There are more than 80 vaccines available for cattle in Canada, most are indicated as an “aid in the prevention of disease.” This label category means the product has been proven to effectively prevent the disease by a clinically significant amount, but not 100 per cent.
Add to that the variability within the environment and among individual animals and it’s understandable why vaccination does not always necessarily equal immunization, says Dr. Claire Windeyer with the department of production animal health at the University of Calgary faculty of veterinary medicine (UCVM).
Immunity is complicated.
There is innate immunity like the mucus in the nose that is always active. Then there’s active immunity that is acquired over time by vaccination or natural exposure to pathogens, and passive immunity that involves the transfer of antibodies in the colostrum or blood from an immune animal.
The immune response can be either humoral, with protection coming from antibodies in the blood or mucus, or cell mediated, with protection managed by immune cells.
The basis for vaccinating as a matter of routine is to try to tip the teeter-totter in favour of healthy animals and improved performance. However, there isn’t a blanket vaccination program for all herds because the risk of disease varies from farm to farm.
“It’s important to speak with your veterinarian to customize a vaccination program that works for your herd,” says Windeyer. “Remember, too, vaccination is not a bandage for poor management.” Nutrition, management, monitoring, early intervention, and good records to track disease are other important facets in a disease prevention strategy.
Windeyer recently searched the scientific journals looking for evidence that supports vaccinating calves against bovine respiratory disease (BRD) upon arrival at the feedlot and pre-weaning at the ranch.
To be of value, a vaccine should have the three Es — efficacy, effectiveness and efficiency.
Efficacy means a vaccine elicits an immune response, and it must be proven in controlled trials and challenge studies involving vaccinated and unvaccinated trials to gain a license for the product. They show the vaccine works but not necessarily under all conditions on all farms.
Effectiveness looks at whether a vaccine works in the real world. Field studies assess the degree to which a vaccine reduces disease in a population.
Efficiency evaluates whether using a vaccine makes economic sense. It’s often difficult to account for all factors in a cost-benefit analysis, so there are few published studies on vaccine efficiency.
Taken one by one, the studies Windeyer found on vaccinating calves at arrival in the feedlot present a confusing picture, offering evidence both for and against vaccinating with single and combination vaccines.
A 1997 literature review of field efficacy studies found positive trials and a few negative ones, but most fell into the neutral zone. Windeyer says the authors, Perino and Hunsaker, touched on the potential for publication bias. Neutral or negative findings were almost three times more likely when none of the study authors were affiliated with the test vaccines.
Doctors R.L. Larson from Kansas State University and D.L. Step from Oklahoma State University recently undertook a meta-analysis of all of the field trials on this subject published in English scientific literature. They looked at the quality of the research and analysed the overall weighted results. Their paper, “Evidence-based Effectiveness of Vaccination against Mannheimia haemolytica (MH), Pasteurella multocida (PM) and Histophilus somni (HS) in Feedlot Cattle for Mitigating the Incidence and Effect of Bovine Respiratory Disease Complex,” was published in March 2012, in the journal, Veterinary Clinics of North America: Food Animal Practice.
MH and MH/PM vaccines provided a slight, but significant reduction in BRD in 18 studies, but nine studies showed no effect on mortality. One study looking at MH/HS vaccines showed a reduction in the risk of having to treat animals for BRD and there were no mortalities among either vaccinated or unvaccinated calves. Only three field studies of HS vaccines were available and they showed no difference in the incidence of BRD.
The evidence for vaccinating calves on the ranch before weaning is also mixed.
Windeyer says this practice isn’t as widely accepted by producers as vaccination on entry to a feedlot largely because of the economics and the old dogma that passive maternal antibodies interfere with the ability of a vaccine to generate antibodies in young calves.
In the early 1990s Alberta veterinarian Joyce Van Donkersgoed found the incidence of BRD was reduced in some groups of calves vaccinated before weaning, but not in calves vaccinated at weaning. In another of her studies, calves that received various combination vaccines against BVD, IBR, PI3 and/or BRSV had higher antibody levels than unvaccinated calves, however, calves with maternal antibodies were less likely to have an increase in antibodies. The responses varied by vaccination protocol. In a third study an HS/MH vaccine given at one and two months of age in the face of maternal antibodies showed an increase in calf antibody levels until six months of age.
In 2001, Dr. John Ellis at the Western College of Veterinary Medicine in Saskatoon found minimal disease in calves with no maternal antibodies vaccinated at 10 to 14 days of age when challenged with BVD at 4.5 months of age. Calves with maternal antibodies when vaccinated became sick when challenged. Antibody levels before the challenge weren’t necessarily predictive of whether the calves did or didn’t get sick.
Windeyer says the Ellis study suggested high levels of maternal antibodies can interfere with the humoral response to vaccination and prevent clinical protection, successful immunity can be achieved through vaccination at an early age if maternal antibodies are not present and high levels of specific antibodies are not essential for clinical protection as it seems that other immune mechanisms may be involved.
Several studies have shown that calves with maternal antibodies vaccinated at less than three months of age may have cell-mediated responses and improved antibody response to a booster vaccination even if they don’t show an antibody response to the initial vaccination.
One U.S. study indicates calves vaccinated against BVD at two to five weeks had fewer clinical signs and higher antibody levels when challenged with BVD at seven to nine months of age.
In other work intranasally vaccinating calves without maternal antibodies at two, six or nine weeks of age with a BRSV or a BRSV-combination vaccine reduced the risk or the severity of disease when they were challenged with BRSV. When calves with maternal antibodies were given a BVD/IBR/PI3/BRSV modified-live vaccine intranasally within the first week of life they showed an increase in antibodies in nasal secretions, but not in the blood.
“Clearly, there is evidence that young calves are capable of mounting responses to immune challenges, whether that be through vaccination or exposure to pathogens in their environments, depending on several variables including age and amount of maternal antibodies,” says Windeyer. “It’s just that a young calf’s response to a vaccine in the face of maternal antibodies may be different from and more difficult to measure than the response of an older animal.”
Many questions remain: At what level do maternal antibodies interfere with vaccination? When is the best time to vaccinate? What is the best route for vaccinating? What are the most important pathogens to target? What happens when calves are given a 5-way modified-live virus vaccine, plus a 3-way bacterial pneumonia vaccine and an 8-way clostridial vaccine at the same time?
“While answers to these questions are being debated by veterinarians and researchers, it doesn’t mean producers should throw out their current vaccine strategies,” says Windeyer. “The risk and cost of a disease outbreak, along with sufficient evidence of efficacy are good incentives to maintain a well-vaccinated herd.”
— Debbie Furber is a field editor for Canadian Cattlemen at Tisdale, Sask.