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VETERINARY VIEWS
LESS USUAL EFFECTS OF EIV
The respiratory tract of exercised horses is often more susceptible to viral infection when compared to rested horses. Any immune- suppressed individual is more susceptible to viral infection – this is well known in people and in horses.
Equine influenza virus infection also has resulted in encephalitis in the natural host, perhaps due to an aberrant host immune response. Affected horses in a 2003 outbreak in the United Kingdom were unvaccinated. On necropsy, a non-suppurative encephalitis was identified similar to what has been described in humans and while not common, is mostly found in kids. Although the mechanism is not clear on how influenza virus induces encepha- lopathy (brain function affected by disease or agent), it is suspected that rather than invading neurologic tissue, it instead replicates in the central nervous system through an immune- potentiating process.
Influenza is also known to cause myocar- ditis (cardiac muscle inflammation), myositis (skeletal muscle inflammation) and recurrent airway obstruction.
CROSS-SPECIES EIV TRANSMISSION
Viruses of aquatic birds are considered the “ancestors” of all influenza virus subtypes. Infection of influenza across species has become more prevalent. The H3N8 influenza virus has infected camels in Mongolia, and canine infection has been reported during a number of equine epidemics, particularly since the 18th century. The virus persists in the canine population in the USA.
The virus can jump across species, as
for example avian flu into humans or cats. While all eight gene segments of influenza are involved in controlling influenza virus species specificity, the influenza hemagglu- tinin protein appears to play a particularly important role as it serves as the receptor binding protein, which allows the virus to attach to sialic acid (SA) receptors on the host cell surface. SA receptors are found throughout the lining of the horse’s respira- tory tract. Horses and dogs have similar sialic acid receptors, so it is thought that this allows horse-to-dog cross-species transmis- sion. In contrast, equine strains do not appear to jump to humans. What is uncom- mon is for the virus to persist in another species once it has crossed.
Influenza may persist in the new popu- lation of a different species because of its ability to adapt during the replication process. Canine flu is now considered to be its own established lineage that has genetically moved from being an equine influenza virus to
being primarily canine. Several studies have examined whether the adapted canine flu can move back to horses, especially those with no vaccine protection. The results demonstrate that ponies only got sick from the equine strain, whereas dogs get sick with both equine and canine strains.
Dogs are commonly present around horse operations, so it is relevant to consider influ- enza vaccination in your canine companions.
BIOSECURITY MEASURES
Equine influenza virus has a 24-72-hour incubation period, so biosecurity measures should include strict quarantine and daily or twice daily rectal temperatures. Horses don’t tend to shed virus until they spike a fever; coughing disseminates virus-laden droplets throughout the barn and shed row to infect others. These droplets don’t stay suspended and only travel a few feet, yet are efficient at infecting other horses. The virus is also known to spread by aerosol, i.e. particles less than five microns in size that stay suspended in the air and travel long distances. Distance between horses and between barns then becomes very important to control an outbreak.
Horses vaccinated with the modified live strain of EIV may shed virus and contribute silently to the spread of EIV. One of the most important biosecurity tools besides quarantine is to take rectal temperatures, and another is to use hand and foot hygiene.
Conclusions stated following the 2007 Australian outbreak of EIV stated: “In future outbreaks, unnecessary contact
and handling of horses should be avoided, especially by those coming from off the premises. Prior to any such contact, persons handling horses should use a footbath (if present), change their clothes and shoes, and wash their hands.” It is also important to consider food distribution around the equine facility as well as barn tools and equipment for food delivery and cleaning.
A healthy horse that has nose-to-nose contact with a sick horse may not show clinical signs, but still may shed virus because it has been exposed. Humans then facilitate spread of disease throughout the farm on their hands, clothes, and equipment by not recognizing this important point.
Besides practical and effective immu- nization protocols, farm management
has a huge impact on the possibility and extent of an outbreak. Biosecurity practices truly decrease the chance of virus enter-
ing the equine population. To achieve the best results, biosecurity practices must be implemented consistently and throughout all stabling on a premise.
EIV is highly infectious and causes high rates of sickness within an equine population. This especially impacts the racing industry and organized competitive events.
H3N8 influenza canine infection has been reported during a number of equine epidemics. The virus persists in the canine population in the USA. Consider influenza vaccination in your canine companions.
Equine influenza virus has a 24-72-hour incubation period, so biosecurity measures should include strict quarantine and daily or twice daily rectal temperatures.
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