This paper describes the change in the annual incidence of bovine leptospirosis in the Narrabri district from 1991 to 2012. Bovine leptospirosis changed from a frequently diagnosed cause of infertility in the early 1990s, to rare by 2000. The description is based on the recollections of the author, personal comments of fellow District Veterinarians and computerised records held at the Narrabri office.
The paper will propose a number of determinants that could explain the changed incidence. The author contends that the most plausible explanation is that the majority of cases are due to L.hardjo, and that vaccination and dry conditions have largely removed sources of L.hardjo.
Beef production in the Narrabri District (defined as the previous Narrabri Rural Lands Protection District), is usually the secondary source of income on any holding, with the primary income coming from cropping, sheep or off farm income for smaller holdings. Reflecting this, most herds are loosely managed, with extended or year round joining and sporadic use of pregnancy testing. Breeding is the basis for most operations, but most will also opportunistically trade cattle, both steers and cows and calves. There are no commercial dairies in the Narrabri district.
Approximately half the Narrabri district is heavy black soil plains that hold surface water after significant rains. The remainder is the foothills of the Nandewar Ranges or the lighter soils of the Pilliga.
Feral pigs are found across the Narrabri district, especially in the western areas and along the Namoi River. Feral pig numbers on the north west plains vary widely from year to year. This variation is strongly linked to seasonal conditions, reaching their highest numbers after seasons with floods, especially if repeated in subsequent years.
The author began duties as District Veterinarian Narrabri in 1991, and has since been located at Narrabri.
The DMS computerised database was used to record disease events at Narrabri from mid 1993 to mid 2010. For a diagnosis of leptospirosis to be entered in the DMS database, the following was required; the herd had to have an infertility syndrome, blood samples from 5-10 animals were submitted to a NSW government veterinary laboratory and the serology was consistent with leptospirosis as the cause of the infertility.
In deciding whether the serology was consistent with leptospirosis, the following were considered; the strain of leptospirosis, time since likely infection, potential interference from vaccination titres and portion of sampled animals that had aborted. In general, this required more than one animal with titres over several hundred. Where appropriate, experienced veterinary pathologists at the testing laboratory were consulted.
Where only L.pomona or L.hardjo was found, the diagnosis was recorded for the respective strain. Where both strains were present on serology, this was recorded. Two leptospirosis cases in 2012 were added to the data set (there were no cases mid 2010 - 2012).
In the early 1990s, bovine leptospirosis was a frequent diagnosis in the Narrabri district. The most common presentation was either less calves born than expected, or a low pregnancy testing rate. Less commonly, cases were investigated after abortions or stillborns. The author's recollection is that in the first few years of the 1990s, several cases a year were diagnosed, being highest soon after commencing as DV Narrabri. Other DVs who commenced duties at this time in the north-west have a similar recollection (Judy Ellem, personal comment, then DV Coonamble).
Leptospirosis was a common discussion topic amongst beef producers in the early 1990s. Thus it is likely that a proportion of infertility cases were not reported, rather just assumed to be leptospirosis. Given the extended or year round joining of most beef herds, it also likely that many cases were unnoticed, masked as delayed fertility.
After 1993 the number of cases reduced, so that by 1996, there were only one to two cases per year. Then between the end of 2001, and the start of 2012 there were only two cases. It is not clear if, or for how long the high level of cases was occurring before 1991. In 2012 there were two cases. Figure 1 shows the DMS data. It should be noted that the DMS figures do not include 1992, when there were a large number of cases, and that the figures for 1993 are for six months only.
Of the 27 cases nearly three quarters (19) were positive to L.hardjo serology. Fourteen cases had only L.hardjo serology, seven cases only L.pomona and five cases both (Figure 2).
There are differences in the time distribution of cases with positive serology to L.hardjo and / or L.pomona. Figures 3 and 4 show the distribution for L.hardjo and L.pomona, including where the other was also present. These suggest that the distribution of L.hardjo is more strongly correlated to the overall case distribution. This suggestion is reinforced by examination of cases where serology found only L.hardjo or L.pomona, not both (Figures 5 and 6).
There are a number of plausible explanations for this significant reduction in annual cases. These include variation in rainfall, variation in feral pig numbers, changes in the level of vaccination, increased herd immunity due to prior exposure and that the majority of cases were due to L.hardjo and that vaccination and dry conditions have largely eradicated sources of L.hardjo.
Variation in rainfall. Figure 7 shows the annual rainfall for Narrabri from 1988 to 2012. The average annual rainfall for Narrabri is 660 mm. A comparison of this with Figure 1 (cases per year) does not suggest any correlation. However, it is noteworthy that the initial reduction in annual incidence coincided with the severe drought of 1994.
Variation in feral pig numbers. Feral pigs have often been implicated in the spread of L.pomona. Therefore variation in feral pig numbers could explain the variation in annual incidence. Both formal studies (Mason et al. 1998) and opportunistic sampling (Libby Guest, DV Narrabri/Walgett and Jillian Kelly, DV Coonamble/Nyngan personal comment) have found a high seroprevalence for L.pomona in feral pigs in NSW. From the authors recollections, working in an office with pest animal staff, feral pig numbers have peaked three times during the study period. These were in the early 1990s, the late 1990s and since 2010. While the early 1990s high feral pig numbers conincided with a period of high annual incidence of leptospirosis cases, the equal high feral pig numbers of the late 1990s and early 2010s did not produce a large number of cases. When the portion of cases that were definitely due to L.hardjo alone is considered, the link becomes more tenuous.
Changes in the level of vaccination. Based on the anecdotal experiences of the author, the level of vaccination rose quickly in the early 1990s, with growing awareness of local cases and promotion by vaccine companies and resellers. By the mid 1990s, most better managed herds vaccinated. From then vaccination reduced, becoming by the 2010s, limited to a minority of herds. If level of vaccination was the cause for the variation, it would be expected that the annual number of cases would have followed this trend.
Increased herd immunity due to prior exposure. The introduction of either L.pomona or L.hardjo in the late 1980s and then the development of herd immunity could explain the changes in annual incidence. However, there is no evidence that either strain of leptospirosis has not been present for decades. Surveys from the 1960s and 1980s (King 1991 and Keast et al. 1964 ) found leptospirosis present in NSW herds, including Narrabri and nearby areas.
Vaccination and dry conditions have largely eradicated sources of L.hardjo. One determinant that occurred for the first time in the early 1990s was the advent of widespread vaccination. It is possible that in Narrabri herds, positive L.pomona serology is only associated with infertility in a minority of cases. If accepted, it follows that L.hardjo caused the vast majority of cases of leptospirosis infertility in the Narrabri area. This raise the possibility that L.hardjo was the cause of the high number of cases in the early 1990s, and that some new determinant then removed L.hardjo, but not L.pomona. One possibility is that increased vaccination levels in the 1990s, assisted by the drought of 1994 largely eliminated L.hardjo, and as a result, case of infertility due to leptospirosis fell. L.pomona was not removed due to feral pigs acting as a reservoir host.
Rather than a peak, high annual incidence in the early 1990s could be explained as being the normal without vaccination. That it was only coming to prominence around 1990, could be explained by the disease largely being masked until the early 1980s by brucellosis, which was endemic in the Narrabri district.
Two key problems with this theory are that it seems unlikely that vaccination would have been widespread enough to remove L.hardjo to levels that would stop its re-emergence, and that with cattle trading being common, L.hardjo should have been reintroduced.
The current leptospirosis survey across the wider north west (led by Jillian Kelly of Central North LHPA) should provide strong evidence to either support or exclude this theory. Should L.hardjo be rare in the regions herd then this would support that the critical change has been the removal of L.hardjo.