What's Going On Blog Silent killers: Bovine Viral Diarrhoea Bovine Viral Diarrhoea is a complicated disease caused by the Bovine Viral Diarrhoea virus (BVDV). The virus is widespread in cattle herds in the UK and very easily spread. Evolution has provided the BVD virus with an incredibly potent ’smart’ weapon in its armoury which enables it to take advantage of an Achilles Heel in the development of the immune system of calves during the first 120 days of development in the cow’s uterus. If a cow which has not previously been infected with BVD virus picks up infection during pregnancy, the virus will cross into the womb and infect the calf. If infection happens during the first 120 days of pregnancy the virus could kill the embryo. However, in many cases, particularly with infection after day 30 of pregnancy but before day 120, the BVD virus fools the immune system into thinking the virus is a normal part of the developing calf. As a result, that calf is infected for the rest of its life with BVD virus. Calves infected with the BVD virus are referred to as ‘BVD Persistently Infected’ (PI) animals and they become ‘virus factories’ for the rest of their lives. BVD virus is shed in every secretion and excretion from PI cattle. Most cattle pick up infection with BVD through direct contact with a PI animal, which can happen simply through contact over boundary fences, in markets or at shows. All equipment which comes in direct or indirect contact with these animals can become contaminated and help to spread the virus. People handling PI animals can also easily become contaminated with the virus and inadvertently spread infection to other cattle. Fortunately BVD virus does not affect people. Most PI cattle die within 18 to 24 months, usually from other secondary infections because of the damage BVD virus does to the immune system. Some also die when the BVD virus they are infected with changes into the fatal form of BVD – Mucosal Disease. There is no treatment for Mucosal Disease and there is also no way of treating PI cattle to clear the virus from their systems. Some PI cattle can live for many years and some can even get pregnant, giving birth to live calves. However, calves born to cows which are PI with BVD are always, themselves, PI. In this way the BVD virus helps to ensure its own survival. If cattle are infected in the womb at any time other than during the first 120 days after conception, they may still show signs of BVD but their immune systems will mount a response to the virus, clearing it from their system, and they will then be protected from reinfection for the rest of their lives. However, they may still show signs of clinical disease after infection with clinical signs sometimes very mild and which may not even be noticed. BVD infection does suppress the immune system and if cattle pick up other infections while they have BVD, e.g. scours or pneumonia, it will make these infections more difficult to treat and some animals may die. BVD can also cause cows to lose their pregnancies and in early pregnancy there may be no signs other than cows coming in heat again when they were thought to be pregnant. BVD can also cause abortions and stillbirths. There are good diagnostic tests and also good vaccines for BVD. You do need to follow the advice on the pack for the particular BVD vaccine you decide to use if it is to be effective. Heifers need to be vaccinated well before their first pregnancy and breeding cattle will need regular booster vaccinations to protect future pregnancies. You can protect your herd by checking to see if any of your cattle are Persistently Infected with BVD. Calves can be tested at birth by testing a small piece of skin for the presence of BVD virus. Most tag companies provide a ‘BVD tag and test’ cattle identification tag which can be used to test calves. Cattle can also be tested by looking for BVD virus or BVD antibodies in blood samples. Any PI animals should be sent to slaughter as soon as possible to prevent them from spreading the infection to other cattle. You can protect your cattle from infection with BVD by preventing any direct or indirect contact with cattle which are not part of your herd. BVD vaccination can be considered if you think there is still a risk the cattle could still contract infection with BVD. All parts of the UK and Ireland now have national programmes to eradicate BVD virus. The programmes aim to identify and remove all the cattle which are Persistently Infected with BVD. It is compulsory to tag and test all calves for BVD in Ireland and Northern Ireland before calves can be registered. There is a compulsory programme for all breeding cattle herds in Scotland and voluntary programmes in Wales (Gwaredu BVD) and England (BVDFree.org.uk). These programmes will only be successful if all cattle Persistently Infected with BVD are identified and eliminated. The risk which BVD poses to rare breeds is multifaceted – valuable pregnancies may be lost, calf mortality can be higher and all PI calves will threaten rather than contribute to the long-term survival of the breed. For more information, contact your vet or visit BVDFree.org.uk.