VN June 2021

Vetnews | June 2021 33 Regulars I Zoetis Livestock Column Control and Prevention As discussed previously, BHV-1 is one of the main viral contributors to multi-factorial bovine respiratory disease. Treatments are thus aimed mainly at combating secondary bacterial infections through the use of various registered antimicrobials from various antimicrobial groups, registered for treatment of the common respiratory bacterial pathogens. In addition to antimicrobials, non- steroidal anti-inflammatory drugs (NSAID’s) are indicated to reduce fever, inflammation and pain. Currently there is no commercially available antiviral treatment for BHV-1, although there is promising research on ivermectin, that demonstrates a dose-dependent reduction of viral replication and viral shedding of BHV-1 infected cells treated with ivermectin. The aim in disease control is rather focussed on prevention through good biosecurity and vaccination. The fact that latent animals can be introduced into a herd makes it difficult to survey via testing prior to introducing new animals into a herd. Especially, since latent animals can harbour the virus without necessarily being seropositive, and since BHV-1 is an endemic disease in South Africa and vaccinated for at large, it is not the aim for a herd to be seronegative. Instead, the aim should be to not introduce an actively shedding individual into your herd, and this can largely be attained by a quarantine protocol where all newly bought animals are vaccinated upon arrival and held separate from the existing herd for a period of 30 - 45 days. This allows time for the immune system to mount a response to the vaccine and also for active or re-activated infections to resolve. There are numerous vaccines available against BHV-1 and a lot of research is continuously being poured into developing new and improved vaccines against this important pathogen. Some of the newer technologies include subunit vaccines (that contain one or more of the important antigens of the virus) and gene-deleted vaccines. In Europe several countries are running eradication programmes with gene-deleted (marker) vaccines, that makes it possible to distinguish serologically between the vaccine strain and the field strain. In South Africa the commercial vaccines available are inactivated vaccines and modified live (MLV) vaccines. Most BHV-1 vaccines are currently in combinations with other viral (and/or) bacterial pathogens. One of the most well-known and widely used combinations, is the “five-way-viral” combination containing antigens against BHV-1, bovine viral diarrhoea (BVD) Type 1 & 2, parainlnfluenza-3 (PI-3) and bovine respiratory syncytial virus (BRSV). Inactivated (killed) vaccines have the benefit of safety, compared to modified live vaccines. This specifically refers to administration of vaccine to pregnant animals (or their nursing calves), which in the case of the modified live vaccines can cause abortions, especially in seronegative animals vaccinated for the first time. Modified live vaccines are considered more efficacious because they elicit a strong cell-mediated response in addition to a humoral response, because they actively replicate inside the cells of the vaccinated animal. As mentioned, they can however cause abortions in naïve pregnant animals. Several manufacturers do have registration for use in pregnant animals, provided they were primed according to label instructions prior to pregnancy. Another benefit of (most) modified live vaccines, is the fact that they only need to be vaccinated once, compared to the inactivated vaccines that require a booster dose to get an optimal immune response in animals vaccinated for the first time. An interesting vaccine currently on the market is a modified live BHV-1 vaccine that is chemically treated to be temperature sensitive, i.e. it does not replicate at body temperature. This vaccine is administered intranasally (compared to the rest of the available vaccines that are administered parenterally) and consequently replicates in the upper respiratory airway but gets inactivated as soon as it reaches body temperature lower down the respiratory tract. It can also be administered safely to calves nursing unvaccinated cows, which is not the case with the parenteral modified live vaccines on the market. Vaccination provides good protection against the disease, provided the animals mounted a sufficient immune response. In developing vaccination protocols, stress and immune suppression must always be taken into consideration. This is a common scenario in feedlot calves, that often arrive at the feedlot recently weaned, having travelled far distances and being co-mingled with various other calves - a series of events that stress these calves and leaves them transiently immuno-supressed. Farmers producing calves for feedlots, replacement heifers and/ or breeding stock, will benefit from vaccinating their calves with a BHV-1 vaccine prior to weaning. Summary The bovine immune system has very pronounced response to BHV- 1 and can be primed to create a strong amnestic response through vaccination. Various laboratory techniques can be employed to diagnose BHV-1, used in conjunction with clinical signs seen in animals. It is, however, important to remember that BHV-1 is often part of a multi-pathogen disease and treatment is mostly aimed at secondary bacterial infection and inflammation. As an endemic disease, proper vaccination and a sound biosecurity protocol should be implemented to minimise clinical disease caused by this pathogen. v Table 1: BHV-1 Diagnostic Techniques Test Antibody Detection ELISA Enzyme-linked Immunosorbent Assay VNT Virus Neutralisation Test IFA Indirect Fluorescent Antibody Test Antigen Detection VI Virus Isolation in Cell Culture DFA Direct Fluorescent Antibody Test PCR Polymerase chain reaction IHC Immunohistochemistry EM Electron microscopy