Viruses are to blame for many of the coughs and colds that develop at this time of year. But what exactly is a virus — and what should we watch out for this winter? Richard Hepburn MRCVS explains

We sometimes hear that a horse “has a virus”, especially in winter when cold, wet weather and more time spent indoors affect the function of the respiratory tract.

Coughing, a runny nose and an off-colour demeanour may well indicate a respiratory tract infection, many of which are caused by viruses.

Viruses are small, infectious agents that possess genes but don’t have any cellular structure. They can only replicate (reproduce) inside the living cells of other organisms.

Invasion by a virus can affect a cell in a number of ways. Viruses may lie dormant, producing no apparent change, or may alter the function of the cell. The infected cell may die, releasing new viruses that go on to infect other cells.

How do viruses spread?

Viruses are most commonly spread when horses inhale airborne particles, or “aerosols”, generated from virus-rich fluids such as respiratory secretions, urine or diarrhoea.

The most infectious fluid may not come from the location of the clinical signs — for instance, equine rhinovirus causes respiratory signs, but is spread via urine.

People attending infected animals and handling yard equipment such as water buckets can act as “vectors”, carrying the virus to other horses.

Good yard hygiene is vital, both to prevent and contain viral outbreaks.

Is that cold a virus?

Respiratory tract conditions caused by viral infection range from relatively common coughs and colds to more aggressive and potentially damaging diseases. But is every cough or cold a virus?

Field studies of horses with a fever and acute respiratory disease have identified viral infection in up to 60% of cases. The viruses include equine influenza, equine rhinovirus and equine herpes virus.

An affected horse typically has a fever (38.5-41°C) that lasts between one and three days. He will probably be off his feed, have varying degrees of nasal discharge — which is initially watery then becomes thick and pus-like (purulent) — and will cough. Less common signs include painful, enlarged lymph nodes and limb swelling.

Latent infections

Infection may be sub-clinical, meaning that it is not severe enough to produce observable signs, and viruses may also be present in healthy horses.

Equine herpes virus in particular is capable of establishing lifelong latent infections, where the virus lies dormant in the body and becomes reactivated or “sheds” (multiplies and becomes transmissable) during times of stress.

The effect of viral replication in the respiratory tract can lead to clinical signs that persist after the horse’s body has cleared the infection.

Reduced clearance of mucus from the windpipe can occur for up to 30 days after infection with both equine influenza and equine rhinovirus, leading to a persistent cough in a healthy-seeming horse.

Normal respiratory function will return, but an extended period out of work may be required.

Overlapping signs

While several of the early signs of these common viral respiratory tract conditions overlap, each condition has distinguishing characteristics.

Equine influenza is highly contagious, leading to significant damage to the lining of the respiratory tract 48hr after exposure.

The disease is endemic in the UK, so most horses will come into contact with it. Infection tends to be linked to horses travelling away or the introduction of new horses. Several outbreaks in the UK have occurred in the past two years.

Clinical signs, including fever, anorexia, lethargy, nasal discharge and a dry, harsh cough, typically last for seven to 14 days. Secondary bacterial infection can occur, and influenza may also predispose to the development of allergic airway disease.

Regular vaccination with a current equine influenza vaccine is crucial to control of the disease.

There are at least five herpes viruses that infect horses — EHV-1, 2, 3, 4 and 5. EHV-1 and EHV-4 are ubiquitous among equines; horses will typically be latently infected within the first months of life. These viruses produce similar but milder respiratory signs to influenza.

EHV-5 is associated with equine multinodular pulmonary fibrosis, where progressive scarring (fibrosis) of the lungs occurs and is often fatal. Early cases can resemble allergic airway disease.

Equine rhinovirus is associated with acute, feverish (febrile) respiratory tract disease, with a copious, watery nasal discharge that later becomes purulent.

Rhinovirus can also be part of a multiple infection with other viruses, such as equine herpes virus, or bacteria.

Diagnosis and treatment

Diagnosis of viral infection is based upon identification of viral particles, their genetic material, or the horse’s antibody response to the infection.

Your vet will choose the most appropriate tests to perform on various bodily fluids or tissue samples, based upon the clinical presentation of individual cases or herd outbreaks.

Routine blood tests such as haematology and acute phase protein testing cannot distinguish between early stage viral or bacterial disease. Repeated testing, however, can suggest the presence of viral disease if changes typical of bacterial disease do not develop.

As most viral disease is short-lived, general supportive care and anti-inflammatories are typically all that are needed. Antibiotics have no effect, although they are used when secondary bacterial infection occurs.

Preventative measures

Hygiene and biosecurity are key. In general, wash your hands between attending to different horses and avoid sharing feeding or mucking out equipment.

New horses should be quarantined and observed for development of a fever.

More thorough biosecurity is needed in the face of an outbreak — your vet can advise you of steps to take if this should occur.

This article was first published in Horse & Hound magazine (20 November 2014)