What is IBR?

IBR stands for ‘Infectious Bovine Rhinotracheitis’ and describes a respiratory disease caused by a viral infection of the nose and upper airways of cattle. The virus that causes IBR is called bovine herpes virus-1 (BoHV-1), but is more commonly known as IBRV (IBR virus).

What are the clinical signs of infection?

IBR can lead to respiratory disease in all ages of cattle, with severe cases resulting in death or long term ill health. The following signs may be associated with IBR. Since these are not unique to IBR, it is important to have any suspect animals examined by your veterinary practitioner.

Dullness and reduced appetite.

High body temperature.

Rapid and loud breathing, sometimes with coughing.

Inflammation inside the nose and in the pink of the eye (conjunctiva) or less commonly on lining of male or female reproductive tracts.

Fluid discharge from nose and eyes.

While most commonly associated with respiratory disease, infection with IBRV can also lead to other clinical signs (either in combination with respiratory disease or on their own), including:

Sudden milk drop.

Conception failure and abortion.

Pharyngitis (inflammation of the throat), most commonly in calves and potentially causing difficulties in swallowing that can lead to inhalation pneumonia due to milk or water entering the lungs.

Nervous signs (normally only in young calves).

Not all animals that become infected with IBRV show obvious signs of disease (sub-clinical infection). This may be influenced by the strain of the virus involved, and the management and immune status of the herd.

How common is infection?

IBRV is regularly detected by the Veterinary Laboratory Service of DAFM in samples from cattle with pneumonia and surveys have shown that infection is widespread in Irish herds, with approximately 75% of both dairy and beef herds containing one or more animals that have been exposed to IBRV.

How does the virus spread between animals and herds?

The virus is mainly spread directly by close contact between animals. Following primary respiratory infection, a susceptible (or naïve) animal can shed high levels of virus in fluid from its eyes, nose and mouth for up to 14 days (this fluid can contain over 10 million virus particles per ml). It is estimated in a susceptible population that each infected animal can transmit infection to up to seven others, leading to the rapid spread of an outbreak. The virus can also be shed from the reproductive tract, including semen, resulting in venereal transmission.

Airborne spread may also occur over distances of up to five metres. Indirect transmission of virus within or between herds can also occur through movement or sharing of contaminated facilities, equipment or personnel.

Recovery from primary infection is associated with the development of immunity, but this does not lead to elimination of the virus. Instead, it enters a latent (sleeping) state, typically in nerve cells in the brain of the animal, which is now a latent carrier. During this period, the latent carrier is not shedding virus.

However, episodes of stress can lead to re-awakening (reactivation) of the latent infection, followed by shedding of the virus (typically without clinical signs in the carrier animal) and the creation of primary infections in other susceptible cattle (Figure 1). Transport, calving and other stressful events such as lameness, nutritional stress, mixing stock and other diseases are all potential triggers for reactivation.

These latently infected carriers play a central role in the maintenance of IBRV in infected herds, where they act as a reservoir of infection.

Periods of reactivation lead to spread of infection to susceptible animals, which in turn then become latent carriers. In dairy herds, the milking herd is often the main reservoir of infection. Since young stock are often managed separately, they may remain uninfected until after they calve and join the main herd.

In the great majority of cases, cattle that have been infected with, and are latent carriers of, IBRV will develop an immune response as a result, which can be detected by testing blood or milk samples for antibodies (they are ‘seropositive latent carriers’).

However, where infection occurs in young calves with maternal (colostral) antibodies to IBR, the calf may become test-negative after the maternal antibodies have disappeared. While this occurs infrequently, these “seronegative latent carriers” (SNLC) represent a risk, particularly to semen collection centres.

Vaccines

There are several different IBR vaccines containing either killed or live virus licensed for use in Ireland.

These are all ‘marker’ vaccines, meaning that when used with an appropriate test, it is possible to distinguish those animals that test positive due to vaccination from those that are positive due to infection. This ability is central to control of IBR, both at herd and national levels.

Vaccines may be used for a number of different purposes. These include emergency use in the face of a clinical outbreak, when intra-nasal vaccination with live vaccine can assist with control; to minimise the occurrence and severity of clinical outbreaks of disease (for example, when purchasing beef cattle for fattening); as part of a control or eradication programme in breeding herds.

Decisions on which product and vaccination strategy to use in a particular situation should be made in discussion with your veterinary practitioner. It is important to read the datasheet provided with the vaccine to make sure that it is stored and used correctly, including being given by the correct route (which may be up the nose, into the muscle or under the skin).

Note that live vaccines are supplied in two parts – powder and liquid. Immediately before use, the liquid should be added to the powder, after which it will have a shelf life of only a few hours.

Diagnostic tests

Antibody tests applied to individual blood or milk samples can be used to screen individual animals to determine their IBR status. The choice of test will be dictated by the vaccination status of the herd. In non-vaccinating herds, either “gB” or “whole virus” tests are recommended, whereas a “gE” should be used in vaccinating herds.

A positive result with the correct test indicates that the animal has been infected with IBRV and is a latent carrier. Note that gE tests are less sensitive, particularly when applied to milk samples and may sometimes give false negative results.

These tests can also be applied to bulk milk tank (BMT) samples to investigate and monitor herd status. Note that a negative BMT result may be obtained with some tests even when 15% to 20% of contributing animals are antibody positive, and should not be taken as conclusive evidence for the absence of infection, particularly on a single result.

All test results, and actions taken as a result, should be discussed with your veterinary practitioner.

Control: international, national and herd level

Several European countries, or regions thereof, are officially recognised by the EU as being IBR free, including Denmark, Germany (Bavaria), Italy (Bolzano), Austria, Finland, and Sweden, while others have EU approved eradication programmes under way. The Netherlands, Belgium, Hungary and Slovakia also have programmes in place, although these have not been submitted for formal EU approval.

Animal Health Ireland is currently working with industry stakeholders to investigate options for a national approach to IBR control. Benefits of eradication would include elimination of clinical and sub-clinical disease, improved welfare, extension of the pool of animals available for entry into semen collection centres and supporting the live export market.

In the absence of a national programme, the decision to implement a control programme in a breeding herd will be influenced by a number of factors, including herd specific goals (e.g. obtaining or protecting a high health status, production of potential AI sires and sale of IBR-free stock) and the levels of clinical and subclinical disease.

Where a decision is taken to implement a control programme, the three recommended steps thereafter are: investigate, control and monitor.

Investigate herd status: where the level of infection is unknown, strategic testing will indicate if there is no current evidence of infection, a low prevalence or a medium/high prevalence. This in turn can help when selecting the relevant control options.

Control: there are three general principles that can be included in herd control strategies for IBR. The most suitable control strategy will vary with the herd status and should be decided under veterinary guidance (Table 1).

Bioexclusion: this is an essential component of all IBR control strategies, aiming to put in place farm-specific measures to prevent introduction of infection. These should address the highest risk activities first, with a focus on introduced animals and direct or indirect contact with cattle in other herds.

Culling/isolation: herds that start with, or achieve a low prevalence of infection, this can be used to remove the last remaining latent carriers.

Vaccination: complete and regular herd vaccination makes it less likely that a latent carrier will reactivate and shed the virus, and reduces the likelihood that a susceptible animal will become ill and transfer infection following exposure. In infected herds, cattle born after the implementation of effective vaccination and bioexclusion programmes should remain free from infection, resulting in an annual decrease in the prevalence of infection.

Monitoring: all control programmes should be monitored to make sure they are working and where this indicates lack of progress, the strategy should be reassessed in consultation with your veterinary practitioner.

The most appropriate method of monitoring will depend on the herd type and status, and may include testing of BMT or individual animals (targeted or whole herd). In herds with a high prevalence, targeted testing of age groups born after the start of the programme will indicate its effectiveness.

Purchasing for store, finishing or export

markets

IBR is a recognised part of the ‘respiratory disease complex’ in herds where animals are purchased from multiple sources and mixed after purchase. These animals are often of unknown health status and have varying levels of immunity.

Transport and mixing can result in outbreaks of IBR following reactivation of latent infection and spread to susceptible animals. The severity of these outbreaks can be made worse by mixed infections with other viruses and bacteria. Vaccination (ideally in advance of movement or on arrival on farm), along with measures to reduce stress during transport and following arrival can help control these outbreaks.

IBR in herds that breed bulls for AI

Animals that have antibodies to IBR (even if as a result of vaccination) are legally prohibited from entering semen collection centres.

These herds are recommended to have eradication programmes in place (if not already IBR-free). Potential AI sires should not be included in vaccination programmes and where these are in place, careful planning to prevent accidental exposure to vaccine virus is required. Recommendations for potential AI sires include:

avoid giving live IBR vaccines (particularly intra-nasally) to animals that they are, or may be, in contact with.

Use dedicated equipment (syringes, handling equipment, etc).

Further information

Detailed information leaflets on IBR and herd biosecurity, along with answers to frequently asked questions on IBR, are available on at http://www.animalhealthireland.ie/page.php?id=126.

AHI is also organising a conference for industry on its priority programmes, including IBR, on 23 th October.

See http://www.animalhealthireland.ie/page.php?id=200 for further details.