This study was prompted as the serological prevalence of leptospirosis and subsequent risk to livestock production and human health in the Central West LHPA was largely unknown. Leptospirosis causing clinical disease has been diagnosed infrequently by veterinarians over the last five to ten years; however cattle producers in the area often enquire whether vaccination is necessary. This study aims to address this question.
The last comprehensive sero-surveillance of leptospirosis in beef cattle in NSW was by Hughes et al (1964), which found that across NSW 13.8% of cows or 19.2% of herds had antibodies (≥1:200) to L.pomona.
Very little testing of sheep flocks for the exposure to leptospirosis has taken place in Australia in a systematic testing program.
The most recent published report of sero-surveillance in feral pigs was published by Mason et al in 1998. In this study, serum samples collected in 1995 from 195 feral pigs throughout NSW were tested for leptospiral antibodies. Across the state, 13.9% of pigs were positive on MAT for L. pomona (titre range 50->6400) and 1% were positive for antibodies to L. hardjo (titre range 50-100).
Pfizer Animal Health Australia has been assisting the Livestock Health and Pest Authority (LHPA) to conduct leptospirosis serological testing on feral pig, cattle and sheep serum samples collected during 2012 over the Central North, North West and Central West LHPAs. An expected 1500 sheep and cattle samples and 150 feral pig samples will be analysed in total by the end of the study. An interim report of the first batch of samples tested from the Central West LHPA is presented below.
Samples were collected from clinically healthy cattle and sheep by CWLHPA staff during routine property visits. Only cattle herds that had not vaccinated for leptospirosis in the past 2 years were included in the study, and producers were asked to complete a survey regarding farm and livestock management practice, and feral animal numbers.
Samples from feral pigs (after death) were collected by local pig hunters or landowners.
In all cases, 10ml of blood was collected into a BD Vacutainer® SSTTM II Advance Gel Seperation Tube, (BD, New Jersey, USA), allowed to stand at ambient temperature for approximately two hours, then refrigerated and transported to the CWLHPA office. The samples were then centrifuged (4 minutes at 400g), and the serum decanted into a serum tube then frozen.
This batch of samples (including 97 feral pig samples, 92 cattle samples and 145 sheep samples) was transported to Pfizer Animal Health Australia, Parkville, Victoria on dry ice in early August 2012. The sera were screened against L. pomona & L. hardjo using a microscopic agglutination test (MAT). Each sample was diluted at 1:20 and then titrated to an end point (dilution showing 50% agglutination) using a series of doubling solutions (Stallman, 1984).
A total of 97 feral pig samples, 92 cattle samples (from 9 herds) and 145 sheep samples (from 16 flocks) were tested across the CWLHPA. The geographical location of samples collected is shown in Figure 1.
An MAT of ≥80 was considered to be positive for exposure to leptospirosis. This figure was used as several serological surveys done in the past in cattle used ≥100 as positive (Elder & Ward, 1978; Spradbrow 1964); and the most recent study in NSW in pigs used ≥50 (Mason et al, 1998).
Given the laboratory commenced serial dilutions from a 1:20 dilution, a titre of 80 represented the third serial dilution where ≥50% agglutination occurred. A MAT titre of ≥640 was considered to be actively infected and/or shedding leptospirosis. This figure was selected after a review of artificial challenge MAT's and urinary shedding observed in control animals post challenge in studies conducted by Pfizer Animal Health Australia (N. Charman, unpublished data).
Forty one percent of feral pigs tested had antibody levels consistent with being exposed to L. pomona at some point in the past, while 16% were actively infected likely to be shedding L. pomona, using the definition above. Thirty one percent of pigs had antibody levels consistent with having had exposure to L. hardjo at some point in the past and none were classified as actively infected with L. hardjo. See Figure 2.
On a mob basis, 32 mobs of pigs were tested (mobs were considered to be groups of pigs sampled in the same paddock on the same day). Seventy five percent of mobs had evidence of exposure to one or both types of leptospirosis and 22% of mobs had pigs in them actively infected with L. pomona.
The actively infected pigs were distributed over the Central West and were not congregated around the Macquarie Marshes or any other particularly wet area. See Figure 3.
When the results were divided based on district, Coonamble had a higher percentage of pigs exposed and actively infected, followed by Nyngan, Warren and Dubbo, as outlined in Table 1.
Eight cattle herds including 82 animals were tested. Overall, 22% of individual animals and 50% of herds had serological evidence of exposure to L. pomona, while 23% of individual animals and 75% of herds showed evidence of exposure to L. hardjo. Twenty five percent of herds contained animals actively infected or shedding L.pomona, while 38% of herds contained animals actively infected or shedding L.hardjo. These results are summarised in Figure 4.
Seventeen sheep flocks including 155 animals were tested. Overall, 12% of individual animals and 59% of flocks had serological evidence of exposure to L. pomona, while 17% of individual animals and 71% of flocks showed evidence of exposure to L. hardjo. Twelve percent of flocks contained animals actively infected or shedding L.pomona, while no flocks were actively infected or shedding L.hardjo. The results are summarised in Figure 5.
Although the study is not completed yet, we can draw some interim conclusions.
At this stage, the interim results indicate that there is a significant amount of leptospirosis (based on serology) circulating in the Central West LHPA, especially in cattle (L. hardjo & L. pomona) and pigs (L. pomona). This poses a risk to the reproductive performance and cattle herd health in the Central West, as well as human health.
It is interesting to note that despite the evidence of serological exposure to leptospirosis in all species tested, there have been few clinical cases of leptospirosis diagnosed in the Central West LHPA in recent years, and the reason for this is unknown.
This is the first study ever done in this region of Australia that measures sheep antibody levels. The implications of our findings for flock health and reproductive performance may be possible avenues for further research.
The survey that accompanied the sheep and cattle testing has not yet been collated with the data, so conclusions relating to farm management and feral animal populations on farm and how they relate to the incidence of leptospirosis are yet to be drawn. The remainder of the samples are due to be tested in late January 2013, and a full report, including analysis of the results and survey data will be produced upon completion of the study.
The authors thank E Walker, A Taylor and G McCann for assistance with collection of samples and surveys.