disease of cattle caused by a virus called bovine herpes
virus-1 (BoHV-1), but more commonly known as IBRV
(IBR virus). Infection in adults can also cause sudden milk
drop, conception failure and abortion, while in calves it
may cause inflammation of the throat or nervous signs.
Not all animals infected animals show obvious signs of
disease (sub-clinical infection).
Infection is widespread in Irish herds, with approximately
75 per cent of both dairy and beef herds containing
animals that have been exposed to IBRV. Generally, older
animals are more likely than younger animals to test
positive. How does the virus spread between animals
and herds? The virus is mainly spread directly by close
contact between animals. It can also be shed from
the reproductive tract, including semen, resulting in
venereal transmission. Airborne spread may also occur
over distances of up to 5m. Indirect transmission within
or between herds can also occur through movement
contaminated equipment or personnel or sharing of
cattle is associated with the development of immunity,
but this does not eliminate 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. Latent
carriers can be detected on a blood test. During this
period of latency, the carrier is not shedding virus.
Episodes of stress can however lead to re-awakening
(reactivation) of the latent infection (Figure 1), followed
by shedding of the virus, typically without clinical signs
other susceptible cattle, which in turn also become
latent carriers (Figure 2). Transport, calving, lameness,
nutritional stress, mixing stock and other diseases are all
potential triggers for reactivation.
These latently infected carriers play a central role in
maintaining IBRV in infected herds, where they act as
a reservoir of infection. In infected dairy herds, this
reservoir often exists in the main milking herd due to
routine management which results in little contact
between the milking herd and young stock. As a
consequence, heifers (in the absence of vaccination) are
susceptible to infection at some point after calving.
In infected suckler herds, the main reservoir of infection
is again typically in the breeding stock. Given the much
closer contact between age groups, calves are at great
risk of exposure than in dairy herds.
In uninfected herds, the introduction of latent carrier
animals is the most likely means by which infection will
be introduced. This can be a particular problem where
cattle are being purchased 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.
Transport and mixing can result in outbreaks of IBR
following reactivation of latent infection and spread to
susceptible animals. Vaccination, (ideally in advance of
movement or on arrival on farm), along with measures
to reduce stress during transport and following arrival
can help can help control these outbreaks.
o cially recognised by the EU as being IBR-free,