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What is new on IBR? Epidemiology and control at the farm level
Guelbenzu M.
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Infectious bovine rhinotracheitis (IBR) is a highly infectious disease caused by the bovine herpesvirus type 1 (BoHV-1). BoHV-1, one of eight herpesviruses known to infect cattle, is an alphaherpesvirus and can also cause infectious pustular vulvovaginitis (IPV) and infectious pustular balanoposthitis (IBP). IBR has worldwide distribution and in addition to the impact on health and productivity also affects the trade of ani- mals, semen and embryos.
In order to be able to apply successful strategies for the control of IBR at the farm level it is necessary to have a good understanding of the epidemiology of this disease.
BoHV-1 is mainly spread directly by close contact between animals. It can also be shed from the reproductive tract, including semen, resulting in venereal transmission. Aerosol transmission typically occurs over short distances but it may also occur over distances of up to 5m. The virus is moderately resistant to environmental factors so indirect transmission within or between herds can also occur indirectly through movement or sharing of contaminated facilities, equipment or personnel.
Clinical signs of BoHV-1 infection most commonly involve the upper respiratory tract and include nasal discharge, hyperaemia of the muzzle (red nose), conjunctivitis, fever and inappetence and on occasions, death. This may be accompanied by decreased milk yields and a range of negative reproductive outcomes depending on the stage of the reproductive cycle at which exposure occurs (failure to conceive, early embryonic death and abortion). However, it is also recognised that in herds with endemic infection the course of infection can be sub-clinical but nevertheless still be associated with a reduc- tion in milk yield and negative reproductive outcomes.
Recovery following initial infection is associated with the development of immunity, but this does not eliminate the virus. Instead, the virus establishes lifelong latent infection in the trigeminal ganglion or pharyngeal tonsils. During this period the latent carrier is not shedding virus. However, at times of stress such as transport, calving, mixing stock etc, the virus may be reactivated and can begin to multiply and be re-excreted. This leads to new infection in other susceptible cattle, which in turn will also become latent carriers. These latently infected ani- mals play a central role in maintaining IBR in infected herds, where they act as a reservoir of infection, and in spreading infection between herds.
Vaccines are used widely for a range of scenarios including to reduce the clinical impact of an outbreak, as part of an IBR control programme and to protect free herds against infection. Both live and inactivated marker vaccines are available which can reduce the clinical signs and the amount of virus shed following infection. When given intra-nasally, live vaccines can give rapid protection in the face of a clinical outbreak. When used as part of a control strategy, the percentage of infected cattle in a herd should decrease over a period of time as older, positive cattle are displaced by younger, uninfected stock. Animals vaccinated with gE-deleted marker vaccines can be discriminated from field-virus infected animals by a negative serological reaction for gE.
Understanding the relevant risk factors for (re)introduction of the virus is key information both for the design of effective control programmes and for individual farmers aiming to maintain their herds free of infection. These risk factors should be then translated into biosecurity measures that farms can apply. The main risk for introduction of infection is the purchase of latently infected animals. Biosecurity options to address this risk include maintaining a closed herd, buying known-negative stock and post-purchase isolation and testing. Other risks, including contacts at boundary fences, shows and sales, and movement of people and equipment, should also be considered.
In summary, control of IBR at the farm level is possible but, especially in endemic areas, this must be accompanied with the implementation of biosecurity measures to protect the farm from (re)introduction of the virus.
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Affiliation of the authors at the time of publication
Animal Health Ireland, Carrick-On-Shannon, Republic of Ireland
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