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Leptospira shedding in dairy herds: new findings

Sunday, 4 June 2017  
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Research by Cord Heuer, Yuni Yupiana, Julie Collins-Emerson, Jackie Benschop, Jenny Weston, David Wilkinson, Emilie Vallee and Peter Wilson, all of IVABS, Massey University, and Roger Marchant of the Farmers Leptospirosis Action Group for Dairy (FLAG-Dairy) and the NZVA, has produced some important findings for the fight to prevent leptospirosis in humans.

While current vaccines are effective in preventing the shedding of Leptospira in the urine of cows for the serovars that are in the vaccines, the non- vaccine serovar Tarassovi has emerged as a risk. Serological evidence of infection with Tarassovi was found in 148 (74%) of the 200 surveyed dairy herds. In addition, 26.5% of the herds had at least one cow shedding leptospires in their urine (PCR positive), and 2.4% of the 4,000 dairy cows were PCR positive.

In 2011 the results of a small-scale pilot study of leptospirosis in dairy herds raised considerable discussion among New Zealand dairy veterinarians. What started as a student research project initiated by Professor Peter Wilson became known as the ‘Parramore study’, implemented by Jennifer Parramore (2011) and Ruth Meenks (2010), two visiting Dutch veterinary students (Parramore et al., 2011).

The urine of 10 cows in each of 44 conveniently sampled dairy herds was tested for Leptospira by PCR and dark field microscopy. A total of 13/44 (30%) herds and 17/445 (3.8%) cows had evidence of shedding. A herd appears to be less likely to have shedders when calves have received their first vaccinations at up to three months old.

Limitations of this pilot study were that no blood was collected for serology, and the herds were not representative of the New Zealand dairy population. The findings, however, led to best-practice recommendations for earlier vaccination of calves (so that they would be protected before they were likely to be exposed and become infected), and a greater urgency for more information that was representative of the New Zealand dairy herd. In particular, it was important to determine whether shedding was due to Leptospira serovars not included in the commercially available vaccines.

The preliminary findings of a nationwide survey of Leptospira vaccine use and its efficacy in New Zealand dairy herds have recently been released, the first study to explore the effects of long-term vaccination in a nationally representative sample of herds. For the research, 200 dairy herds were randomly but proportionally selected from all regions in New Zealand, and samples of urine and blood were collected from 20 milking cows in each herd from January to March 2016.

No evidence of Leptospira in urine was found in 97.6% (3,908/4,000) of cows, thus they were regarded as non- shedders. These cows came from 73.5% (147/200) of herds. The 94 (2.4%) cowswith evidence of Leptospira in urine (‘shedders’) came from 53 (26.5%) herds, suggesting that one in four New Zealand dairy herds has at least one cow out of 20 (≥5%) shedding. Most of these herds had only one PCR-positive cow, but 10 herds had between two and six cows with urine that was positive to PCR. Higher MAT antibody titres to any of the vaccine serovars (Hardjo-bovis, Pomona, Copenhageni) did not increase the probability of a positive urine-PCR. On the contrary, there was a small but significant trend that higher Hardjo-bovis titres correlated with negative urine- PCR. These results indicate that animal vaccination programmes continue to be an effective measure to prevent shedding in dairy cattle and thus reduce human exposure to the Leptospira serovars in the vaccines. Hardjo-bovis and Pomona are the most common serovars in New Zealand livestock and in human notified cases.

Antibodies to non-vaccine serovars Ballum and Tarassovi were detected in 3.4% and 17.1% of cows and herds respectively (MAT cutoff 1:48). Whereas Ballum titres were not associated with shedding, the probability of shedding increased substantially with the serum antibody titre for Tarassovi. Figure 1 demonstrates the increased risk of shedding at Tarassovi titres equal to or higher than 1:96.

This strong and highly significant finding raises concerns for herds where cows with high titres against Tarassovi are present. There were 11% of cows with such elevated titres, and 133/200 (67%) herds had ≥1/20 PCR-positive cows and/ or one or more cows with a high Tarassovi titre. Regression analysis indicated that most of the shedding results based on urine-PCR were attributable to Tarassovi. Therefore, serology offers an opportunity to determine the shedding status of a herd and identify individual high shedders. None of the participating farmers had observed signs of clinical leptospirosis or noticed unusually high frequencies of abortion in their stock.

These findings around Tarassovi are novel, as this serovar has not been seen in high frequency in dairy cows previously, although the last significant investigations occurred in the 1970s/’80s. However, in a recent and yet unpublished Massey study of dry stock, Tarassovi was found in 18% of beef cattle, 14% of sheep and four percent of deer (Mannewald et al., 2015, unpublished). This serovar was formerly associated with pigs as the preferred host. It may be hypothesised that Tarassovi has emerged in dairy cattle as competing serovars Hardjo-bovis and Pomona were removed by vaccination. On the other hand, the similar prevalence in beef cattle and its prevalence in sheep and deer suggest that the serovar has developed an ecological niche, as few ruminant dry stock herds are vaccinated.

The key question, therefore, is how dairy cattle become infected and whether cattle have become a reservoir host for Tarassovi as a consequence of genetic mutation and adaption.

The association found in the pilot study of shedding being less likely in herds with calves vaccinated by three months of age was not significant in this survey, obviously because most shedding was associated with the non-vaccine serovar Tarassovi. Of the 90% of dairy farmers responding to this question, 40% had vaccinated calves for the first time prior to three months of age (Figure 2). This was similar to the 43% of farmers in the 2010-11 pilot study. Recent scientific evidence suggests that maternal antibodies do not interfere with vaccination (Klaasen et al., 2012). Therefore, early vaccination of calves is current best practice in New Zealand. The survey found that most New Zealand dairy farmers (76%) start vaccinating calves between October and February. Only 42% followed best- practice guidelines of vaccinating early and completing the first course of two vaccinations before the end of December.

Veterinarians involved with the 21 herds with a seemingly high level of shedding were contacted to discuss the survey findings, possible consequences and optional preventive measures. To our knowledge, all contacted colleagues shared with farmers and managers the individual herd results and a series of remedial actions for preventing human exposure.

Further analyses are underway to describe the practices of vaccinating cattle and their potential association with shedding and serology. The results will be used to review and possibly update best-practice guidelines for vaccinating cattle against leptospirosis.

Meanwhile, given that we now know that there is a non-vaccination serovar on many dairy farms, the following precautions are recommended for dairy farmers in addition to continuation with current vaccination:

  1. Control rodents and wildlife: seek advice on baiting and trapping, vermin-proof buildings and feed stores.
  2. Minimise contact with animal urine. ‘Keep it off. If splashed then dry and wash.’
    a. Personal hygiene: hand washing, no smoking/eating/drinking in the shed.
    b. Personal protective equipment: wear aprons, gloves and boots and keep wounds covered. Make sure that this equipment is effective at keeping you safe.
    c. Exposure outside the milking shed: effluent spraying, assisting calving, home kill, hunting.
  3. Be aware and share your awareness: remember, others can be exposed when not working eg kids playing in puddles. If anyone in contact with dairy cattle has a ‘flu-like’ illness, seek medical help early, raise the suspicion of leptospirosis with your GP, and insist on getting tested.

Note: The 4,000 sera from the survey were tested for L. interrogans sv. Pomona and Copenhageni and for L. borgpetersenii sv. Hardjo-bovis, Ballum and Tarassovi.


References

Klaasen HL, Van-Der-Veen M, Molkenboer MJ, Sutton D. A novel tetravalent Leptospira bacterin protects against infection and shedding following challenge in dogs. Veterinary Record doi: 10.1136/ vr.101100:1-6, 2012

Mannewald A, Heuer C, Sanhueza J, Collins- Emerson J, Emanuelson U, Wilson PR. The role of atypical-host Leptospira serovars in beef cattle, sheep and deer in New Zealand. Degree research thesis, Swedish University of Agricultural Sciences, Faculty of Veterinary Medicine and Animal Science, Department of Clinical Sciences, 2015

Parramore J, Wilson PR, Hooijer GA. Leptospirosis in dairy cattle: The effectiveness of long-term vaccination of dairy cattle on New Zealand farms. Student research project, Massey University, December, 2011