; the remaining five pups
were delivered by caesarean. In the other household, the
female (P3) tested seropositive when her offspring were
12 months old; only one of the two offspring (full siblings)
was seropositive on subsequent testing. Another dog in
this household (M2) was inseminated by a dog that later
tested positive (M1), although the former was found to be
seronegative on subsequent testing.
Pig hunting status was not known for the three
remaining household clusters. In two of these clusters,
index cases had a history of being fed raw feral pig meat
(Fig. 3: M3, M4) and were the only seropositive cases
detected in these households. The remaining dog (O2) had
no known contact with feral pigs or infected dogs and
was not fed raw feral pig meat according to the current
owner, although this dog was adopted from a pound and
exposure history prior to this was unobtainable.
Clinical presentation and outcome in dogs
Tables 1 and 2 show the signalment and clinical
presentation of dogs, respectively. The median age of affected dogs
was 2 years (range 5 months to 12 years). There were no
clear relationships between age and clinical presentation,
with dogs aged ≤24 months just as likely to present with
certain clinical signs as those aged >24 months (p > 0.05
for all signs). De-sexed dogs were more likely than
reproductively intact animals to present with back pain (50 %
versus 10.3 %; p = 0.074) and lameness (50 % versus 7.4 %,
p = 0.045), while females were more likely than males to
be sub-clinically affected (53.6 % versus 31.8 %, p = 0.087).
No other significant associations existed between gender,
reproductive status and specific signs. Dogs with clinical
signs were just as likely as dogs without clinical signs
to have an inconclusive sero-status (35 % versus
37.5 %, p = 1.0).
Signs consistent with reproductive tract involvement
were the most common presenting problem and
occurred across a range of ages (1–9 years). Among 21
dogs with orchitis/epididymitis, presentation was
unilateral in 12 cases and bilateral in eight (one unspecified).
Serology conducted on one female at the time of
abortion was negative; retesting 8 months later however
showed an inconclusive result.
Nine dogs presented with back pain, four of which also
presented with intermittent lameness, while one
presented with concurrent reproductive tract signs. Back
pain was mostly localized to the thoracolumbar junction
(five out of six cases). Two dogs were diagnosed with
discospondylitis with localized empyema adjacent to
affected vertebrae. While there was no bacterial growth
following culture of cerebrospinal fluid, B. suis was
cultured from soft tissue material collected during
decompressive hemilaminectomy in both animals.
Three dogs presented with lameness without back
pain. In one case (Bull-Mastiff, 22 months), the dog
presented with a history of shifting lameness and swollen
joints followed by an acute episode of dyspnoea. A large,
oedematous mass cranial to the larynx was palpable and
considered to be impeding airflow. Generalised
lymphadenomegaly was also evident and unilateral
orchitis/epididymitis and pyrexia developed one week later. The dog
was castrated and euthanised following culture of B. suis
biovar 1 from the affected testis. A second dog (Kelpie
X, 33 months) presented with painful, dorsally-swollen
left carpus. B. suis biovar 1 was cultured from joint fluid
collected from the inter-carpal joint.
Public health considerations
In two separate clusters, veterinary records indicated
that the dogs’ owner had been diagnosed with
brucellosis prior to presentation of the dog. Both households
practiced pig hunting. Other potentially risky practices
identified from veterinary records included assistance
with whelping and on-sale of live-born offspring prior to
diagnosis in the parent dogs. A woman in at least two
households was reported as being pregnant at the time
of canine diagnosis.
Opportunities for potential occupational exposure
were noted in records of several cases. Four dogs were
de-sexed during the episode of illness as part of clinical
management of reproductive tract signs, while two
underwent spinal surgery. Infection status of these dogs
was unknown at the time of the procedure, and thus the
surgical teams are presumed to have taken no special
precautionary measures to minimise the risk of
infection. Likewise, bacterial culture was performed by
unsuspecting laboratory staff involved in the diagnosis of two
This article comprises the largest and most detailed
compilation of canine B. suis cases reported to date.
Previous cases have been recorded in Bulgaria [
25, 28, 29
], Germany [
], Hungary [
], India [
the US [
22, 24, 30, 32
]. Many of these were documented
in the mid-20th century when commercial piggeries
constituted the main reservoir of B. suis biovar 1. With
eradication having been achieved in many of these
countries, infection dynamics have shifted considerably, as
have exposure risks .
We observed a 17-fold increase in the number of canine
cases detected in NSW between 2011 and 2015. A similar
trend has been documented in Georgia, US in association
with recreational pig hunting activities [
extent to which this increase reflects true emergence
versus enhanced detection of previously unrecognized
foci of infection is unclear. Certainly awareness for
the disease is growing in NSW, as evidenced by the
increasing number of dogs presented for testing.
However, the incursion of seropositive feral pigs in
NSW lends biological plausibility to a novel source of
infection, a finding supported by the high degree of
spatial overlap between feral pigs and canine cases
reported here. We suspect B. suis was introduced
following deliberate transportation of feral pigs across
state borders by recreational hunters, a practice
which is illegal but, anecdotally, widely practiced [
Natural migration of infected feral pigs across the
NSW-QLD border is also likely [
]. Movements of up
to 12 km per day as well as swimming across rivers and
off coastlines have been observed in feral pigs and seem
to be largely driven by external factors such as human
disturbance, weather conditions, food and habitat
Given co-occurrence of exposures in the same
households and potential for casual contact between dogs (e.g.
through sharing feed bowls), it is difficult to draw
definitive conclusions about transmission. Direct involvement
in pig hunting was the most plausible mode of
transmission for the majority (90 %) of dogs, while feeding of
raw feral pig meat resulted in infection of dogs not
involved directly with hunting. Evidence for vertical
transmission was limited in cases reported here, although we
presume it occurs by extrapolation from other Brucella
]. Pig-to-dog transmission of B. suis through
hunting or co-habitation with domestic pigs has been
]. Precisely how dogs acquire the
infection from pigs is not known. Given the nature of pig
hunting – which involves a high frequency of injuries
to both animals and humans – we suspect transfer
likely occurs through blood-borne contact and/or direct
inoculation by contamination of wounds, transmission via
mucous membranes or via ingestion of pig offal or meat.
Ingestion of aborted foetal material has also been
proposed . A number of dogs in this study presented for
treatment of sub-cutaneous abscesses most likely
sustained as a result of pig hunting. Cutaneous lesions due to
B. suis have been described in humans [
] and may be
consistent with traumatic inoculation at these sites.
Little is published on the risk of foodborne transmission
of B. suis biovar 1 via meat, although the first canine case
was linked to this practice [
]. Further, Hellmann and
] postulated that raw pet food obtained from
Eastern European countries was the likely source of
infection for dogs in Germany. The likelihood that B. suis may
be transmitted via feral pig meat has implications for
human and animal health. At one time Australia supplied
as much as 30 % of wild boar meat consumed globally,
with a smaller quantity being sold domestically and
byproducts/substandard carcasses used as pet food [
Proper cooking and/or canning will destroy Brucella spp.,
thus the main risk comes from handling and consumption
of undercooked meat [
]. Raw pet food diets are
becoming popular in Australia and elsewhere, and concerns have
been raised following detection of a number of zoonotic
foodborne hazards in raw pet food [
]. We are not
aware of any studies that have specifically tested
commercial raw food diets for B. suis in Australia or elsewhere.
It is generally assumed that hunters acquire brucellosis
following direct contact with blood and other fluids and/
or aerosols during field slaughter of feral pigs [
Risk reduction strategies have therefore focused on
preventing transmission at slaughter, including promoting
use of personal protective clothing, such as mesh gloves
to prevent abrasions and cuts to the hands . The
extent to which infected dogs present an ongoing risk to
humans in the household is unknown but plausible in
the view of the authors. A single 1967 case report found
that a woman in Massachusetts most likely acquired B.
suis following unprotected disposal of aborted dog
]. The dog, which was confirmed at necropsy to
be infected, had been allowed to roam freely and likely
acquired the infection from a swine farm nearby. Other
case reports have implicated dogs as the source of
human infection with B. abortus [
], B. canis [
B. melitensis [
], establishing that pathogen
excretion and/or contact sufficient to lead to human infection
In reviewing veterinary records, opportunities for
human exposure through contact with body fluids within a
household environment (e.g. aborted foetuses, placenta)
as well as occupational exposure of veterinary staff via
routine (e.g. diagnostic specimen collection and
processing, castration and ovariohysterectomy) and advanced
procedures (e.g. spinal surgery) were identified. High
speed burr was used in the latter case, increasing risk of
aerosolisation. Veterinary staff should be encouraged to
adopt strict precautions, including use of masks, gloves
and eye shields when handling dogs with a history of pig
hunting. Further, following presumptive diagnosis by
serology, preliminary antimicrobial therapy may make
subsequent surgery less hazardous by reducing viability of
organisms in vivo. We also recommend that specimens
collected from pig hunting dogs be clearly identified as
such, so that laboratory staff can perform subsequent
manipulations under safe conditions even if specimens
are not specifically tested for brucellosis. Finally, we
advise that pregnant women (in the household and in the
workplace) should avoid contact with hunting dogs.
Dogs with brucellosis presented with one of three
syndromes consistent with involvement of the
reproductive tract, axial skeleton or appendicular skeleton,
although combinations were possible also. Reproductive
tract involvement is recognized as a clinical feature of
brucellosis in a number of animal species, including
25, 27, 29, 33
]. Discospondylitis due to B. suis has
also been previously described in one dog  and
spondylitis is a well-known complication of B. suis
infections in humans [
]. The detailed clinical findings
and successful treatment of one discospondylitis case
reported here (Fig. 3: O2) as well as two orchitis cases
is described elsewhere (James et al. 2016, in review).
We found that dogs that had been de-sexed were more
likely to present with back pain and/or lameness than
reproductively intact animals, suggesting that B. suis
may display tropism for bones or joints when the
preferred site (reproductive tract) is not present.
A considerable number of dogs without clinical signs
were reactive on RBT/CFT and were presumed to be
sub-clinically infected. These cases present a particular
challenge in terms of detection and clinical management.
Sub-clinical infection has been reported in both
naturally occurring [
] and experimental infections in dogs
]. The finding that females were more likely to be
sub-clinically infected may reflect a bias towards
detection in males, with owners more likely to recognize and
investigate enlarged testes. Alternatively, veterinary
records may have failed to adequately document a history
of abortion in healthy female dogs that present later for
testing. Given the high frequency of sub-clinical
infection, regular testing of pig hunting dogs and in-contact
animals, particularly prior to invasive surgeries or
breeding, is advised.
This study has a number of limitations. Clinical and
exposure histories were limited for some dogs, making
conclusions about source of infection and onset of illness
difficult in these cases. Further, the location and timing of
hunting trips was not disclosed. Such information would
be useful in guiding surveillance and control activities in
the feral pig population, as well as updating knowledge on
incubation period in dogs. Diagnosis remains a particular
challenge with a number of dogs deemed to have
inconclusive serological results. To our knowledge, necropsy
was not performed on any dog after euthanasia nor was
sequential serology performed on animals that were not
euthanised (with exception of the case described by James
et al., in review). Culture is the gold standard for
diagnosing brucellosis but was not pursued in the majority of
cases. The sensitivity and specificity of the serological tests
used has not been determined, nor is it known whether
seropositive cases are actively infected or rather
previously exposed but with elimination of all foci of disease.
This distinction has major implications for evaluating
public health risks associated with canine infection and,
for this reason, molecular-based methods are being
pursued by DPI.
In conclusion, brucellosis is an emerging disease of dogs
in NSW with the principal risk being involvement in feral
pig hunting. Veterinarians should consider this diagnosis
in any dog that presents with reproductive tract signs,
back pain or lameness, particularly if the dog has a history
of feral pig exposure. The extent to which infected dogs
present an ongoing risk to humans and/or other dogs in
the household is unknown but plausible. Therefore all
people in close contact with infected dogs such as hunters,
household contacts and veterinary personnel should take
precautions to prevent zoonotic transmission. More
research into the natural history of infection and treatment
is needed to formulate more evidenced-based advice on
clinical management of infected dogs.
The authors wish to thank Kelli Johnson and Sally Spence (NSW DPI) for
assistance with the canine data and Peter West (NSW DPI) for sharing data on
feral pigs from NSW DPI pest surveys. Richard Malik’s position is supported by
the Valentine Charlton Bequest to the Centre for Veterinary Education.
Availability of data and materials
The data supporting the conclusions of this article are available upon request.
SM coordinated the case series, planned the study design, conducted the
data analysis and drafted the manuscript; AW contributed to the study
design, reviewed the literature and drafted the manuscript; AL, BM, DJ and
RM provided case data, contributed to the study design and revised the
manuscript. All authors read and approved the final manuscript.
SM is a public health epidemiologist and veterinarian-researcher based at
The University of Sydney. Her interests include emerging zoonoses, and
informing evidence-based practices to controlling the same.
The authors declare that they have no competing interests.
Consent for publication
Ethics approval and consent to participate
This research was reviewed and approved by the Human Research Ethics
Committee at The University of Sydney (2015/537), which granted permission
for the researchers to approach private veterinarians to request the veterinary
records of affected animals. All data furnished to the researchers were
de-identified such that owners of affected dogs remained anonymous.
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