In cattle, spirochaetes of the genus Leptospira cause several syndromes: haemolysis, chronic interstitial nephritis, mastitis, abortions and stillbirths. They are also an important zoonosis (Parkinson et al. 2019). In this case an unusually high number of Angus heifers were found to be not pregnant, associated with high titres to Leptospirosis interrogans serovar Pomona (L. Pomona) and a heavy infestation of feral pigs.
Leptospirosis was provisionally diagnosed as the cause of reduced fertility in a mob of Angus heifers in which 47/191 were found to be not pregnant at routine pregnancy testing. None of these cattle had been vaccinated against leptospirosis. Feral pigs are an increasing problem on this property in the Oberon district of the NSW central tablelands.
Four of eight nonpregnant heifers had L. Pomona MAT titres of 3200 or greater. Two heifers had evidence of previous exposure to pestivirus while the remainder were naïve. None of the eight heifers tested had serological evidence of exposure to Neospora caninum.
Another ten non-pregnant heifers and cows from across the herd were subsequently tested. Four had L. Pomona levels of 800-3200. Leptospira borgpetersonii serovar Hardjo type Hardjobovis (L. Hardjobovis) levels were either low (200 or lower) in six cows or negative in four. Four of 10 cows had evidence of previous exposure to pestivirus but again, not recently. One of 10 was positive for Campylobacter on a vaginal mucous agglutination test.
While isolation of the causal organism is normally necessary to diagnose leptospirosis, these high antibody levels, in unvaccinated cattle, are consistent with a recent L. Pomona infection with subsequent abortions. These cattle were about five months pregnant when pregnancy tested. L. Pomona is reputed to be able to cause abortion any time after the fourth month of gestation, most commonly in the sixth and seventh months. It is therefore possible that these cattle failed to conceive rather than aborted and that the high L. Pomona levels indicate exposure prior to the risk period for abortion.
These test results are not sufficient to prove that L. Pomona was a major factor in this case of reduced fertility. However, they show that L. Pomona is a risk to the property and that a vaccination program against leptospirosis is warranted. The finding of a low level of Campylobacter is of uncertain significance but given the modest cost and high efficacy of vaccinating bulls against Campylobacter, a Campylobacter vaccination program was also recommended. It was also recommended that a vaccination program against pestivirus be commenced given that 6/8 heifers and 6/10 cows were naïve.
Outbreaks of leptospirosis L. Pomona have occurred across NSW in the past but not in the Central Tablelands recently. Despite routine surveillance as part of infertility and abortion investigations, L. Pomona is rarely diagnosed as a cause of abortions in the Central Tablelands.
Feral pigs are an increasing problem on properties near timbered country on the Central Tablelands. There is a well-known association between feral pigs and L. Pomona with feral pigs acting as a reservoir for L. Pomona, which spills over from feral pigs to cattle (Riddout et al. 2014). A recent study of feral pigs in the Central Tablelands found three positive samples from 83 pigs, with all positive samples from pigs on a single property. This finding suggests that some local feral pig populations could be significant reservoirs of L. Pomona while others may not (Nigel Gillan pers comm.), which would lead to differences in the risk of infection of local cattle populations.
Ongoing surveillance for L. Pomona in both feral pig and cattle populations is important. While L. Pomona has been rarely diagnosed in cattle on the Central Tablelands in the past, feral pig numbers are increasing, vaccination against leptospirosis is not routine and, importantly, leptospirosis is a zoonosis.