At least 18 horses in Pinjarra and the South West have been diagnosed with a mystery disease that may have transferred on to humans.
The Department of Health is struggling to identify the disease, which causes small, solid lumps to appear inside the mouth of those infected.
The symptoms were first found in horses three weeks ago at the Pinjarra Veterinary Clinic, but concerns have been raised after a number of reports of people developing similar lesions after handling the animals.
"We've been working a lot with the Department of Agriculture to determine what exactly the disease is," Pinjarra veterinarian Michael Gibbs said. "We think it's viral - but we need to get the results from the testing back before we know for sure.
"So far we've been taking tissue samples and doing electron microscopy to get a better idea of what we're dealing with."
It’s uncertain at this stage where the lesions have come from, but those affected aren’t showing any other signs of infection.
"It could be that it's spread from one species to another," Mr Gibbs said. "From what we've seen so far it's not particularly pathogenic or harmful to either horses or humans."
The Department of Agriculture and Food is currently conducting tests on several groups of horses throughout the South West to finalise a diagnosis.
"Testing has already ruled out the main virus of concern, vesicular stomatitis.," said a department spokesperson. "Preliminary testing suggests the lumps might be a response to a viral infection."
Possibly links have been identified between horse or equipment movements between the affected properties.
The Health Department is advising people to wear gloves and masks when coming into contact with horses.
Horse owners are being encouraged to contact their vet if any unusual symptoms are present in their animals.
