A preliminary study of the production and financial impacts of clinical salmonellosis outbreaks in lactating cows in seasonal calving, pasture-based dairy herds in New Zealand, from which Salmonella enterica subsp. enterica were isolated.

AIMS: To quantify the production effects of outbreaks of clinical salmonellosis in seasonal calving, pasture-based dairy herds in New Zealand, from which non-Brandenburg Salmonella enterica subsp. enterica were isolated, and to use these results to undertake a financial analysis of their consequences.

METHODS

In this retrospective cohort study, we collated routinely available data for cow demographics, disease treatment, and accumulated whole of lactation milk solids (MS) yield from five spring-calving, pasture-based dairy herds that experienced naturally occurring clinical salmonellosis outbreaks in milking cows over the study period from 1 June 2021 until 31 May 2022. We used multivariable generalised linear models to estimate the effects of cow-level risk factors for clinical salmonellosis and the effects of disease on accumulated whole of lactation MS yield and risk of removal by culling or death. The financial consequences of salmonellosis were estimated with a partial budget model using the study results and publicly available information.

RESULTS

The median herd incidence risk of clinical salmonellosis over the study period was 2.6% (min 0.5, max 7.0%). The adjusted marginal mean incidence risk for disease for cows of age 2, 3-4, 5-6 and ≥ 7 years of age were 0.93% (95% CI = 0.39-2.18%), 0.98% (95% CI = 0.49-1.93%), 3.5% (95% CI = 2.15-5.63%) and 4.8% (95% CI = 3.06-7.40%), respectively. In three of the four herds with milk production records, there was no measurable effect of disease on MS yield and in a single herd, the adjusted marginal mean effect was a reduction in affected cows of 150 (95% CI = 98-202) kg (p < 0.001) accumulated MS yield per cow (36%). In a study of median-sized hypothetical herds, under six different scenarios differing in the level of impact of salmonellosis on MS yield and with differing incidence risks of disease, we estimated a total herd-level increase in expenditure and decrease in income for the season in which the outbreak occurred of NZ$1,873-NZ$13,444.

CONCLUSIONS

This preliminary study suggests that the financial consequences of clinical salmonellosis in New Zealand dairy herds are highly variable and are driven mainly by the incidence risk of disease and its effect on milk production.

CLINICAL RELEVANCE: Our results provide a preliminary guide to the range of biological and financial impacts of salmonellosis outbreaks in New Zealand dairy herds, which can aid in the planning of future research and inform farmer decision-making on control measures.