Tri-State Neighbor: South Dakota Farm Newspaper

Spring is here. Calving is progressing nicely and most areas have had sufficient moisture to begin the grazing season. This year it appears that the cattle industry has declared war on BVD.

Bovine Virus Diarrhea (BVD) is a relatively common disease in cattle that has been around for many years. Originally, BVD usually manifested in a feedlot situation and was associated with the “feedlot wreck.” Recent improvements in vaccines and the subsequent development of newer, better, more efficient testing procedures have made eradication seem possible. Scientists have improved everyone’s understanding of the disease syndrome and now we can determine the economic losses for all facets of the bovine economy.

BVD can affect a herd in two ways. Horizontal transmission is the way we once thought of the spread of BVD from animal to animal. It means an animal becomes sick and begins to shed the BVD virus. All other animals that come in contact with the sick animal are exposed to the disease and may become infected. Attempts to stop this form of spread were by timely booster vaccinations where it was believed animals may become exposed and their immunity may be waning. Sometimes this procedure worked well, but many times there were problems with control.

Breeding problems have also been diagnosed in naive and even vaccinated animals. Even with good biosecurity and no new sources of virus entry into the herd, problems continued.

Vertical transmission of BVD has also been noted. This means a cow that is infected during her pregnancy may pass the disease to her calf. The outcome of the infection may result in the death of the fetus, which results in the infertility we see in BVD infections. Sometimes the calf becomes infected in the uterus and continues to mature. The virus multiplies in the fetus inside the uterus and the calf’s developing immune system recognizes the virus as a normal inhabitant of the calf, therefore, the calf forms no immunity to the virus.

These calves are called persistently infected (PI). They shed BVD virus in large amounts in virtually all body secretions including nasal discharge, saliva, semen, urine, tears, milk and feces. Some PI calves are weak at birth and may die shortly after, while others may appear healthy and normal. These animals will mature normally and enter feedlots and even breeding herds constantly shedding virus to any commingled and adjacent animals.

In the feedyard, these PI animals expose other animals daily. As other animals’ immunity resulting from vaccination decreases over time, the contact with the virus may result in infection. This explains BVD outbreaks in started, vaccinated cattle that seem to have no outside means of exposure. PIs are very detrimental to the feedlot industry, but only cause horizontal transmission.

In breeding herds, a PI bull or cow will constantly circulate the virus in the herd. If the herd has never been vaccinated, there may be a marked decrease in conception rates as well as some abortions and even some cow and heifer losses. Clients with vaccinated cows usually see no adult death loss or clinical signs. Typically, a greatly decreased conception rate is the most common sign. In the breeding herd, PI animals cause both horizontal and vertical transmission.

PI calves are uncommon - generally less than one-half of 1 percent of the United States cattle population; but their presence in a herd greatly affects the economic potential of the herd or lot. In the next article, I’ll focus on the removal of PI cattle from the herd and management procedures which will help eradicate BVD.

Comments »

Comment on this story

Comments will be approved within 48 hours