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Understanding the value of vaccines

Although vaccines have been used commercially for more than 70 years, the bulk of vaccines available on the market has not changed significantly. They are still mostly the products of either live or killed whole virus or bacterial culture. Research has yielded improvements to adjuvants (components that stimulate immunity), introduction of genetically engineered subunit and DNA vaccines, vaccine markers, and methods to deliver vaccines that increase effectiveness.

Vaccine research vastly improved our understanding of the body’s immune system and opened doors to new technology allowing modification of how the immune system responds to disease organisms.

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The rationale of preventing disease through appropriate and timed vaccination programs becomes a critical part of veterinary guidance to producers. Vaccination strategies need to be well thought out and continually reviewed by the producer and veterinary consultant. Three basic principles become a part of most vaccination strategies:

  • Need: Is risk of exposure to a given disease high enough, or the level of stress great enough to become a health issue?
  • Efficacy: Is the vaccine effective and is enough evidence available to support claims?
  • Safety: Is the vaccine safe and timing of administration appropriate?

The Beef Cattle Institute, Kansas State University surveyed the current recommendations by 23 consulting feedlot veterinarians who care for a collective 10 million head on feed in Canada and the U.S.

The seven factors commonly used by them to predict morbidity and mortality in feeder cattle ranked as follows:

  1. Risk of health-related events
  2. Amount and quality of labour
  3. Quality of nutrition when cattle were received
  4. Weather
  5. Class of antibiotic used for metaphylaxis (treatment of large groups on arrival)
  6. Class of antibiotic used for treatment
  7. Vaccine choices

The choice of vaccines, though considered important as a tool for managing health, ranked seventh in both 2009 and 2014. Vaccination protocols vary with veterinarians’ personal experience, variance in how herds and feedlots are managed, and type of cattle being handled i.e. high risk or low risk. Antibiotic use is more common in feedlot replacements considered “high risk” for contracting bovine respiratory disease (BRD), especially newly weaned calves and those that have been transported long distances.

Disease control in beef herds has traditionally centred on vaccination to prevent clinical disease. While reduction of the severity of clinical disease can be directly related to immunization, decreasing the transmission of pathogens between individuals in a herd is a primary objective. Herd immunity, rather than individual immunity, becomes the target in most vaccination programs. Herd immunity is achieved by:

  • Reducing the number of individuals shedding pathogens
  • Decreasing the amount of pathogen being shed by individuals
  • Decreasing the duration of pathogen shedding
  • Increasing the infective dose needed to initiate disease

Sound vaccination programs reduce the efficiency of transmission between individual animals and collectively across herds, while reducing the number of individuals shedding infectious organisms. The percentage of immune individuals in a herd to achieve herd protection varies, but generally ranges between 83 and 94 per cent.

An exception being the percentage of immune individuals limiting the spread of bovine virus diarrhea (BVDV) within and between herds. Without persistently infected (PI) individuals it’s calculated to be 57 per cent. With PIs the figure jumps to 97 per cent. The value of vaccine in herds with PIs is low (i.e. a high percentage of immune animals is needed to limit transmission) if not supplemented by testing animals at risk and culling all PIs.

Modern livestock systems have adopted the practice of giving vaccines containing multiple antigens, either as multivalent combinations or individual vaccines administered concurrently. Use of vaccines in this manner is often a result of only being able to handle calves once or twice between birth and weaning. Preventive health strategies can include administration of vaccines containing as many as eight or more antigens at the same time. The value of the “shotgun” approach to vaccination is often questioned; not knowing whether calves are capable of responding to all vaccine components at the same level, or if some antigens actually interfere with response to other components.

Shipping, handling, processing and most importantly, commingling are major stressors on commercial calves entering today’s marketplace. The value of vaccines and vaccination protocols are inversely proportional to the risk of exposure to infectious disease and stress, especially commingling. Slowing the rate of exposure through commingling provides susceptible animals time to respond to vaccination.

At present, calf health and weaning programs emphasize BRD vaccination protocols and weaning for at least 45 days. Although vaccination is still considered an important component of providing healthy calves to the marketplace, key factors are low-stress weaning and the interval between weaning and sale. Vaccination in this case probably plays a secondary role to good management practices.

Presently there are no broad-spectrum antiviral pharmaceuticals available. Good production practices and biosecurity plans that limit exposure combined with vaccination are the only means to prevent or control viral infections.

The value of vaccines will probably become a decisive factor in the global struggle against antimicrobial resistance. Learning to work effectively with the immune system through effective vaccination certainly provides a stepping stone to more prudent drug use and antimicrobial stewardship in both animals and man.

Sound vaccination programs also fit with a growing sentiment by consumers that the livestock industry must adopt clearly understood and justifiable means of rearing animals for food. Codes of practice promoting proper care and welfare of animals make effective vaccination against common production diseases a valid part of our social licence to operate.

About the author

Columnist

Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).

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