Bovine viral diarrhoea and mucosal disease

INTRODUCTION

According to recent surveys the virus causing Bovine viral diarrhoea (BVD) occurs throughout South Africa amongst cattle, but only now and then causes severe problems.  The virus is transmitted via nasal discharges, saliva, faeces, urine and blood or through contact with contaminated objects.  It penetrates the membranes of the nose and mouth whereafter it multiplies in the body.

CAUSE

Two types of virus have been found :

1. The cytopathogenic virus that causes abortion, resorption and malformation of the foetus in pregnant cows.
2. The non-cytopathogenic virus that causes so-called "persistently infected" or PI calves in cows infected during the first three months of pregnancy.

These PI calves act as carriers that transmit the virus to other animals in the herd.  If such an animal is exposed to the cytopathogenic virus at some stage, it develops "mucosal disease", a severe fatal form affecting the intestinal tract.   The BVD-virus together with other viruses and Mannheimia (Pasteurella) haemolytica could predispose animals to pneumonia in feedlots.

SYMPTOMS

The most common symptoms are:

1. Reproduction.

Symptoms vary from embrional death to abortion or the birth of weak calves.  Calves may be born with an underdeveloped cerebellum leading to poor coordination (Fig. 1 en 2).

2. Weak calf sindrome.

These calves are usually carriers.  They grow poorly and are very susceptible to pneumonia and diarrhoea.

3. Acute BVD.

In herds not previously exposed, a large number of animals will become infected.   Affected animals show fever, nasal discharge, lowered milk production and diarrhoea.   Usually few animals die.  See Fig. 3.

4. Respiratory disease.

Suppression of the animal's immune system makes it much more susceptible to pneumonia.

5. Mucosal disease.

This deadly form of the disease occurs in young carriers.   It is nearly always fatal and is characterised by severe bloody diarrhoea and ulceration of the intestinal tract.

Fig. 1: Young calf with underdeveloped cerebellum.   Note the wide-based stance.

Fig. 2: Severely underdeveloped cerebellum (small brain) in a calf infected before birth.   The cerebellum is normally 3 to 4 times bigger than in this case.

Fig. 3: A large ulcer on the mouth of an animal caused by the BVD virus.

CONTROL

There are two approaches:

1. BVD-free herd

All animals in the herd are to be tested and any persistently infected calves culled.   Only animals that test negative are to be introduced into the herd.   The virus can also be transmitted via semen, so care should be taken that cows are not exposed to foreign bulls.  Since most herds test positive in South Africa, this is only a theoretical solution.

2. Vaccination for control of BVD

If cows are vaccinated early in pregnancy with the inactivated vaccine, abortion and fetal abnormalities are minimised. Weaned calves are vaccinated with the live vaccine two weeks after weaning and again four weeks later.

TYPES OF VACCINE

The live vaccine

This vaccine could be used with safety in feedlot cattle.  It should not be used in pregnant animals or in suckling calves.  The live BVD-virus is usually combined with other viruses causing respiratory symptoms: Infectious bovine rhinotracheitis (IBR), parainfluenza 3 (PI3) and respiratory syncitial virus (BRSV).  The primary vaccination is given followed by a booster four weeks later.

The following commercial vaccines include live BVD-virus:

• Bovi-Shield 4 (Pfizer AH)  (IBR, BVD, PI3 en BRSV)
• Pyramid MLV (Fort Dodge(Intervet))  (IBR, BVD, PI3 en BRSV)

Inactivated vaccine

This vaccine may be given to any animal, pregnant ones included.  Since calves from vaccinated mothers will have passive immunity, they should only be vaccinated at six months of age.  Calves that have been vaccinated before six months, should receive another dose at six months old.

The following commercial vaccines include inactivated BVD-virus:

• Bovilis BVD-MD (Intervet)  (BVD only)
• Bovilis Qarat (Intervet)  (IBR, BVD, PI3 and BRSV)
• Cattlemaster 4 (Pfizer AH)  (IBR, BVD, PI3 and BRSV)

In practice replacement heifers should be vaccinated at eight months with the live vaccine followed by a booster four weeks later.  These animals should have a lifelong immunity.  Cows may be given the inactivated vaccine annually, or the live vaccine after calving and before breeding.  Two vaccinations with the live vaccine a year apart should also provide lifelong immunity.

CONCLUSION

BVD does occur in the Rustenburg area and it is wise to take precautions.  There are many other diseases causing similar symptoms, so it is advisable to seek advice from the vet in your area should you experience problems.

Acknowledgement for photo's:
Figg. 1 and 2: Bristol Biomedical Image Archive © 1999 University of Bristol
Fig. 3: Compendium on Continued Education for the Practicing Veterinarian Feb 1997