Bovine viral diarrhoea is a contagious disease and is better known as simply “BVD”. The viral disease affects cattle and death can occur in some cases especially where the immune system has been compromised. The bovine viral diarrhoea virus (BVDV) can result in significant losses to farmers economically.
Bovine viral diarrhoea is a viral infection caused by a virus from the genus pestivirus. Once it has entered the animal, this pathogen attacks the body’s systems including the respiratory and reproductive organs. The immune system is also adversely affected and this often results in the cow becoming more at risk to infection from other diseases taking advantage of the animal’s suppressed immunity.
All cattle are susceptible to contracting the disease although very young calves, before the age of three months old, are generally protected by the maternal antibodies provided by the colostrum ingested from the mother’s milk. The disease is transmitted via contact with fomites such as contaminated feed troughs but also from infected animals that are found to persistently shed the virus.
Signs and symptoms include acute diarrhoea, high temperatures, abortions, and infertility. The two latter signs and symptoms are mostly responsible for the consequential economic losses as well as the cost of the treatment required for recovery. The disease is not zoonotic which means that humans are not at risk of infection from the BVDV. Vaccinations are available to protect against bovine viral diarrhoea.
Transmission of Bovine Viral Diarrhoea
Transmission usually occurs as a result of what are known as “persistently infected” cows. These animals continue to shed the virus, often without showing any signs of infection, and therefore spread the disease throughout the herd. Contact between one of these cows and an unaffected cow can lead to the virus begin transmitted and infection occurring. Such cattle include shared bulls used for breeding which can also transfer the disease via semen. Foetuses are able to contract the disease whilst still inside the mother’s womb and contaminated milk is able to contribute to the spread of the virus. Additionally, transmission can be a result of contact with infected nasal and ocular discharges, saliva, urine, and faeces.
Other animals such as sheep are also able to transmit the disease via direct contact and this usually occurs if there is mixed grazing with infected sheep. Fomites such as clothing, footwear, and surgical tools are also sources of infection. Needles that have had contact with infected blood may also infect other cows. Outside of the host however, the virus cannot survive for long periods of time. It only survives for a few days but often this is enough time for transmission to occur.
Signs and Symptoms of BVD
The most well-known sign of infection from the BVDV is, as in inferred from the name, acute diarrhoea. This is more commonly called scour especially when present in calves. The economic consequences of bovine viral diarrhoea are such that it is a relatively feared disease on cattle farms. This is due to the fact that infertility and abortions are often occurrences following contraction of the virus. Calves may also be born deformed and there can be some nasal discharge. A decrease in milk yield may be observed in dairy cattle. Infected calves usually die before they are two years old due to mucosal disease (MD).
Some cows may present completely different signs. This is as a result of the disease leading to a compromised immunity, and so the signs and symptoms will suggest an infection in the cow from different diseases that the animal becomes susceptible to. This produces especially severe infections in calves that already have a lower immune system than their older counterparts. Some calves will experience signs of pneumonia. Such infection can prove fatal for both calves and adult cows.
Many become persistently infected with the bovine viral diarrhoea virus, usually obtained while still in the mother’s uterus. The virus survives in the cow for the rest of its life which could be as long as the average life of a cow or shortened due to the infection. Without showing any symptoms, the persistently infected cow will be a continuous source of infection by permanently shedding the virus.
Treatment of Bovine Viral Diarrhoea
Unfortunately, there is no cure against bovine viral diarrhoea. The main treatment plan simply consists of supportive therapy. This can include fluid therapy and in the administration of anti-inflammatory medication. In addition, there is neither a cure for mucosal disease (MD) and those with an acute infection of it are usually euthanized.
Prevention of BVD
Prevention is better than cure and since there is no cure for BVD then it can be argued that preventative methods are vital to ensure the safety of cattle herds. Some countries have now actually eradicated or almost eradicated the disease by imposing particular biosecurity methods.
Vaccines are available to protect against the spread of bovine viral diarrhoea. This preventative method is often used on farms where there are a high number of infected animals in the farm or general area. Commonly, it is the breeding cattle in the herd which are normally vaccinated before the breeding actually occurs. This includes bulls, cows, and heifers. Annual boosters are sometimes appropriate in order to provide long term protection against BVD to the animal, thus reducing the spread of the disease. There are a couple of different vaccination methods which can be farm specific, each with different time schedules and veterinarians should be consulted for advice.
In addition to vaccination, other preventative measures can be put into place. These include double fencing, verifying that incoming animals are disease free and keeping high levels of hygiene all over the farm. Contaminated areas should be disinfected and pregnant animals should be especially protected. Mixed grazing should be restricted to only animals of which there is no infection and any dead animals or foetuses should be removed from the property. New additions should be isolated for a period of thirty days. Regular testing of the herd can be accomplished via sampling the milk in bulk and attempting to detect any relevant antibodies or antigens which suggest the presence of the virus. Persistently infected cows may be culled to prevent further transmission.
Diagnosis of BVD
There are various different methods for the diagnosis of bovine viral diarrhoea. It is initially diagnosed by observing the signs and symptoms presented and by gaining knowledge of any possible history of the disease in the herd or in the area. Blood and milk samples are generally taken in order to detect the presence of the virus. This can be done via isolating the virus and identifying it, polymerase chain reaction (PCR) testing, and serology. Milk can be tested in bulk for the presence of antibodies and antigens in the herd. This is useful for farms which require regular checks on their animals.
Prognosis of Bovine Viral Diarrhoea
The health of the animal is often adversely affected and death usually results when dual infections occur. This is due to the compromised nature of the infected cow’s immune system. Infertility and abortions are the main sources of economic loss. When a calf has contracted mucosal disease (MD) then generally every one in four can be lost. When MD is acute then most are culled.