Farm Ireland
Independent.ie

Friday 9 December 2016

Step up your methods to help eradicate BVD from the herd

Liam Fitzgerald

Published 23/03/2010 | 05:00

Bovine Viral Diarrhoea (BVD) is a complicated viral disease and is widespread in the cattle population in Ireland.

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After infection, the virus targets a wide range of sites in the body, mainly rapidly dividing tissues such as lymphoid tissues, the mucosa of the intestines, sperm-producing cells in the testes and embryonic and foetal tissues. This results in a range of clinical effects and a complex set of diagnostic tests and control measures.

Animal Health Ireland (AHI) has, in turn, targeted the disease for control and eventual eradication, as has been achieved in Scandinavian countries. AHI recently published a BVD information leaflet, which sets out a step-by-step procedure for controlling the disease. This is available on the AHI website, www.animalhealthireland.ie.

The eradication of BVD would have a major economic benefit for the beef and dairy sectors. BVD seriously suppresses the immune system and, especially in animals of low-immunity status or in young calves where the immune system has not fully developed, the severity of other diseases, such as pneumonia and scours, is intensified by BVD infection.

The main manifestation of BVD is in breeding herds, where it causes a range of fertility losses, such as repeat breeding arising from embryo and foetal loss, the birth of mummified, deformed or dead calves and the production of persistently infected (PI) calves.

These PI animals, as the name implies, remain infected with BVD all their lives, continue to shed the virus and are a source of constant infection while in the herd. The calves appear normal up to weaning as they have the protection of the maternal antibodies. From eight to 18 months of age many develop signs of ill-thrift, a persistent scour and sores on the mouth and feet. Once a PI is discovered it should be slaughtered as apart from spreading infection, it will eventually die. Some, however, grow normally and may go undetected. These are the ones that need to be identified and slaughtered -- a PI cow will always produce a PI calf.

Pregnancy

If a cow gets infected in the early stages of pregnancy, up to about 70 days, it will usually lose the embryo and repeat. It is then likely to have developed sufficient immunity and will breed normally, but will have slipped back in calving date. If the infection occurs from about 70 to 120 days of pregnancy, the virus will get into the foetus and, at this stage, the foetus is not capable of developing its immunity as it doesn't recognise it as a foreign body, accepting it as part of its own tissue.

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When this animal is born it gets immunity from the cow in the colostrum and is thus protected for a time. It is antibody negative -- it doesn't produce antibodies and so blood testing for antibodies will not find it.

Finally, if the foetus becomes infected after four months of pregnancy, the birth of a normal calf, the abortion of the foetus or a range of abnormalities can occur. These could be deformities of the eyes, bones and nervous system, or the calves may be of low stature and have poor balance.

The AHI leaflet sets out a four-step approach to control: (1) planning; (2) investigating; (3) controlling; (4) monitoring.

Planning a control programme could be initiated when you have a suspicion that your herd is not performing to its potential or when, despite good husbandry, there is an above average level of disease, or you have seen some of the symptoms outlined here.

Investigating involves screen testing, followed by further intensive testing. For beef herds, the recommendation on the screen testing is to bleed a number (15) of non-vaccinated adult animals from the herd and test for antibodies, and do a pooled test for the virus. Similarly, do an antibody test and a pooled virus test on five to 10 young animals between eight to 24 months of age.

If the weanlings do not have antibodies it indicates that the herd has not been recently exposed and the BVD level is low. Following screen testing, herds are classified into 'never exposed' -- where no virus is found in the adult or young stock -- and various levels of risk going from low-level antibody in the herd to active infection, where the virus has been detected in adult and young stock.

For high-risk herds and those wishing to attain BVD-free status, the farmer should move to individual animal testing, which includes both antibody and virus examinations. The virus can be detected in blood, milk or ear-notch tissue samples. Testing pregnant cows doesn't detect BVD in the foetus so calves should be tested post-birth.

PI animals should be culled immediately. It has been found that vaccination alone will not keep BVD out but it will protect the herd from the devastating effects of BVD introduction into a clear herd. Two vaccinations are required in the first year so complete them a month before the breeding season starts.

Stringent bio-security is vital to maintain long-term control. This entails keeping a closed herd or the testing and quarantine of bought-in stock, maintaining stock boundaries, and strict hygiene and disinfection of visitors' clothing, footwear and machinery. Finally, diagnostic herd monitoring is recommended to ensure the control measures are working.

Irish Independent