This February, equine influenza generated mainstream media attention when a wide-spread outbreak of the disease caused a six-day racing industry shutdown in the United Kingdom. Coverage by news organizations such as The Guardian newspaper, the BBC and CNN highlighted the national scope of the outbreak that – as of Feb. 17 – included 29 separate outbreaks in 17 counties across England and Scotland.

In addition to the race meet cancellations, a total of 174 racing stables were placed under full lockdown by the British Horseracing Authority (BHA) after the disease was found to be present in horses that had been vaccinated against influenza. The BHAs move prompted Ireland to place a temporary ban on entries from all UK-based racehorses.

Although racing in the UK resumed by the end of the month, the February outbreak resulted in the loss of 23 racing meets and an estimated $350 million in revenue for the British racing industry.

These losses have had a few in the industry questioning the necessity of the BHA’s actions. A handful of trainers have objected to the racing ban and calling the mandatory quarantines an overreaction to a manageable and common disease. The BHA has defended its actions, saying its response to the influenza outbreak was pragmatic and evidence-based, and done for the welfare of racehorses, as well as the entire UK’s horse population.

The BHA’s response to this year’s influenza outbreak is not unprecedented. A similar situation occurred in Australia in 2007 when an outbreak of equine flu resulted in the cancellation of all racing events in Sydney for the entire spring racing season.

So why does equine influenza trigger such a strong response from racing authorities? After all, the disease is rarely fatal and, in Canada at least, vaccination against it seems to be prevalent.

About Equine Influenza

The quick answer lies in the pathology of the disease. Transmitted through airborne particles, Equine Influenza Viruses (EIV) are highly contagious viruses that infect the nasal passages, the bronchial tubes and the lungs for a period of two to three weeks, thereby severely impacting a horse’s ability to breathe and oxygenate.

And, since the respiratory tract takes 21 days to regenerate, horses can be highly susceptible to the development of secondary bacterial complications such as pneumonia, and chronic bronchitis for almost a full month after the initial onset of disease symptoms.

That’s one month of lost training and racing time.

Horses between the ages of one to five years old seem to be especially susceptible to EIV, and it’s the bacterial complications that can sometimes be deadly for foals, and for horses that have chronic respiratory problems such as asthma.

Signs and Symptoms of Equine Influenza

Typical EIV symptoms are identical to the symptoms of human flu , including:

  • fever
  • dry, harsh coughing (which can persist for several weeks)
  • bilateral nasal discharge that is usually white or yellowish in colour
  • decreased appetite
  • lethargy or depression
  • muscle weakness or soreness
  • ocular discharge

More severe infections tend to cause higher and long-lasting fever, heavier nasal discharge that is yellow or greenish in colour, and a complete loss of appetite. Audible breathing that sounds ‘wet’ or rattling is a sign of the onset of a more serious secondary infection and requires immediate medical attention.

Treatment for Equine Influenza

There is no cure for EIV, but the disease usually passes with complete rest from strenuous physical activity for two to three weeks, along with the consumption of nutritious food and plenty of water.

NSAIDs (nonsteroidal anti-inflammatory) can be used in cases of high or persistent fever, and antibiotics are often prescribed when fever persists beyond three or four days, or when nasal discharge is thick and yellowish or greenish (a possible indication that pneumonia is present).

Prevention of Equine Influenza

Vaccination is the best option for tackling EIV. Although the flu occasionally affects vaccinated animals, vaccinated horses experience milder symptoms, and recover faster. At risk adult horses can benefit from booster injections every six months, but always consult a veterinarian to determine which vaccines and what vaccination timetable works best.

The spread of EIV can be limited by hygienic management practices. Sick horses should be isolated from healthy horses, and equipment such as halters, brushes, and feed buckets should not be transferred between animals. New arrivals should be kept in quarantine, particularly if coming from locations where influenza cases are suspected or confirmed.

Equine Influenza Strains and Outbreaks

The reason why some horses contracted EIV despite vaccination is due to disease varieties, or strains. Much like horse pedigrees, the influenza viruses can be divided and classified by family strains and clades. (A clade is a group of organisms believed to have evolved from a common ancestor).

For example, this year’s outbreak in the UK was caused by Florida Clade 1 H3N8. H3N8 was first isolated in 1963 during a widespread outbreak in the US and is thought to have originated from an avian source in South America. Since 1963, H3N8 has evolved into American and European strains. In the 1980s, the American strain further evolved into the Kentucky, South American and Florida sub-strains and in early 2000s, the Florida sub-strain split into clades 1 and 2.

It’s the two Florida clades that have become the dominate forms of EIV outbreaks since the year 2000, but the other EIV strains continue to circulate as well.

Knowing which strain and clade is circulating in a horse population or geographic territory is the job of governmental ministries of agriculture, veterinary colleges and veterinarians. Vaccinations are then administered based on this knowledge because it has been scientifically proven that, in the case of influenza viruses, the more closely a vaccine matches a strain or clade the more effective the vaccine is at preventing infection.

Unfortunately, due to the transient nature of horse racing, an unexpected EVI strain or clade can popup in a population that has previously not needed vaccination against that particular form of the virus. Without vaccination against that specific strain, a disease outbreak of some size inevitably occurs.

It is important to note that the biggest causal agents of wide-spread, severe and industry-stopping EVI outbreaks are horses that are completely unvaccinated.

Equine Influenza or Worse?

EVI is rarely fatal, but there are two other, much more serious equine diseases that can be mistaken for the flu.

EVA, or Equine Viral Arteritis, presents the same clinical signs of respiratory disease, but causes vasculitis that destroys blood vessels by inflammation. It can also cause abortion. Most infected stallions become carriers of the disease and are responsible for the persistent circulation of the virus through infectious seminal fluid.

Equine herpesvirus (EHV) is actually two distinct groups of viruses – EHV-1 and EHV-4. Primary infection with both types presents as acute respiratory disease. EHV-4 usually attacks the lymph nodes and only occasionally causes abortion or neurologic disease. EHV-1, on the other hand, causes abortion anywhere from two to 12 weeks after infection. The neuropathic strain of the virus can cause lesions of the brain or spinal cord. These lesions can be mild, producing incoordination, or severe which can leave a horse unable to stand.

Vaccinations are the best prevention of EVA, EHV-1 and EHV-1.