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Bovine Viral Diarrhoea (BVD)
It has many different manifestations in a herd, depending on the herd’s immune and reproductive status. Transient diarrhoea, mixed respiratory infection, infertility or abortion and mucosal disease are the most common clinical signs of the disease and can be seen separately or simultaneously in a herd. Due to its varied manifestations and subclinical nature in many herds, the significance of the disease has often been underestimated in terms of its affects on animal health and the economics of the farm.
Causes
There are two sub-types of BVD virus reported, Type 1 and Type 2. Type 2 BVD infection is associated with more severe clinical signs of haemorrhagic enteritis at the time of introduction to a herd.
With the more common Type 1 infection, apart from causing an outbreak of acute diarrhoea at the time of introduction into a herd, the following short- and long-term effects are caused by a herd infection: infertility, embryonic death, foetal mummification, abortion, congenitally damaged calves, weak or ill-thriven calves and persistently infected (PI) calves.
Effects
nfection of a naïve dam in the first half of pregnancy will lead to infection of the calf in-utero before development of the calf’s immune system. Thus, the calf fails to recognise the virus as foreign and remains infected and, importantly, is a source of infection throughout its life. Some of these PI calves die of mucosal disease before two years of age due to infection with a cytopathic BVD virus strain.
Mucosal disease is characterised by oral ulceration, diarrhoea, and interdigital lesions in the feet. It is rapidly fatal and untreatable. An important differential diagnosis in severe case of mucosal disease is Foot and Mouth Disease.
BVD virus has also been identified as an immunosuppressive agent, increasing the risk of infections such as respiratory disease in calves, salmonellosis, interdigital dermatitis and mastitis.
Epidemiology
To be able to understand the epidemiology of BVD infection in a herd it is important to know how the virus affects the host at different stages of life (if animal is not immune):
|
Stage of production or age |
Effect of infection with BVD |
|
Adult, non-pregnant cows |
Acute diarrhoea or no clinical signs |
|
Adult, pregnant cows, <120 days of gestation |
Acute diarrhoea or no clinical signs Foetal loss or abortion PI calf |
|
Adult, pregnant cow 100-150 days gestation |
Acute diarrhoea or no clinical signs Foetal loss or abortion Congenital defects in calf and possible PI calf |
|
Adult, pregnant cows, >120 days of gestation |
Acute diarrhoea or no clinical signs Foetal loss or congenital damage to calf or no effects Calf is BVD antibody positive at birth |
|
Non-immune calves |
Acute diarrhoea or no clinical signs Respiratory infections or scour outbreaks possibly due to immunosupressive effect of BVD |
|
PI animals |
Mucosal disease |
It has been estimated that approximately 1% of cattle are persistently infected with BVD, with this relatively low percentage of animals the key to maintenance of infection in a cattle population and therefore also key in its control. Dairy herds positive for BVD virus antibodies have been reported to have an increased abortion rate and to have an increased duration to first calving in heifers.
The reproductive effects of BVD have been suggested to be the most important economically with estimates of the cost of infection varying, depending on whether the herd is endemically infected or susceptible.
This information is taken from:
http://www.vetsweb.com/diseases/bovine-viral-diarrhoea-d217.html
For more information see:
http://www.organicvet.co.uk/Cattleweb/disease/bvd/bvd1.htm
http://www.agresearch.teagasc.ie/moorepark/Articles/BVD.pdf
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