A Report On Bovine Viral Diarrhea Virus: Epidemiology, Diagnosis, Prevention, And Treatment
Disease Epidemiology
Bovine Viral Diarrhea virus (BVDV) transmission is spread through nose to nose contact, fomites, many bodily fluids and secretions such as saliva, respiratory secretions, feces and semen, as well as congenitally. The virus does not persist in the environment but can survive long enough to be transmitted via infected equipment, needles, and palpation sleeves. Acute infections are cleared within 11 days. After this time, there is usually no shedding of the virus. However, the reproductive consequences of this disease lead to calves with CNS disease (if exposed 150-200 days gestation) or who are persistently infected if exposed to the virus at less than 125 days gestation. Cows can give birth to transiently infected calves that can either clear the virus or become persistently infected (PI). These PI calves shed the virus FOREVER and in high amounts, and can spread BVDV throughout the herd. Two genotypes, BVDV type 1 and BVDV type 2, are identified as distinct species within the genus, with further classification as cytopathic and non-cytopathic. Type 2 is considered more virulent, causing severe disease, including thrombocytopenia in some cases, but type 1 can cause the same manifestations of disease. The non-cytopathic strain is the strain that results in PI cattle. PI cattle can get mucosal disease if exposed to a strain of cytopathic disease. BVDV can also cause abortions in cattle
The only way to get complete “immunity” from BVDV is to cull all persistently infected calves. Persistent infection arises by the unique ability of BVDV to survive by inducing immune tolerance in the bovine fetus through evasion of both innate and acquired immunity in utero. Adaptive immunity is avoided by immune tolerance through infection of the fetus prior to development and maturation of the adaptive immune system. Good biosecurity is key to disrupting the interaction between the host, agent, and environment (maintaining a closed herd, raising your own replacements, etc). Also, it is imperative to clean and sterilize equipment between cows and maintain good sanitation and hygiene.
Disease Surveillance and Diagnostic Testing
I would first do an antibody/antigen screening test via ELISA of young cattle. If they were found to be positive, I would then begin a more extensive search for antigen-positive animals (ear notch samples) within the herd and cull any positive animals immediately. The pitfalls of this strategy would be having no animals test positive, so the herd is deemed negative, but in reality, you missed a positive case. If the animals are particularly valuable, if a calf is found to be positive on an ear notch sample, some producers may opt to retest in 2-4 weeks because transiently infected calves may give a positive result on ELISA initially.
I would choose the antigen-capture ELISA which has a reported sensitivity and specificity of 94. 5% and 100%, respectively. This means that 100% of the time the test can detect animals who DO NOT have the disease, therefore we can have confidence in a positive result.
If I wanted to know with complete confidence that a cow that tests negative is truly negative, I would test in parallel. This would ensure that any positive is truly positive and would increase my negative predictive value.
Disease Prevention and/or Elimination
Persistently infected (PI) animals serve as the reservoir of BVDV and a source of infection. Persistent infection is induced by infection of the fetus with NCP BVDV early in gestation before development of the humoral immune system. To persist, both the innate and adaptive immune response are averted by the virus. BVDV interferes with Type I interferon signaling and this has been thought to be a contributing factor in the establishment of PI. Work has shown that dams of calves had activation of the innate response during the first trimester as a result of ncp BVDV infection, but this is only partially effective in halting viral replication. An adaptive immune response is required to eliminate infection. Recent advances allow better detection of PI animals and thus improve the possibility of enhanced control.
Control strategies have focused on identification and removal of PI animals from herds. Acutely infected animals do not serve as a source of infection to herd mates. Strategies of control include vaccination and removal of PI animals. The prevalence of PI animals is reported to be between 1-2%, so large numbers of animals need to be tested to identify those few that PI with BVDV. However, BVDV can infect other species such as White Tail Deer, alpacas, and mountain goats, so this poses a risk for re-infection of BVDV if eradicated from a herd. Also, maintaining an open breeding herd is the primary risk factor for disease introductions. A way to combat this issue is to purchase older cattle, as many PI cattle do not live past the age of 3. Cattle movement in general is associated with increased risk of herds being seropositive for BVDV. We would expect that older dams are less likely to be PI and more likely to be immune from previous exposure. In dairy cattle, however, the purchase of a pregnant dam was not a risk factor at all. This is thought to be due the prompt removal of calves from their mothers after birth. Therefore, if calves were PI, they have a much lower chance of spreading the disease throughout the herd because of the isolated rearing of dairy calves. However, if an animal is persistently infected with BVDV, no length of isolation of new additions will be long enough. A 30-60 day isolation period will, however, allow any transiently infected animals to clear their infections before contacting the existing herd.
Depending on whether we are dealing with dairy or beef cattle, there are different strategies that I would use for cleaning and disinfection. If we are dealing with dairy cattle, that would be much easier than beef cattle, as there are surfaces that are better cleaned in a dairy operation than in a beef cattle operation. In general, I would choose to use an alcohol or phenol. BVDV is an enveloped virus and both alcohols and phenols are effective against this type of virus. These also have the least disadvantages associated with them. First, with an alcohol, the advantages of this type of disinfectant are it is fast acting and it leaves no residue, which is better for a dairy milking parlor. The disadvantages of this disinfectant are that it evaporates quickly and can be flammable. With phenols, the advantages are they have good efficacy against organic material, which is great for BVDV because the virus is shed through secretions and the like, it is also non-corrosive, and stable in storage. The disadvantages are it can cause skin and eye irritation so the handler will need to use cause caution and wear the appropriate PPE for protection when using this particular disinfectant. Phenols are also effective with soaps and detergents mixed in and with hard water. Having all this information, I think a phenol would be the best choice as a disinfectant, but beef cattle herd may have a hard time disinfecting fields and fences and what have you but could disinfect feeding and water troughs, as well as hay rings. d. Modified live and killed vaccines have been available for more than 50 years, yet the incidence of BVDV-induced disease remains significant, suggesting a need for improved vaccines. There are 4 structural proteins of BVDV but the E2 glycoprotein is the major target of neutralizing antibodies. Newly designed vaccines have addressed the issues related to virulence of E2-expressing vaccine viruses by incorporating the E2 gene into non-virulent vectors. Due to antigenic variability, there is potential for a lack of cross-protection against wild-type BVDV viruses, even though cross-neutralization studies have shown reactivity to differing genotypes. However, since disease due to BVDV remains widespread, additional measures beyond vaccination are needed. The goal of the vaccination program in the breeding herd is to prevent fetal infections. Common recommendations are for MLV vaccines used 30 days pre-breeding. PI animals can easily shed enough virus to overwhelm a proper vaccination protocol. When on pasture during the breeding season, they can efficiently cause the creation of more PI calves by infecting cows in the right stage of gestation.
Disease Economics
Major economic losses result from reproductive losses and exacerbation of concurrent bacterial or viral infections. During acute outbreaks cattle infected with BVDV may exhibit non-specific clinical signs of depression, inappetence, fever, and diarrhea, leading to transient declines in milk production, growth performance, and animal fertility. Additionally, few PI calves survive past the age of 2 or 3 years of age due to underlying immunosuppression and development of mucosal disease.
