by Dr Charlie Blackwood

Bovine pestivirus and Bovine Viral Diarrhoea (BVDV) are the same disease – different people call it different things. We will call it BVDV in this article. BVDV is one of the most complicated cattle diseases we deal with. Don’t worry if you need to ask or read articles a few times to understand the disease. We often do as well.

BVDV can be a serious threat to dairy farm production and profitability. This article, some of which comes from the Western Australian Department of Agriculture, explains the effect of BVDV on different groups of animals.

Table 1: What immunity status definitions mean
Naïve	Never infected Cattle that have never been infected or challenged by BVDV. They have no antibodies to the virus.
Immune	Have been infected but are not carriers Cattle that have been infected with BVDV but are not carriers. They have antibodies and are immune to future infections. There are two ways this immunity can occur: infection as BVDV circulates through the herd – immunity is lifelong (‘natural immunity’) vaccination – annual boosters needed to maintain immunity.
Persistently Infected (PI)	Carriers Carriers of BVDV secrete the virus for their entire life. They exhibit variable signs and degree of disease and most die 1–2 years after birth. The calf of a persistently infected heifer or cow will also be persistently infected. Persistently infected bulls are also potent spreaders of BVDV.

What happens when BVDV occurs?

The effect of BVDV depends on whether it occurs in unborn calves, pregnant cows, feedlot or other cattle.

Unborn calves

Most of the harm done by BVDV is to unborn calves and depends upon timing of infection.

• Infection of a naïve cow (one not previously exposed to or vaccinated for BVDV, see Table 1) in early pregnancy causes loss of the embryo.
• Infection of naïve cows in mid-pregnancy can cause abortions, birth defects and live-born calves to be persistently infected with BVDV. These calves spread the disease within and between herds. Most of these calves die within two years of mucosal disease.
• By the last third of pregnancy, the calf has developed sufficiently to produce immunity. Some of these calves may be aborted, but most are born healthy.

Pregnant cows

Infection of a naïve cow results in a mild 2–3 week illness that suppresses the cow’s immune system and reduces disease resistance. Most important are the effects on her unborn calf noted above. After recovery these cattle are immune to BVDV.

Feedlot cattle

If a persistently infected animal is introduced to a feedlot, infection will spread to all naive cattle in that pen and adjoining pens within days. It will rarely spread further because usually only persistently infected animals shed an infective dose.

The exposed naïve cattle will develop mild illness and suppressed immunity. As this coincides with transport and social stress, diet change and challenge by other diseases, the health impact of BVDV in affected pens can be significant.

All other cattle

Other cattle exposed to BVDV will also experience a 2–3 week illness followed by lasting immunity. Exposing non-pregnant cattle to persistently infected animals can stimulate immunity and protect future pregnancies in those cattle.

Table 2 Effect of BVDV infection on cattle according to pregnancy status
Non-pregnant naive cattle	2–3 week infection short-term immune suppression lifelong immunity to BVDV following infection
Pregnant naive cattle	1–4 months gestation 2–3 week infection embryo death, abortions, stillbirths, mummified foetuses any live calves are persistently infected 4–5 months gestation 2–3 week infection abortions, viable or non-viable calves delivered at full term congenital deformities and defects some calves born normal and immune to BVDV 5–9 months gestation until birth 2–3 week infection abortions any calves born alive at full term immune to BVDV

Are Your Cattle Immune?

As you can see from the points above, the most significant problems caused by BVDV occur when cattle have no immunity and a carrier animal is introduced early in the joining time. A blood test of 6-10 animals prior to joining allows you to get an idea of the overall status of your cattle. Of particular interest are the unjoined heifers who have had less chance to develop immunity. Even if there is some BVDV in your herd, there is no guarantee that all animals will be immune

Signs of BVDV Infection

• Poor fertility, including low calving rates and a high rate of pregnancy tested cows not in calf.
• Reproductive abnormalities, such as abortions, stillbirths, mummified foetuses, congenital deformities and defects and calves that die soon after birth.
• Oubreaks of disease, such as fever, not wanting to eat, dullness, lethargy, pneumonia, scours or other disease. Production losses can be high.