So how do so few cause such a major problem? The key to understanding this comes down to two factors. The first is knowing the amount of virus particles a PI animal will shed. PI’s shed over a billion virus particles every day they are alive, whereas acutely infected animal will only shed around 100,000 virus particles per day for up to 2 weeks. The second factor is to recognize how Prevalence, Incidence and Exposure Rate relate to each other and how the random placement of PI’s expose a large number of your cattle population.
Prevalence Rate – The number of BVD-PI cattle within a total population of cattle. Our data shows this number varies by weight but the average we have found over the past 10 years for 4, 5 and 6 weight cattle is 0.4%. This means that 0.4% of a total population of cattle are PI, which is 4 PI’s per 1,000 head of cattle. We got our data from over 5.7 million head tested through CST.
This number by itself does not sound very impressive. Why test the entire population to only locate 0.4%? It may sound like a waste of money until you realize how Prevalence Rate relates to both Incidence and Exposure Rate.
Incidence Rate - The frequency of groups that have a PI animal. In the feedyard setting, this is normally broken down by lots or loads. Our data shows an average Incidence Rate somewhere in the range of 26-32%. In other terms, 26-32% of the incoming loads will have a PI on them. We got our data from 28,500 lots of 4, 5, and 6 weight cattle. The Incidence Rate percentages are based off 4 PI’s per 1,000 and 100-115 head lot sizes.
Exposure Rate – The percent of cattle that get exposed in your operation. We conducted a study back in 2009 where we tracked all PI’s coming into a feeding operation and how many cattle they exposed. A PI will expose not only it’s entire pen, but all adjacent pens. What we found was that at a 0.4% Prevalence Rate, 62% of the feedyard was exposed to BVD! This is because at 4 PI’s per 1,000 head and 100-115 head per lot, every 3rd lot will have a PI.
The exposure rate also takes in to account how many adjacent pens you have before a break like an alley and the number of head per pen. The more pens that are adjacent, the higher your exposure rate. The more cattle you have in each pen, the higher your exposure rate.
By itself Prevalence Rate does not seem like a number you need to worry about, but once you understand how it relates to Incidence and Exposure rate then you can see how just a few PI’s can cripple your operations herd health.