Seroprevalence and herd-level risk factors for seroprevalence of Leptospira spp. in sheep, beef cattle and deer in New Zealand

AIMS To determine seroprevalence of Leptospira borgpetersenii serovar Hardjo and L. interrogans serovar Pomona in beef cattle, sheep and deer in New Zealand and the association between farm-level risk factors and seroprevalence. METHODS Between June 2009 and July 2010, 20 serum samples per flock or herd were collected from 162 sheep flocks, and 116 beef cattle and 99 deer herds from 238 farms, along with farm data by interview. Samples were tested for antibodies to serovars Hardjo and Pomona by microscopic agglutination testing, with a titre ≥48 being positive. Species-specific associations between herd-level seroprevalence (number of seropositive animals, for each serovar, divided by the number of animals tested) and herd-level risk factors were determined by multivariable logistic regression analysis. Vaccinated animals were excluded from seroprevalence estimates but included in multivariable analyses. RESULTS For sheep (n=3,339), animal-level seroprevalence was 43.6 (95% CI=41.9-45.3)% for serovar Hardjo and 14.1 (95% CI=12.9-15.3)% for serovar Pomona; for beef cattle(n=1,886), it was 45.6 (95% CI=43.3-47.9)% for Hardjo and 19.6 (95% CI=17.9-21.5)% for Pomona; and for deer (n=1,870), it was 26.3 (95% CI=24.3-28.4)% for Hardjo, 8.8 (95% CI=7.6-10.2)% for Pomona. In sheep flocks (n=161), flock-level prevalence for Hardjo varied from 77.9-91.3%, and for Pomona from 40.4-73.9%, when ≥1, ≥2 or ≥3 animals were seropositive. In beef herds (n=95), herd-level prevalence for Hardjo varied from 79.0-90.5%, and for Pomona from 42.1-68.4%. In deer herds (n=93), herd-level prevalence for Hardjo varied from 45.2-59.1%, and for Pomona from 22.6-48.4%. For sheep flocks, herd-level seroprevalence for Hardjo was associated with flock size (OR=1.56) and number of dogs (OR=0.75), and for Pomona, seroprevalence varied with region. For beef cattle, herd-level seroprevalence for Hardjo was associated with herd size (OR=1.4), presence of dams (OR=0.6) and vaccination (OR=2.9), and for Pomona, co-grazing with deer (OR=0.4), vaccination (OR=3.22), presence of dams (OR=0.2) and streams (OR=2.7). For deer herds, seroprevalence for Hardjo or Pomona was associated with herd size (OR=1.6 and 1.8) and varied with region, and for Pomona seroprevalence varied with season (summer vs. winter: OR=4.8). CONCLUSIONS Serovars Hardjo and Pomona were highly prevalent at herd and animal levels, with serovar Hardjo highest in all species. Larger herd size was the common risk factor for seroprevalence in all livestock species.