Vet Times The website for the veterinary profession https://www.vettimes.co.uk Infectious bovine rhinotracheitis: causes, signs and control options Author : Adam Martin Categories : Farm animal, Vets Date : September 5, 2016 Bovine herpesvirus type one (BHV-1) was first identified and reported as the cause of infectious bovine rhinotracheitis (IBR) in the early 1950s. Blood sample to establish serological status of a herd is essential if eradication is to be considered. Since then, BHV-1 has been found to be responsible for a wide range of conditions, including reproductive diseases (infectious pustular vulvovaginitis in cows, infectious pustular balanoposthitis in bulls), abortion, infertility, mastitis, conjunctivitis, enteritis, encephalitis, dermatitis and milk drop (Straub, 2001). In part, the type of disease depends on the strain of BHV-1 that infects a diseased animal. The pathogen causes considerable economic loss and has severe impact on an individual animal s welfare. Despite this, IBR, and other diseases caused by BHV-1, are endemic in the UK and most parts of the world. Most-common presentation IBR is BHV-1 s most common clinical presentation. IBR is an upper respiratory disease that affects 1 / 7
cattle generally aged six months or older. It can present as a relatively mild disease, causing transient pyrexia with or without conjunctivitis, causing a slight drop in appetite or milk yield. In more severe outbreaks, coughing is common and typically lasts for two to three weeks before subsiding. However, in severe cases, animals may die from a severe pneumonia complicated by secondary bacterial infection. It is worth noting IBR can affect young calves, causing encephalitis, enteritis and pneumonia, although this is rare. If calves are affected in this way, mortality is high. After animals have undergone primary infection with BHV-1, they become lifelong latent carriers of the virus. This latency generally establishes itself in the trigeminal ganglion or pharyngeal tonsils, although the sacral ganglia is the site of latency after genital infection. Latently infected animals need not shed virus, although, in all but very rare cases, they produce antibodies against the virus. Periodically, the latent virus will reactivate and shedding will recommence. Under these circumstances, viral transmission can occur. Reactivation of the virus typically occurs in stressed animals (poor weather, poor husbandry) or after stressful events (calving, transport, mixing of animal groups). Treatment of latently infected animals with corticosteroids has been shown to result in viral shedding. European and UK prevalence of IBR Infectious bovine rhinotracheitis is rarely seen in calves younger than six months of age. Herd level prevalence of BHV-1 in the UK has been found to vary between 43% and 84%, depending on the region and class of cattle surveyed. Six countries in Europe (Austria, Denmark, 2 / 7
Finland, Norway, Sweden and Switzerland) have managed to eradicate BHV-1 from their cattle populations following the introduction of control programmes in the 1980s. Regions of Germany and Italy are also BHV-1 free. Voluntary or compulsory control programmes are underway in several other European states. Countries with eradication programmes in place are granted additional guarantees with respect to intra-community trade, which has the potential to severely impact the export markets of other member states. As an example, prior to Belgium introducing an eradication programme, the Republic of Ireland exported 20,877 cattle there. In the 12 months following the 2014 granting of additional guarantees, the number fell to 405 a 98% reduction. Consequently, the control of BHV-1 has important economic consequences. Many of the risk factors for BHV-1 infection are similar to many infectious diseases. Increasing age increases the time at risk for exposure to the pathogen, thus BHV-1 seropositivity increases as animals age. Interestingly, the peak time for seroconversion appears to be in animals aged younger than two years of age, which might be linked to management practices. Bulls generally have a higher seroprevalence than cows. This is thought to be the result of increased mixing with animals. Increased seroprevalence Generally, increasing herd size has also been shown to be associated with increased seroprevalence. The reason for this is each animal has a larger number of transmission contacts. Furthermore, previous work, not specific to BHV-1, has shown herd size is frequently a proxy measure for other disease risk factors, such as stress, other pathogens and increased purchase of animals. Unsurprisingly, direct contact with animals from other herds is a risk factor. However, the number of herds, distance between adjoining herds and cattle density in any area also affect the risk of seropositivity to BHV-1. This has been recognised in some of the UK-based control schemes, where double fencing is required to reduce the potential for inter-herd contact. Transmission BHV-1 survives relatively poorly in the environment, so the majority of infection occurs following the direct contact of animals or animal fluids, for instance, mucus or semen. While BHV-1 is considered to be pathogenic exclusively to cattle, some evidence shows other ruminants can be infected by, and shed, the virus. It is not thought species other than cattle play a role in the epidemiology of the disease. However, similar herpes viruses sharing a number of antigens are known to be found in other species. Serologically, it is impossible to determine the difference between BHV-1 and the closely related herpes viruses of other species for example, caprine herpesvirus type one, or elk herpesvirus one. 3 / 7
Problems in control programmes have also arisen when non-bovine species have tested positive for BHV-1 antibodies, especially if the positive result represents an active BHV-1 infection or if it represents a related herpes virus adapted to another species. Control options Producing BHV-1 free breeding stock is the first step towards eradication of the pathogen. Essentially, two main strategies are used to control BHV-1: an infection test and slaughter strategy or a vaccination-based strategy. If prevalence in a population is relatively low, the most effective strategy for disease eradication is to test the animals and cull the positives.fundamental to this strategy is the idea a disease-free pool of breeding animals is developed and, therefore, protected from the disease as seropositive animals are culled from the herd and seronegative ones replace them. Eradication schemes are far from perfect, however. The danger always exists a latently infected seronegative animal will remain in the population and spread disease. One study investigated how this was possible and identified latent infection was possible in young animals that had been passively immunised against IBR as calves. Vaccination The alternative strategy of vaccination is most appropriately targeted at populations with a high prevalence of virus, or where there is a low prevalence, but the population is at risk of being infected. So, despite a low prevalence of on-farm BHV-1, vaccination may still be an appropriate control measure if the risk of exposure to the virus is high. The ability of BHV-1 to become latent initially caused problems when differentiating between 4 / 7
seropositive vaccinated animals that did not have the ability to shed virus and animals infected with wild virus that did. However, for many years marker vaccines have been available that allow for vaccinated and wild infected groups to be differentiated. Differentiation between these groups is possible as the vaccine contains viral strains from which one or more glycoproteins have been deleted when compared to the wild strain. This means wild BHV-1 strain infection results in an antibody response to the glycoproteins that are not present in the vaccinated strain. This response can be measured using diagnostic tests, such as ELISAs. While the development of marker vaccines has aided the control of BHV-1 enormously, it is worth noting differentiation between vaccinated and wild infected animals can only be made if serology is performed. Diagnostic tests performed on milk, such as monitoring bulk milk samples, make no differentiation between vaccinated and wild infected cattle, which is problematic. Furthermore, as with all diagnostic ELISAs, diagnostic thresholds (cut-offs) have been assigned to each diagnosis. With these cut-off values comes associated sensitivities and specificities between tests. These can vary considerably, something that makes effective surveillance and evaluation of vaccinated populations challenging. Viral shedding Vaccination against IBR does not prevent shedding of virus from animals that were latently infected with it prior to vaccination. However, vaccination of latently infected animals reduces viral shedding when infected animals are challenged. Controlled studies and field studies have shown major IBR outbreaks are still possible, and do occur, in vaccinated herds although, typically, vaccines are effective in controlling IBR irrespective of whether live or attenuated vaccine has been used. It is worth stressing, however, vaccines are, on the whole, extremely effective at controlling the disease and marker vaccines should always be considered when tackling an IBR problem. 5 / 7
BHV-1 status of European countries in the final quarter of 2014. In the UK, several laboratories provide a testing and Cattle Health Certification Standards (CHeCS) accreditation service. Testing and accreditation as free from BHV-1 is a goal UK producers of breeding stock should definitely aspire to. Additionally, they should also consider a number of husbandry requirements, such as double fencing of fields with adjoining livestock and quarantining of new animals prior to them entering a herd. CHeCS accreditation of a herd begins once all animals older than 12 months are found to be negative in two tests conducted not less than four weeks and not more than one year apart. After this, disease-free status is maintained through regular testing combined with targeted testing of animals, including any that have aborted or have given birth to a stillborn calf, clinically sick animals that may have had IBR or any purchased animals. The CHeCS system allows farms to be accredited as BHV-1 eradicated from the herd or as having vaccinated monitored free status. The former is regarded as having the higher health status. The short-term benefits of BHV-1 eradication may not be immediately apparent, or may not exist, for all farmers. However, it is likely the long-term cost will be far lower than the losses particularly as international trade will increasingly be linked to IBR status. Summary IBR is a serious and endemic disease in the UK. An increasing number of trading partners in Europe have begun national disease control and eradication programmes. While these programmes might be complicated and face difficulties, little doubt exists they will improve the health and welfare of the cattle in these countries. If disease-free status can be maintained once it has been achieved, it will prove to be financially beneficial too. 6 / 7
Powered by TCPDF (www.tcpdf.org) Increasing pressure from Europe, plus an increasing understanding of the benefits and importance of disease eradication, will hopefully mean a growing number of progressive farmers begin to control IBR. References Straub OC (2001). Advances in BHV-1 (IBR) research, Deutsche Tierärtzliche Wochenschrift 108(10): 419-422. 7 / 7