Bovine Viral Diarrhoea (BVD) is a viral disease which can have a number of consequenses. It is a widespread problem with over 90% of herds and 60% of cattle in NZ having exposure to the disease. Many of the clinical signs of BVD are common problems which may have many different causes and are therefore not easily identified as being caused by BVD.
The virus can be transmitted by contact from cow to cow through body fluids (eg – urine, faeces, saliva and semen), and from cow to foetus (via the placenta). Any newly purchased cow, calf, or bull may introduce the infection, but the disease can also cycle within a herd.
If a cow catches the virus when not pregnant, she will usually not show any symptoms of disease and will mount a strong immune response. This will leave her resistant to the disease. The problem arises when a pregnant cow becomes infected. Pregnancy is the worst time to get infected – and cows are pregnant for 75% of the year! A cow in the first trimester of pregnancy will mount a strong immune response and becomes immune, but is likely to abort or resorb the foetus. You may notice low conception rates, late returns to service or a long Calving to Conception Interval (CCI). An infection later in pregnancy may produce a dummy calf or a sickly calf possibly with neurological or ocular defects.
The other scenario for a pregnant cow is that the foetus gets infected before its immune system has developed enough to tell the difference between “self” and “other”. This foetus / calf thinks the virus is part of itself and will never mount an immune response – it is a Persistently Infected (PI) animal. Often these calves will be ill thrifty, scoury and die of Mucosal Disease (a severe form of BVD). However, some survive for years. A PI will shed virus all its life, infecting the rest of your herd. PI cows always produce PI calves. It is estimated that 1% of bulls are PI animals, and these may spread the virus in their semen!
BVD in calves. Like healthy non – pregnant cows, most normal calves that are exposed to the virus will be transiently infected, usually show little clinical signs, and then mount an immune response leading to long term immunity. However, there is some evidence to suggest that BVD can cause problems in calves suffering from other diseases – eg – Yersinia, excessive parasitism or feed stress. These diseases or states of health may combine to lead to severe ill thriftiness or death.
Antigen (Ag) – Antigen are particles that the body recognises as foreign and tries to get rid of. Some examples of antigens are bacteria, viruses, allergens (eg - dust or pollen) and parasites.
Antibodies (Ab) – Antibodies are small proteins made by the animal that are designed to attach to an antigen and destroy it. The presence of antibodies to a certain disease causing agent (eg BVD virus) indicates that the animal has been exposed to that disease, recognised it as a foreign particle and amounted an appropriate immune response to the disease.
Diagnosis of BVD
Bulk milk tests give an antibody level, which relates to the proportion of your herd which are immune (i.e. have been infected in the past and mounted an immune response). Low Ab levels in a bulk milk sample indicate that most cows in the herd have not been exposed to the virus. High Ab levels in a bulk milk sample indicate that the herd has recently been exposed to the virus. Moderate bulk milk Ab levels are more difficult to interpret and follow up tests may be required.
Individual milk tests will show the level of Ab in that cow – if very high it is likely that she has been infected recently. These milk tests do not confuse vaccination with infection, as the antibodies are different.
Blood samples (or tissue samples) are taken from the individual and can be tested for
· Antigen i.e. virus present in animal.
· Antibody (again, high levels suggest recent infection). The Ab blood test does not confuse vaccinated animals with infected animals as the antibodies are different.
PCR tests can be done on either bulk milk samples (up to 800 cows per sample), individual milk samples or blood samples (pooled or individual). PCR tests look for the DNA of the virus and are therfore not affected by maternal antibodies and can therfore be used in very young animals. Vaccination also has no influence on the outcome of the test. These tests are useful for screening large numbers of animals to identify PI’s, because as the samples are able to be pooled, the cost of testing large groups is reduced considerably.
Blood samples can identify a PI. A PI animal is antibody negative (Ab-) and antigen positive (Ag+). A PI animal is infected with the virus, excretes large amounts of the virus into the environment but mounts no immune system to the virus. Alternatively, to detect PI’s a second blood test 2 weeks after the identification of an Ag+ animal will reveal if that animal is still Ag+. A normal animal would have got rid of the virus by then and be Ag-, but a PI will never get rid of the virus and will always be Ag+. Many people condemn an animal on a single Ag+ test, due to improbability of testing a normal transiently infected animal during the very short time that it is infected.
There are a number of blood test options available. For sick animals, the best option is a test with a fast turnaround time. For whole herd screening, pooled blood samples can be used for PCR testing. These pool 10 samples together before testing. If the pooled sample is negative, then all the animals represented in that sample are negative. If the pooled sample is positive, each of the original 10 samples need to be sampled individually to determine which animal is positive. The turnaround time for the pooled sample testing is slower and can take up to 2 weeks.
Options for control
‘Natural Vaccination’ - hope that natural immunity is induced by the presence of a PI animal. This is a bit ‘hit and miss’ as not all animals will be exposed prior to mating and naive animals may get infected when pregnant. PI animals are also expensive and inefficient to raise and maintain in the herd. This is how most herd owners deal with the disease (either knowingly or unknowingly). However, with this method of ‘control’, disasters can happen. Also, it has been suggested that up to 50% of abortions in these dairy herds may be caused by BVD and BVD can also contribute to poor mating performance. These sorts of ‘subclinical’ effects of the disease can be hard to measure and quantify economically.
Monitor and Assess – A bulk milk test will indicate the level of infection in your herd and how at risk they are. If you are experiencing herd problems with fertility, a milk test may help rule BVD in or out. Alternatively, taking blood samples from different mobs may reveal “at risk” ages (e.g. heifers are often mostly naive).
Vaccinate with Bovilis (2 doses 4 weeks apart at least 1 month before mating, with annual boosters). You can target susceptible groups, or blanket vaccinate the whole herd. Bovillis BVD vaccine provides full protection against NZ strains of the virus, and claims to protect the foetus from infection in a pregnant cow. Over a number of years, PI animals would theoretically disappear from the herd.
BiosecurityBlood sample all new animals (especially the bull) to test for PIs. This will not completely eradicate the disease, as BVD can cycle through a herd if PI animals are already present, but it may help prevent disastrous matings.
Eradication – this is difficult but can be done. It requires identification and culling of all PI’s. This can be done over a number of years (for example, just testing the calves every year) but this would obviously take longer. Without vaccination, this would leave a nieve herd that would require strict biosecurity, with testing of all animals that enter the herd. Hefiers at grazing may be at risk of exposure to the virus and infection during pregnancy and vaccination prior to mating may be indicated in this group.
Gold Standard control – test all animals, cull PI’s, and vaccinate the rest of the herd (and replacement stock and bulls) every year. Obviously this is an expensive method of control, and the economics of this approach would have to be looked at closely.
As a bare minimum, I reccommend that all herds blood sample and vacciante all their bulls prior to mating.
If you would like to discuss BVD, or organize milk or blood tests, please phone our clinic
Last updated – 26/8/09