Dairy Pipeline: March 2004
Extension Area Dairy Agent,
In a past issue of the Dairy Pipeline, we looked at BRSV-it's symptoms and treatments. This month we tackle a real threat to our industry, because many people think they are safely vaccinated against it. This month, we investigate BVDV. Ironically, one would think that the Bovine Viral Diarrhea Virus would manifest itself as diarrhea, but that is one of the rarest symptoms. BVD (Types I and II) attack the nervous system and some common outward signs include hairlessness (especially around the poll area,) weak calves, calves that are born blind, calves that have trouble with front legs that knuckle under, and abortions in cattle.
Type I vs. Type II-What's the Deal? BVD Type II is sometimes thought as the new kid on the block; however, that is not the case. BVD mutates very quickly and there are many different types "out in the woods." Type I and Type II vaccines have shown to have the most success preventing the disease in healthy calves (more on that later.) For a time, the Type I vaccine had success controlling BVD, but Type II has changed so that the "old" vaccine is not as effective. BVDV commonly infects a fetus when the mother is 60 to 180 days of gestation. The newborn calf is born with very low immunity and is usually described as "unthrifty" or a "poor doer."
But I Vaccine Against BVD! Cattle that are most seriously infected are known as PI (persistently infected) calves and cows. They rarely live very long lives, and they do not respond well to vaccination against BVD. Thus if you have PI calves born in the herd, your vaccination program does not have a firm foundation on which to build an "immunity shield."
How do I tell I have a problem? The simplest way to test for BVD in your dairy herd is with a bulk tank test. That will tell you there is a presence of the disease in the herd. If you do have a positive tank test, your next step would be to test the herd via blood samples or skin samples. Ear notching can work well in young calves. Sampling is not cheap ($15 per slide;) however, you can help lower the cost by placing as many as five notches per slide so that brings the price down to $3 per animal. Cornell University offers testing and you may obtain testing through your local veterinarian.
So I find it-Then what? PI animals need to be removed from the herd as soon as possible. Shedding animals can spread millions of pathogenic particles each day. The term "ticking time bomb" is used to describe PI animals. Enough said. BVD is spread throughout the herd as PI animals expose pregnant cows and infect the fetuses. A vaccination program is imperative to helping control BVD. But that brings up the question of Killed versus Modified Live Virus vaccines.
Killed vs. MLV Killed products are "safer" to use because they contain just the "peel" of the organism in question. They contain no pathogenic properties of the original bug. Modified Live vaccines have been altered so that while the bug is still intact, it is far less virulent than the original. Modified Live vaccines build stronger resistance, but will cause spontaneous abortions in pregnant cattle. It should be avoided if there is any doubt the animal is pregnant. MLV works well in a control program with young heifers. Interesting, Susan Silver, Fort Dodge Animal Health Representative suggests that a MLV vaccines work best as a follow-up to a killed virus product. So you might consider vaccinating weaned calves with a killed product and do follow-ups with a modified live product.