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Little evidence vaccines reduce Danish antimicrobial use

Regardless of vaccination status, almost equal proportions of herds experienced a decrease and an increase in animal daily doses resulting in no overall change in animal daily doses. Furthermore, only minor effects were found, when vaccinations were used in combination.

Source: American Association of Swine Veterinarians

Livestock production and the use of antimicrobials within are under the microscope worldwide, with European producers having to work under stricter regulations years before the U.S. industry.

Due to its large production, the Danish pig sector accounts for 76% of the total amount of antimicrobial substances used for livestock production per year in the country. Official focus on reducing antimicrobial use has, therefore, been on pig production. The Danish government and the swine industry have put in place several initiatives to try to mitigate the potential risk related to the development of antimicrobial-resistant bacteria. One of these initiatives, “the Yellow Card Scheme,” which identifies and warns livestock farmers using above a given permitted limit of antimicrobials, was introduced in 2010 and is managed by the Danish Veterinary Authorities. The antimicrobial use decreased after the introduction of the Yellow Card Scheme. From 2010-14, there was a 14% reduction in the antimicrobial treatment proportion, measured as defined animal daily doses per 1,000 animals per day across the total Danish pig production. The pig industry’s goal is a further reduction by 10% before 2020. To achieve this, relevant and effective strategies that can minimize the need for treatment with antimicrobials in pig production are needed.

It is often said that vaccines may help reduce antimicrobial use in swine production. However, limited evidence is available outside clinical trials.

A group of researchers from the University of Copenhagen in Frederiksberg, Denmark, and from the Danish Agriculture & Food Council studied the change in amounts of antimicrobials prescribed for weaners and finishers in herds following initiation of vaccination against five common endemic infections: Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, porcine circovirus type II, porcine reproductive and respiratory syndrome virus and Lawsonia intracellularis.

Comparison was made to the change after a randomly selected date in herds not vaccinating against each of the infections. Danish sow herds initiating vaccination during 2007-13 were included (69-334 herds, depending on the analysis). Danish sow herds with no use of the vaccine in question were included as non-exposed herds (130-570 herds, depending on the analysis). Antimicrobial prescriptions for weaners in sow herds and for finishers in receiving herds were extracted from the VetStat database for a period of 12 months before and six to 18 months after the first purchase of vaccine, or random date and quantified as average animal daily doses per 100 animals per day.

The herd-level difference between animal daily doses in the period after and before vaccination was the outcome in linear regression models for weaner pigs, and linear mixed-effects models for finishing pigs, taking into account sow herds delivering pigs to two or more finisher herds. Three plausible risk factors (baseline animal daily doses, purchase of specific vaccine, purchase of other vaccines) and five confounders (herd size, export and herd health status, year and season) were initially considered in all 10 models. The main significant effect in all models was the baseline animal daily doses; the higher the baseline animal daily doses was for weaner and finishing pigs, the larger the decrease in animal daily doses was following vaccination (or random date for non-vaccinating herds).

Regardless of vaccination status, almost equal proportions of herds experienced a decrease and an increase in animal daily doses resulting in no overall change in animal daily doses. Furthermore, only minor effects were found, when vaccinations were used in combination.

In conclusion, this study provided little support for the hypothesis that vaccination against five common endemic diseases provides a plausible general strategy to reduce antimicrobial use in Danish pig herds. It was felt, however, that vaccination can be an asset in some situations. The full report on this research can be found online.

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