Page 70 - Speedhorse November, 2018

INFLUENZA OUTBREAKS
Mark Crisman, DVM, MS, Diplomate ACVIM, works for Zoetis, but also teaches at the Virginia- Maryland College of Veterinary Medicine in Blacksburg. He says equine influenza has been around for centuries, with outbreaks recorded throughout North America and Europe. “One incident that focused our attention was the outbreak in Australia in 2007,” says Crisman. “Australia had been influenza- free, so horse owners there did not vaccinate for this disease, and it came into that continent with an imported horse. The native equine population
was completely susceptible. More than 10,000 premises and all horses on those premises were affected. Australia spent more than a billion dollars getting that outbreak resolved.
“The incubation period for this respi- ratory disease is very short—just a couple of days. We’ve seen outbreaks sweep through a stable or barn, and in a teach- ing hospital, where within 48 hours every horse in the barn was coughing.” he says.
There are outbreaks now and then
in various regions of the world. Thomas Chambers, PhD, Professor, is part of the international expert surveillance panel for equine influenza. This panel meets every year to review the situation from the previous year and share what was learned about outbreaks in the U.S. and around the world.
“One of the curious features about outbreaks is that their pattern is different in different parts of the world, and seems to be associated with vaccination. In the U.S., there is a certain level of vaccination in the equine population, and the disease is enzootic, which means it is always around at some low level,” Chambers says. Horses in the U.S. are likely to be exposed to this disease at some point in their lives.
“Generally, we do not see huge out- breaks in the U.S. We will see a few out- breaks at racetracks affecting maybe 20 horses, or an outbreak at a veterinary hos- pital like we saw in Oregon in October 2015 that affect a small number of horses before we get it under control. There was also an outbreak in New York a few years ago that affected about 35 horses,” says Chambers. But these are small spo-
radic outbreaks and generally localized,
although traveling horses can carry the virus and start another outbreak.
“This is what we have been observ- ing in the U.S. for a long time,” says Chambers. “By contrast, some other countries, like India, have a very different pattern, perhaps because they do not vaccinate. They have had huge outbreaks involving many thousands of horses,
and afterward the disease will disappear for about 10 years or so. Then they’ll have another huge outbreak, followed by another long period of quiet.
“They don’t routinely vaccinate in India, so we think what’s happening is that when they have a huge outbreak, just about every horse is exposed and develops some immunity. This high level of ‘herd immunity’ apparently stays in place for some years after. This has been adequate to keep the lid on the disease for a period, even in the absence of vaccination.
“That herd immunity eventually wears off and then they experience the next big outbreak. This is a striking contrast to what we see in the U.S. Here in North America, we continue to have
a few low-level outbreaks and nothing dramatic. It’s only been in the last few years that the vaccines have been catching up with the recommendation that we made in 2010—that the vaccines should contain both the Florida Clade 1 and Clade 2 strains.
“Fortunately, in the U.S. we have not had any outbreaks of the Florida Clade 2 strain for a long time. The fact it has taken so long for the updated vac- cines to reach the market has not really hurt the U.S. But because the Florida Clade 2 strains do circulate elsewhere, primarily in Europe, and because there is international horse traffic, we can’t expect that we will only have to worry about Florida Clade 1.
“Dr. Nicola Pusterla at the University of California at Davis identified some imported horses that were infected with the Florida Clade 2. Fortunately, they were quarantined. The disease was caught before these horses could start any outbreaks in the U.S. But at some point, there will probably be a re-introduction of Florida Clade
2, which makes it desirable for owners and practitioners to use vaccines with updated virus strains,” says Chambers.
68 SPEEDHORSE, November 2018
VACCINATION
There are several influenza vaccines avail- able, and these are updated periodically. “The influenza virus strains mutate, like in humans, and change slowly over time,” says Wilson. “This enables them to evade the immune system, or the protection offered by the vaccines we’re giving. If the vaccine is not the current strain, it won’t be able to prime the immune sys- tem to fight off the virus that infects the horse. The strains don’t change as quickly in horses as they do in humans. For people, a new vaccine is created every year based on what was the most common strain of influenza the previous year.”
While the virus changes don’t happen that quickly in horses, vaccine manufacturers try
to use the strains that have been involved in most of the outbreaks within the previous five years. “The equine vaccines do get updated, but sometimes not that often,” she says.
One is an inactivated, killed virus, given as an intramuscular vaccine. “The recom- mendation for those vaccines is that the horse should have an initial three-dose series of injections, a month apart, if it has never been vaccinated before,” she explains. “The horse is not fully protected until about two weeks after the final injection.” After that, the recommendation is for boosters twice a year for young horses that will be exposed and co-mingled with other horses.
The second class of influenza vaccines is
the modified live virus (MLV) vaccine. “The virus has been modified, so it is not infective. It won’t cause the disease, but it will stimulate the immune system to create antibodies. Generally, the MLV vaccines stimulate the immune system better than the killed virus vaccines because they appear more like the actual influenza virus. The MLV is an intranasal vaccine, squirted up the nose and into the pharynx. When these vac- cines are packaged, long tubes are included to attach to the end of the syringe,” she explains.
The intranasal vaccines induce immunity locally in the pharynx and nasal passages rather than stimulating the immune system to develop antibodies that circulate through the blood- stream and lymph systems. Local immunity from the MLV vaccine is probably more effec- tive because it attacks the virus where it enters the body.
For the intranasal MLV vaccine, the horse only needs one dose, even if it has never been vaccinated before. “This vaccine does not need initial boosters because it induces a totally dif- ferent type of immunity and is very effective,” Wilson says. “The recommendation is for vac- cination every six months after the initial dose.”
The third class of influenza vaccine, which should be administered every six months, is
a modified canary-pox virus that serves as a vector to carry the influenza virus parts, and
EQUINE HEALTH