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 of Bovine Viral Diarrhoea >
- Effects of Bovine Viral Diarrhoea >
- Treatment & Control of Bovine Viral Diarrhoea >
- Medication/Vaccination for Bovine Viral Diarrhoea >
Causes of Bovine Viral Diarrhoea
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 of Bovine Viral Diarrhoea
Infection 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):
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Stage of production or age
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Effect of infection with BVD
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Adult, non-pregnant cows
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Acute diarrhoea or no clinical signs
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Adult, pregnant cows,
<120 days of gestation
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Acute diarrhoea or no clinical signs
Foetal loss or abortion
PI calf
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Adult, pregnant cow
100-150 days gestation
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Acute diarrhoea or no clinical signs
Foetal loss or abortion
Congenital defects in calf and possible PI calf
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Adult, pregnant cows, >120 days of gestation
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Acute diarrhoea or no clinical signs
Foetal loss or congenital damage to calf or no effects
Calf is BVD antibody positive at birth
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Non-immune calves
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Acute diarrhoea or no clinical signs
Respiratory infections or scour outbreaks possibly due to immunosupressive effect of BVD
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PI animals
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Mucosal disease
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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.
Treatment & Control of Bovine Viral Diarrhoea
Treatment essentially, entails eradication of disease (with or without the use of vaccine) and maintenance of an accredited disease-free herd or the control of exposure to disease by continuous use of vaccination. Control and monitoring of BVD infections should be an integral part of the farm’s Herd Health Plan in association with the veterinary surgeon.
Several European countries or regions have established BVD control or eradication. The advantages of BVD eradication are both improved herd health and the ability to sell disease free stock, which may be important to some pedigree herds. Establishment and maintenance of an disease-free herd is dependent on a high standard of biosecurity, which must be assessed in each herd’s circumstances.
The main risk factor for BVD infection in a herd is the existence or introduction of a PI animal in the herd. The PI animal then infects in-contact animals. Before the introduction of good vaccines, the PI animals were knowingly allowed contact with non-pregnant animals to allow them to be infected and develop natural resistance. However, this method was unreliable and is not recommended as it frequently resulted in disease outbreaks, reproductive loss and sub-optimal immunity in the herd.
Bought-in animals are the greatest risk of disease introduction, either as PI animals or if cows are bought in in-calf, where they may be carrying a PI calf. Another major risk factor for BVD is the introduction of an acutely infected animal into the herd, such as a purchased breeding or fattening animal, a herd member returning from a show or a hired bull. Therefore, all new animals should be tested and/or quarantined on arrival to the herd, especially breeding bulls if their health status is not BVD accredited-free.
Other significant risk factors can be direct contact with cattle from a neighbouring, infected herd. Less important but, in certain cases, contact with other ruminants (sheep, deer) or contact with infected materials, such as veterinary instruments, veterinarian’s boots or clothing, other visitors, semen etc.
Medication/Vaccination for Bovine Viral Diarrhoea
With several effective vaccines currently available, there are a number of approaches to vaccination, which may be the preferred option if high levels of biosecurity are not possible to maintain and disease prevalence is high. The overall aim of vaccination is to prevent transplacental infection of the virus and the development of PI animals, which can contribute to more rapid management of the herd because of foetal protection. Vaccination has been shown to be highly effective in controlling BVD in an endemic situation, although it can still mask underlying infection.
Vaccines available contain the Type 1 strain of BVD, although this has been shown to be effective in minimising the effects of Type 2 BVD infection.
There is no treatment for subclinical BVD or acute mucosal disease. Support therapy in the form of fluids and anti-inflammatory agents can be used.
This info was adapted from Defra’s Compendium of Animal Health & Welfare in Organic Farming, which is a collaborative effort led by Duchy College, Cornwall with VEERU at The University of Reading and The Faculty of Veterinary Medicine, University of Glasgow.
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