Signs of Primary infection
Following primary infection, the virus damages the internal surface of the nose and the upper airways and may enter the blood to spread to other parts of the body. Some primary infections have no apparent clinical signs while others can be very severe.
The following clinical signs may be observed during primary respiratory infection, (which is the most common form of disease resulting from BoHV-1 infections):
- Dullness and reduced appetite
- High temperature for 4–5 days
- Milk drop
- Inflammation inside the nose
- Inflammation of the pink of the eye (conjunctiva)
- Fluid discharge from nose and eyes
- Heavy and loud breathing
Sign of Latent infection
All animals that have a primary infection subsequently develop a ‘latent’ or hidden infection.
During the primary respiratory infection virus enters the nerves of the head. After recovery from the clinical signs, the virus is able to survive for the lifetime of the animal in these nerves. The virus is said to be in a ‘latent’ state and the animal becomes a lifelong carrier. However, as the virus is ‘latent’ (i.e. not replicating or causing disease) the animal shows no ill effects and does not spread virus to other animals until reactivation occurs.
In almost every case, animals with latent infection will have antibodies against BoHV-1 that can be detected in blood and milk by suitable tests. In very rare cases an animal can be latently infected but have no detectable antibodies, and may not be identified by a laboratory test on their blood serum. These are called ‘sero-negative latent carriers’.
Reactivation and secondary infection
BoHV-1 virus within latently infected animals can re-activate to start a secondary infection and spread to new animals. Secondary infections are almost never accompanied by any clinical signs in the latently infected animal, because the animal already has some immunity resulting from the primary infection. Reactivation may occur when an animal is under stress. Calving, transport, and mixing stock have all been shown to stimulate reactivation. Other stressful events (such as lameness, nutritional stress, high doses of immune-suppressive drugs [e.g. corticosteroids] other disease etc) can also stimulate reactivation of latent infection.