Animal health is more than just the absence of disease. The presence or absence of disease is usually not the best measure of individual or herd health. Every producer should have well defined production goals. The best indicators of a truly healthy herd are whether these production goals are being met. If the herd is a cow-calf operation, the weaning percentage might be a good indicator.
In a feeding operation, rate of gain might be the best gauge of health. Subclinical disease like parasitism or trace mineral deficiency can often reduce production indices in a herd and yet there is no real “disease” in the obvious sense. If a calf producer’s weaning rate is only 60% then there’s a problem whether there is mortality in the cow herd or not.
In a herd with optimal health there is optimal production; regardless of how it’s measured, in pounds of meat or number of calves. Productivity is the best overall measure of health. Therefore, optimizing herd health leads to maximum productivity and profitability.
It is important to remember that disease occurrence is more complicated than just a bison meeting a disease-causing organism (a pathogen). Understanding the exposure to pathogens and the outcome of the exposure in your herd is important. Whether or not disease results from the exposure is determined by a complex interaction between the animal, the environment, and the pathogen.
Healthy bison are disease resistant, and clean healthy environments don’t support pathogenic organisms. The nature and strength of pathogen exposure is more difficult to control and so it’s easier to avoid the pathogens than to modify or neutralize them.
Health is an expression of many interrelated factors, each one contributing to the well-being of the animal. Similarly, disease is almost always the result of many contributing factors. Some factors are “necessary” or specific to a disease before it can occur and others are only “sufficient” or general causes for disease occurrence.
For example, a necessary factor for Malignant Catarrhal Fever (MCF) is the presence of the virus OvHV2, which causes MCF. If that specific virus isn’t present, then whatever the disease affecting the animal, it’s not MCF. On the other hand, “sufficient” causes are more complicated and usually have several components or factors.
In the example of MCF disease, the OvHV2 virus is necessary to cause MCF, but it usually isn’t sufficient or enough to cause disease in a herd all by itself without other factors like suitable environmental conditions for transmission from sheep and a compromised immune system in the host bison; perhaps from stress.
Environmental factors that can become sufficient cause for disease include climate extremes (too hot or too cold), wet and/or dirty conditions such as those found in poorly drained pastures with heavy fecal contamination, and overcrowding the herd. Environmental factors exert their effects on the bison by causing stress, which decreases resistance to disease.
Stress can have a cumulative effect and may involve nutrition, physiology, environment, social order, and management such as handling events. Bison health is dependent on all of these factors and minimizing stress on your animals will be a major factor in the well-being of your bison.
In summary, productivity is a better indicator of bison herd health than disease occurrence and mortality. When disease appears in herd it is the result of a complex interaction between animal (host), environmental and pathogen factors.
To some extent environmental influences can be minimized but the main disease prevention strategy should be to create disease resistance in bison through minimizing stress, the presence of sub clinical diseases like parasitism, and optimizing nutrition.
BISON ARE NOT CATTLE
Bison are a unique species when fenced and raised for production or as a recreational herd. In the past they have been treated and managed by cattlemen and have been considered by many who did not understand their biology and nature to be a “bad ruminant” because of temperament and manageability issues.
We have learned much about bison management over the years. The first lesson we have learned is that bison are not cattle. This fact is easy to forget because they are ruminants like cattle and form herds like cattle.
But sheep and deer are also ruminant animals and nobody is tempted to compare the two. The problem lies in the fact that bison look roughly look like cattle and have similar looking calves. There are significant physiological and behavioral differences between cattle and bison that are important to bison health and productivity.
Differences include the age at which first breeding occurs in bison (2.5 years), nutritional requirements over winter, nutrition for slaughter animals, social structure and longevity. For example, bison are seasonal breeders and eaters whose metabolism slows in the winter to accommodate the relative scarcity of food for growth and lactation.
Bison have relatively good resistance to many pathogens that affect cattle, but are naïve to more recently introduced microbes. Bison are susceptible to some cattle diseases because they did not co-evolve with these pathogens and have no innate or genetic resistance to them.
Secondly, we still have much to learn about what makes a bison a bison. Bison are still behaviorally wild animals.
Keeping them behind fences for a few generations without deliberately selecting quiet, tame temperament for breeding has not served to make them domestic animals. They are genetically hard-wired to defend themselves when trapped or threatened and have well developed survival instincts that create dangerous situations for nearby humans. Bison are not yet domesticated and many bison owners are reluctant to do so.