BVD virus can cause congenital brain condition

The other day one of my veterinary students showed me a video on his smartphone of a calf from his father’s ranch.

The calf was walking with a very unusual gait in which its movements seemed somewhat exaggerated. All four limbs were affected and the calf seemed to sway while walking.

I couldn’t give any definitive diagnosis by just looking at the video, but one possibility that we considered was that this condition might be caused by a disorder of the cerebellum.

The cerebellum is a structure at the back of the brain, tucked under its two main hemispheres. The name is Latin for “little brain” and is also sometimes referred to as the hind brain.

The cerebellum plays an important role in control of motion and helps to regulate and control muscle movement and co-ordination.

Animals with cerebellar problems are often diagnosed at a very young age. They will show clinical signs such as an exaggerated gait in all four limbs. In fact, all voluntary movements may be exaggerated.

In addition, they may stand with their legs in a wide based stance and show other nervous symptoms such as a head tremor, head tilt and unusual eye movements.

The most common way that the cerebellum is affected is through a viral infection.

Bovine viral diarrhea virus is the usual suspect when we see these symptoms in young calves. It is a common virus in cattle throughout North America and can result in many different aspects of disease, depending on the timing of infection and the age of the animal, such as diarrhea, abortion and in this case congenital defects of the cerebellum.

If the fetus becomes infected with BVD virus between 40 and 120 days gestation, this infection will coincide with the development of the fetal calf’s immune system.

One of the first tasks of the immune system is to recognize the normal tissues and cells of the calf so that it will not develop antibodies and attack itself.

The calf’s immune system will not recognize BVD virus as “foreign” if it is present, and although the calf may be born completely normal, it will be tolerant to BVD virus or persistently infected.

These persistently infected calves are permanent carriers of the virus. They are often the source of BVD outbreaks and excrete copious quantities of the virus in their saliva and manure.

The calf will not develop immunity to BVD virus but will always be a carrier and shedder of the virus. In some cases, these calves are poor doers with stunted growth, but they can also appear completely normal.

Only .3 to .4 percent of calves in North America are persistently infected with BVD virus, but these animals can cause devastating results if they enter a herd and spread the virus among cattle that are not appropriately vaccinated.

In the scenario where the cerebellum is affected, these calves are also infected as a fetus but are exposed to the virus at 125 to 180 days gestation, just after the timing where the virus would cause a persistent infection.

When the virus infects a pregnant unvaccinated cow at this stage of gestation, the BVD virus causes a failure in development of the cerebellum, and these calves can be born with varying levels of cerebellar dysfunction. In some cases, we see these calves also affected with cataracts on their eyes.

They are not usually persistently infected with BVD virus and as a result, the normal skin test that we use to identify infected calves will probably give a negative result.

There are certainly other things that can cause cerebellar dysfunction in calves besides BVD virus.

Schmallenberg virus was first recognized in northern Europe in 2011 and caused outbreaks of disease in adult cattle and abortions and stillbirths in sheep, cattle and goats.

Many of the young animals affected had brain abnormalities with cerebellar damage. This virus is not yet present in North America and is carried by biting insects such as midges and mosquitoes.

There are also rare hereditary conditions in some breeds of cattle that can result in damage to the cerebellum.

An appropriate BVD vaccination program can easily prevent the persistent infection of BVD virus and the cerebellum condition caused by BVD virus. In both cases, we are vaccinating the cow to ideally protect the fetus from the infection.

The most effective means of control is to vaccinate the cow herd before the breeding period with a modified live viral vaccine, but other approaches can also be used, depending on the management of the herd.

Most vaccines have been proven to provide reasonably good levels of fetal protection if used appropriately. Consult with your veterinarian to design a strategy that fits your herd’s management system.

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