What is 'swamp cancer' and how could a vaccine protect countless horses?

For the many horses that call Chincoteague home, a vaccine for the ominously named "swamp cancer" has provided hope for many of the cases that first appeared there in 2017.

Pythiosis, as it is officially called, is actually neither a cancer nor necessarily fatal in equine cases. It is caused by a fungus-like germ called Pythium Insidiosum, and can infect plants, mammals and birds. In horses, it appears as skin lesions when they swim through standing contaminated water and spores enters through a cut, even as small as a mosquito bite. While it can manifest on any part of their body, it commonly affects legs or hooves.

In 2017, seven Chincoteague ponies were diagnosed with the illness and eventually humanely euthanized. Just three years later, a handful of Pythiosis cases were diagnosed and successfully treated. A key to containing the illness was a breakthrough vaccine developed through a partnership between veterinarians Dr. Richard Hansen and Dr. George Marble, who had been treating the cases when they first appeared in the refuge.

"The (vaccine) was in development since late 2008," said Hansen. "The vaccine stimulates the immune system of the animal so that when exposed, the disease is prevented. In the case of SolidTech’s Pythiosis Vaccine, the zoospore component hopefully stimulates resistance against the initial infection, and the hyphae component to help the immune system to fight off the early hyphae stage that emerges from the zoospore."

According to Hansen, the vaccine targets an infection already there, and since Pythiosis lesions vary in severity, treating early before they get extensive is key. Also, augmentative treatment is almost always required since the lesion often is secondarily infected by bacteria, where antibiotics are needed. This means treating areas of affected tissue and removing hard structures in the skin lesion.

"The dosage is an initial three doses given at monthly intervals, starting during April 2019, then yearly boosters given each spring since then," Hansen said. "There were some ponies that either were too young to be vaccinated or could not be rounded up, required later in the year. Since SolidTech is a USDA Center for Veterinary Biologics, and not the FDA-licensed developer and manufacturer of biologics, there are multiple hoops that must be jumped through to obtain licenses."

Hansen noted only the federal Food and Drug Administration "approves” veterinary drug products while the United States Department of Agriculture “licenses” veterinary biological products, like the vaccine, the category under which SolidTech products fall.

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Biological products must also be shown to be “pure, safe, potent and effective."

"SolidTech initially worked with Dr. Leonel Mendoza of Michigan State University, who is a worldwide authority, to begin working toward an improved immunotherapeutic. Dr. Mendoza continues to be a valuable resource of information and experience," Hansen said.

'Swamp cancer' cases moving increasingly northward

While no cases of Pythiosis have been found in herds in Maryland, experts have been surprised by the illness occurring this far north. "Swamp cancer" is common in tropical climates due to overall temperatures, so multiple cases in the Delmarva Peninsula and even in Pennsylvania have been a concern.

"It's very interesting to have cases in (Virginia)," said Janak Dhakal, associate professor of Animal Sciences at the University of Maryland Eastern Shore. "Some causes that could be a factor are global warming and temperate climates that are coming up. They normally are in Florida and Texas and coastal areas like that. But now, Pythiosis is coming as far north as Minnesota."

According to Dhakal, it can be prevalent in cattle, with far fewer cases in dogs and a miniscule and rare number of cases in humans.

"The most important aspect of the vaccine is early detection," Dhakal said. "We must take note even when it appears to be a normal wound like from barbed wire. When the diagnoses is made as early as 15 days after the wound, the vaccine is 100% effective. Until 60 days, the vaccine can still have a good prognosis."

Taking note of swampy areas in the environment should be the start of consulting veterinary professionals for those who tend to herds of animals either in the wild or on a traditional farm.

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Virginia staying vigilant against cases

Steven Escobar, a veterinarian and president of Virginia Horse Council, echoed Dhakal's sentiments that cases of Pythiosis are being recorded in new locations where it was previously unseen.

"As we see climate change issues occur, we see warmer weather and more tropical diseases like Pythiosis. Gulf states and areas of warmer climate generally see this in stagnant water or pastures that have standing water. Veterinarians will be the best resource to provide a differential diagnosis," Escobar said.

Escobar assured that it is not yet a "significant concern" in the state, but that condition could change.

The state veterinarian, he noted, as well as the Virginia Department of Agriculture both consider the matter as one of "heightened awareness" with additional monitoring.

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This article originally appeared on Salisbury Daily Times: What a vaccine means for 'swamp cancer' in Delmarva Peninsula horses