Veterinarians have warned horse owners to be on the lookout for symptoms of Japanese encephalitis as widespread flooding and warmer days creates “the perfect storm” for mosquito-borne diseases across eastern Australia.
It comes as the Australian Medical Association has urged people to protect themselves against mosquito bites and to get vaccinated for Japanese encephalitis if they are eligible, after a slow vaccine uptake in at-risk communities.
The Japanese encephalitis virus (JEV) is transmitted by mosquitoes and was first detected in New South Wales, Victoria and South Australia earlier this year.
The virus multiplies in waterbirds and pigs, but horses and humans are both susceptible to the disease. Most cases are asymptomatic but it can also cause severe infection and death.
As of 19 October, there have been 42 cases of the virus reported in humans, with seven deaths.
There have also been 35 “highly probably” positive cases of the disease in horses in Queensland and NSW, including three deaths.
The president of Equine Veterinarians Australia, Dr Hadley Willsallen, said it was difficult to protect horses against the disease without access to a vaccine, which has not yet been approved for use in Australia.
“It has the potential to be very significant for horses, as the highest concentration of horses in Australia are located within the ranges of infected mosquitoes,” she said.
The majority of reports of the disease have been around the NSW-Victorian border, Willsallen said. The perimeter of the known outbreak now covers 600,000 square kilometres on both sides of the great dividing range.
The Charles Sturt University Equine Hospital at Wagga Wagga saw 10 likely cases of the disease this year, including three in which the horse died from the disease or had to be euthanised.
It’s possible there were more deaths that were not reported, Willsallen said.
Protecting horses from being bitten by mosquitoes is a difficult task. Willsallen recommended horse owners use a combination of rugs, masks, insect sprays, and stabling where possible.
“Only time will tell how significant the outbreak will be, however knowing how difficult it is to prevent horses from mosquito exposure, a vaccine is our best defence,” she said.
The federal agriculture department said it is working closely with the Australian Pesticides and Veterinary Medicine Authority to submit an emergency use permit application for vaccine for use in domestic horses.
But, Willsallen said, the mosquitos are already here – and notwithstanding further delays in approval and procurement, it takes four weeks after the vaccine has been administered for a horse to be fully protected.
“We are entering the perfect storm for mosquito-borne diseases, with severe and very widespread flooding and summer fast approaching,” she said. “It is a breeding haven for mosquitoes.
“The disease affects animals and humans and we need a collaborative, multidisciplinary approach to address the problem,” she said.
Dr Ken Jacobs, an equine vet who lives on the Southwest Slopes in NSW, said Australia was “woefully underprepared” for an outbreak of Japanese encephalitis.
“We are going to see more cases,” Jacobs said. “My view is that any horses that are of value should be vaccinated as soon as the vaccine becomes available.”
Most horses who contract the virus will be asymptomatic, but in some cases it can lead to severe neurological conditions and even death. Symptoms include fever, not eating, sweating, muscle twitches and muscle weakness that may progress to the point they cannot stand.
Diagnosis is difficult, as the symptoms and antibodies are similar to other mosquito-born diseases like Kunjin virus and Murray Valley encephalitis. There are tests available, but Jacobs said they frequently showed false negatives as the testing window was short.
Roughly 99% of JEV infections in humans cause no symptoms, but in serious cases it can cause swelling of the brain. Among those who have a symptomatic form of the virus, the fatality rate is 25-30%.
Researchers have estimated almost 750,000 Australians could be at risk of the virus this year.
A free vaccination is available for people who live or work in 41 local government areas of NSW, and who spend at least four hours a day outside for work or other unavoidable activities, are living in temporary or flood-damaged accomodation or are working on flood recovery efforts.
Jacobs said the free vaccine should be extended to other local government areas.
NSW Health said the vaccine supply was limited and they have placed orders for additional vaccines, which are expected to arrive in early 2023.
Dr Cameron Webb, an Associate professor of medical entomology with New South Wales Health Pathology said preventing mosquito bites could “greatly reduce your chances of being exposed” to the virus.
This includes applying mosquito repellants to all areas of exposed skin and wearing loose fitting clothing, because “mosquitoes can bite through clothing”.
“There are other mosquito-borne pathogens that are spread by mosquitoes that we would be equally on alert for at the moment, given the environmental conditions,” he said.
“Everybody, irrespective of your vaccination status, should be taking steps to avoid as many mosquito bites as possible.”
