Flu outbreaks a jolting reminder of ever-present threat

February 12, 2008

Thomas M. Chambers, PhD, backgrounds the threat posed by equine influenza and says effective quarantine measures remain the best prevention against its introduction in a world in which horses travel globally.

The 2007 outbreaks of equine influenza (EI) in Japan and Australia are a jolting reminder that influenza viruses use simple but very effective strategies for their own survival, and that our lack of attention allows this virus to create epidemics.

One of those strategies is very high contagiousness, with the capacity to infect large numbers of horses quickly within stables or across continents. Another is the ability to cause sub-clinical infections in partially immune animals.


A veterinarian takes a nasal swab for testing.
Equine influenza is a common upper respiratory disease, which prevents the horse from being exercised. Typical clinical signs include fever, nasal discharge and a dry, hacking cough. Outbreaks can produce significant problems for horse industries by disrupting exercise and horse transport.

Equine influenza is enzootic (constantly present though affecting only a small number of animals at any one time) in the United States, Canada, Europe, Scandinavia and South America. Most other parts of the world have seen repeated outbreaks.

The historic exceptions were Australia, New Zealand and Iceland. Australia and New Zealand, which import and export horses, have mandatory vaccination and quarantine systems designed to stop the viral disease, whereas for horses entering the United States, there are no specific regulations regarding EI.

Where influenza is enzootic, its level of morbidity (relative severity of disease) can range from severe to inapparent, depending on the animal's prior exposure. But even in previously unexposed horses, with the exception of young foals, fatalities are rare.

Once horses are infected by EI, opportunistic bacterial invaders can cause secondary infections that produce more severe complications.

The EI outbreak in Japan started on August 15, 2007, and was largely under control within about one month, although scattered cases persisted through the autumn. The number of horses affected and severity of disease during the outbreak were markedly less than Japan experienced in its most recent previous outbreak in 1971-72.

The Japan Racing Association attributed the decrease to its long-standing requirement for twice-annual EI vaccinations. The virus isolated was highly similar to a known reference strain of EI, Wisconsin/03.

In the Australian EI outbreak, clinical signs of the index or earliest documented case were first observed on August 17, 2007, in horses newly imported from Japan, the United Kingdom, Ireland and the United States that were quarantined in Sydney.

These horses remained in quarantine, but on August 22, other cases were observed elsewhere in Sydney. How the virus got past the quarantine may never be known. The virus was most likely transmitted on clothing or equipment of persons who had contact with the imported horses, or on a contaminated horse trailer.


Decontamination of horse floats during the Australian outbreak.
It appears that EI was unknowingly transmitted to at least one resident horse that carried it to an event held 100 miles away at Maitland on August 18-19. Once there, it spread to many other horses and, in short order, EI was distributed to hundreds of premises in the states of New South Wales and Queensland in eastern Australia.

Unlike Japan, resident horses in Australia were never vaccinated for EI. Thus, the population was highly susceptible, and once introduced, EI was able to spread very rapidly. Movement restrictions on all horses were rapidly instituted, restricting the spread to eastern New South Wales and southeastern Queensland. No cases were identified elsewhere in Australia.

Within the affected areas, over 50,000 horses contracted EI. Some fatalities were reported and though the actual number is unknown, it appears to be very low.

The Australian government has pursued a goal of eradication and return to EI-free status. It instituted a strategic vaccination program on September 17. However, the government's aim is to stop vaccinating once the outbreak has burned itself out. The last known cases were reported on December 24.

Influenza is a moving target. The viruses mutate and gradually change so that the vaccinated horse's immune system no longer recognises them. Vaccines need to be updated to keep up with the changing virus.


Australian horses were microchipped following vaccination against EI during the 2007 outbreak.
The Australian EI virus (Sydney/2007) is also very similar to the Wisconsin/03 strain. Vaccine strain recommendations since 2004 include a Wisconsin/03-like strain, so these latest outbreaks do not change the recommendations.

At present, no EI vaccine has a strain more recent than 2002. The United States Department of Agriculture has recently adopted a policy to streamline the updating of EI vaccines, so it is to be hoped that vaccines with strains similar to Wisconsin/03 and Sydney/07 will appear in short order.

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