Neonatal immunology—practical vaccinology (Proceedings)
There are a plethora of commercially available vaccines available for use in cattle, the majority of which target viral and bacterial pathogens of the respiratory and enteric tracts. As veterinarians we often find ourselves positioned somewhere in the middle between vaccine manufacturers with data to share and product to sell, and producers anxious to realize both a health and economic advantage from the use of that product. The issue becomes even more challenging when we are faced with an ongoing outbreak of disease or the recent memory of an etiologic diagnosis for which a product or products may be available and a client anxious to do something different prophylactically to prevent future recurrence. Rarely (although it still can occur) these days does a vaccine result in specific harm or worse still death, but a more relevant issue is whether or not it actually confers a protective advantage. This presentation will focus on immunology of the calf from birth through the first year of life and the products available commercially. Wherever possible evidence based medicine from the literature will be cited to support/refute the applicability of using products against specific pathogens at certain ages. Inevitably however, there are considerable holes in the literature, and many vaccine programs on farm are the result of compromise and the collective experience of veterinarians and producers.
Decades old basic immunologic research underscores what all veterinary graduates should know –namely that the new born calf is immunologically best served and protected against infectious disease by passively acquired antibodies in colostrum and the provision of good biosecurity and general hygiene during the first few weeks of life. In the broadest terms, vaccination of calves should ideally take place after the waning of maternally derived antibody, typically at or beyond weaning age in dairy calves. However, the strict implementation of this dogma has become more and more challenging as time goes by. Challenges to the success of programs that rely entirely on this approach are wrought by several common de facto features of modern dairying and heifer replacement rearing. In no specific order, comingling wet or recently born calves from multiple sources on contract heifer rearing farms, feeding alternatives to colostrum as part of herd health/biosecurity for infectious diseases such as Johne's Disease or BLV, and endemic infection and environmental contamination with organisms such as Salmonella, are all commonly encountered factors, amongst others, that can challenge or overwhelm the long favored traditional and simplistic approach. Inevitable, ubiquitous, and high dose exposure to numerous pathogens, literally from the moment of birth onwards, allied with the still significant proportion of calves for whom adequacy of passive transfer is not achieved combine to produce higher than targeted morbidity and mortality rates. The perception that enhanced frequency, increased antigen number and precocious vaccine timing will overcome or reverse this trend is understandable but flawed both in science and in reality. Overall it is important to remember that there is an inverse, but linear relationship between the calf's age and immune responsiveness (the older the calf the better the response to vaccination), and maternally derived antibodies and protection against disease (the older the calf the poorer the protection afforded by passively derived antibody); and that at some point in the first two to four months of life there is cross over between the two. The precise timing of this is quite individually variable and we do not have a good, practical and economic way to estimate it in even general terms let alone for specific pathogens.