The BVD virus is usually contracted via direct transmission. While indirect transmission of the BVD virus via needles, clothes or vectors needs to be taken into consideration, it is less epidemiologically important. In most cases the virus infests the cattle through oral-nasal, conjunctival and genital mucus. Direct transmission of BVD is mainly airborne. In addition, two kinds of transmissions need to be considered.
First of all, acutely infected animals also known as transiently infected animals which tend to shed the virus over a very short period of their lives, usually four to ten days. Secondly and even more importantly, persistently infected and high-risk animals, the PIs, can almost continuously excrete very large quantities of the virus for the rest of their lives. This means up to 10.000 virally infectious particles per millilitre of blood. As soon as PI animals are introduced into a naïve herd, BVD transmission can be observed as early as three to six months after their introduction. The progress of BVD transmission depends on several factors such as the life-cycle type, the geographical location of the infected animal groups in respect to the naïve herd and the contact possibilities between them.
All in all, the major risk of BVD transmission arises from the presence and/or repeated contact with PI animals. Therefore, it is paramount to quickly detect them, control them and to eliminate these animals as soon as possible.