Veterinarians advised to watch for drug-resistant ringworm infections

U.S. public health officials are asking veterinarians to be vigilant in watching for antifungal-resistant ringworm infections caused by a novel fungus that emerged on the Indian subcontinent a decade ago and now poses a threat to humans and animals worldwide.

Human cases of ringworm infection attributed to Trichophyton indotineae, a highly infectious and often drug-resistant species of fungus, have been reported on six continents, and for the first time in March 2023 in the U.S., according to a May 12, 2023, Morbidity and Mortality Weekly Report.

Infections of T. indotineae are characterized by widespread inflamed or dry and scaly pruritic plaques of tinea corporis, cruris, or facia on the body, crural fold, pubic region and adjacent thigh, or the face, the report’s authors wrote. Overuse and inappropriate use of topical antifungals and corticosteroids is believed to have contributed to the current epidemic.

Dogs and cattle in India and Iran have also been diagnosed with T. indotineae infections. While no animal cases have been reported in the United States, the U.S. Centers for Disease Control and Prevention (CDC) is encouraging veterinarians to be on the lookout for patients with severe ringworm infection that’s difficult to treat.

“We want to encourage U.S. veterinarians to be aware of potential animal infections of T. indotineae because of its high person-to-person transmissibility and its isolation in dogs and cattle,” said Jeremy Gold, MD, a medical epidemiologist with the CDC’s Mycotic Diseases Branch.

The CDC is not aware of any cases in which T. indotineae spread from humans to animals, but given the fungus’s infectiousness, it cannot be ruled out, Dr. Gold told AVMA News.

In humans, itraconazole is generally effective against T. indotineae infections, but rare cases of itraconazole-resistant cases have been reported.

Culture-based techniques cannot distinguish the fungus from other Trichophyton species. Identification of T. indotineae requires advanced molecular testing, which is only available at select laboratories.

The CDC acknowledges that data on T. indotineae infection in animals are limited. As a result, it is not known whether and how often transmission of the fungus between animals and humans is occurring—or vice versa—or whether and how the clinical presentation of T. indotineae infection in animals differs from ringworm caused by other dermatophytes.

“There’s a lot we are still learning about drug-resistant ringworm, and veterinarians are often like the eyes and ears of public health,” Dr. Goldman said. “So, it’s important for veterinarians concerned about possible antimicrobial-resistant ringworm to reach out to their public health officials or state public health veterinarian for support because they can provide useful information.”