Australia’s Mysterious Mosquito-Borne Disease: Over 100 Horses Suffer as Few Answers Surface

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This special report aired on Australian television news last night and includes footage of horses exhibiting symptoms of a strange neurological disease that has affected over 100 horses so far in New South Wales alone.

You know them by the letters: there’s WNV. EEE. WEE. And in Central and South America, VEE.

What are they? They are all mosquito-borne diseases, known as? variations of “arbovirus” in the world of contagious disease specialists.

And this time of year, you know you’re going to be getting a lot of reminders to vaccinate your horse against the mosquito-borne diseases that threaten horses in the United States. But what if those pesky mosquitoes are carrying a disease that isn’t even on our radar screen yet?

After all, there are arbovirus forms of mosquito diseases that you’ve probably never encountered, like Japanese encephalitis, or Ross River virus, or Murray Valley encephalitis. But hang on, you’re about to learn!

That’s what horse owners in Australia are going through right now. A learning process. Call it Arbovirus 101. With their horses’ lives on the line.

As you’ll recall, Australia was under siege back in the fall (their spring) by torrential rain and the inevitable floods they created. As a result, their summer months were plagued with massive swarms of mosquitoes. And some horses have been referred to vet clinics with disturbing neurological symptoms of a disease that has veterinarians, researchers and the government wondering just what is going on: is there a new disease affecting these horses? And how can it be prevented from affecting more horses, and from recurring in future years?

To make matters worse, some horses have died from the effects of the disease.

Call out the disease cavalry! Australia takes infectious diseases very seriously, and the memory of the 2007 Equine Influenza (EI) outbreak is still fresh in their memories. That was the first time that EI had been seen in the nation, and the horses were completely defenseless. The disease–which is like a mild flu and passes quickly through most normal horses but is also highly contagious–shut down the horse industry in Queensland and New South Wales: no racing, no showing, no breeding. But that was 2007. It’s over, right?

Maybe not; or maybe another disease is threatening to reawaken those biosecurity blues. The prospect of losing horses to an unknown mosquito virus is disquieting anywhere on earth, but it seems like the Australians have enough to deal with without this extra punch. I’ve been collecting what information is available to keep you up to date on this story; in other words, I’ll share with you everything that no one knows.

Australians take biosecurity seriously. So should you. Flickr photo thanks to Steve Reid.

According to the Australian Horse Industry Council’s information page on the disease outbreak, veterinarians around Australia have reported 100 cases in New South Wales, 60 in South Australia and about 90 in Victoria.? On Monday, the total in New South Wales was upped to 110 by the New South Wales Department of Primary Industries. No cases have been seen in Queensland.

In the Victorian community of Cardinia, 15 horses have died. In the horse-dense Hunter Valley of New South Wales, Dr Deb Racklyeft reports treating ten cases in an article on Monday’s ABC News web site. One of her cases has died.

Australian Veterinary Association President Barry Smyth believes that a mosquito-borne virus has caused the deaths. “The weather patterns of the past summer have created ideal conditions for the spread of mosquito-borne disease,” Dr Smyth said, regarding the situation.

He further remarked, “The symptoms of the disease are depression, mild colic and nervous signs, including increased sensitivity to touch and sound, facial paralysis, difficulty chewing and exaggerated limb movement and weakness. “

The Australian Veterinary Association is now warning horse owners to take precautions against what they are calling an “unidentified mosquito-borne disease with unusual neurological signs”.

“These cases are still under investigation; however, they may result from infection with a mosquito-borne virus in the Kunjin-West Nile group,” said Dr Christine Smith, President-Elect of the Australian Equine Veterinary Association.

“Most of the horses involved appear to be recovering slowly, however there have also been reports of a few deaths,” said Dr Smith.

According to information for horse owners posted by our friends at Equestrian Australia, necropsies are scheduled for the bodies of infected horses who died or were euthanized; these tests should help to determine the cause of the infections.

“It is important that veterinarians and any assistants take stringent precautions when performing necropsies on horses showing neurological clinical signs,” warned the announcement from Equestrian Australian. “Great care should be exercised when handling brain and spinal cord tissue and appropriate personal protective equipment should be utilized as part of a risk management approach to personal safety.”

All of the horses tested by the New South Wales Department of Industry and Investment so far have been negative for the deadly Hendra virus.

The Australian authorities seem to have some idea of what this disease isn’t, even if they don’t yet know what it is. According to the Australian Equine Industry Council:

  • Laboratory testing of samples from the horses with soreness indicates that most infections are probably due to Alphavirus infections, including Ross River virus.
  • Laboratory testing of samples from horses with unusual neurological signs suggests that a majority of cases are due to infection with one or more viruses belonging to the Flavivirus group of viruses that includes viruses like Murray Valley encephalitis virus and Kunjin virus. Testing of samples to date has ruled out Japanese encephalitis virus.

An interesting, and unsettling, point made by a report from the New South Wales Primary Industries group is that it is possible that many horses would test positive for a virus but only some horses are showing symptoms.

It’s easy for authorities to advise horse owners to protect their horses with fly spray and sheets and not to turn the horses out at dusk, when mosquitoes seem to be hungriest. And it’s just a matter of being pro-active to remove standing water from around your property. But what can horse owners do to ease their anxiety?

The air is full of mosquitoes, and every one of them is a question mark until someone unravels the mystery of what’s behind the disease affecting so many of Australia’s wonderful horses.

But knowing the Aussies, they’ll get it done.

Thanks to the Australian Veterinary Association, Equestrian Australia and the Australian Horse Industry Council for their excellent reports to horse owners. The best resource I can recommend to horse owners and anyone tracking this problem is the New South Wales Primary Industries article, Flavivirus Nervous Disease in Horses, which was published this week.




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