At least seven ponies at Chincoteague have recently contracted the illness and undergone surgery. The volunteer fire company said the fungus “looks exactly like sesame seeds.”

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It “bunches up in one area like grapes and then [spreads] to other areas,” it said.

Swamp cancer, or pythiosis, is a fungal-like infection that typically involves lesions on a horse’s lower limbs. Horses can contract it through a wound when they stand in water in which the pathogen is present.

The other ponies that have had surgery seem to be recovering. Volunteer fire officials said in one Facebook post that “it’s a waiting game.”

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Nancy Finley, who manages the Chincoteague National Wildlife Refuge, said the fungus-like organism can cause an infection in animals if they have a break in their skin. The cases at Chincoteague are unusual for Virginia, she said, with most cases occurring in Florida.

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Success rates for treating the illness are higher if it is caught early, but the horses at Chincoteague are allowed to roam free and are not observed daily.

Officials are trying to determine where the horses are coming into contact with the organism.

Finley said the spores could be on plants or in pools of water, adding that officials might be able to contain the outbreak by keeping horses away from certain areas or changing grazing patterns. U.S. Fish and Wildlife Service officials will work with veterinarians and biologists to find the source.

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Experts are expected to collect samples of plants, soil and water to “see if we can get this mess contained,” according to the Facebook post.

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Last year, there were isolated cases in which ponies at Chincoteague contracted the illness, but this is “the first incident at this magnitude,” Finley said.

Chincoteague and nearby Assateague Island are known for their free-ranging herds of wild horses. An annual pony swim in the summer between the two spots draws thousands of visitors.