Diphtheria-Like Bacteria Shared Between People and Pets, Utah and Colorado, 2022–2023

Toxigenic Corynebacterium ulcerans is an uncommon bacterium, spread between animals and people, that can cause an illness resembling diphtheria. How it passes between humans and animals is poorly understood. Two investigations reported in CDC's MMWR shed new light on it.

Utah, 2022

In April 2022, the Utah Department of Health and Human Services learned that toxigenic C. ulcerans had been found in a non-healing leg wound of a Utah resident with diabetes. The patient lived with a spouse, another housemate, three cats and a dog.

• Health officials advised covering the wound, wearing masks and disinfecting household surfaces.
• Testing of the household — all without symptoms — found the bacterium in the patient's spouse and two of the cats. Whole genome sequencing showed the cats' isolates and the patient's were the same type.
• The three people were treated with penicillin and the pets with amoxicillin–clavulanic acid, but afterward the bacterium was still found in the patient and all the pets.
• Susceptibility testing then showed the organism was susceptible to erythromycin but only moderately susceptible to penicillin, which would have needed a higher dose. After everyone — people and pets — was treated with erythromycin, no one in the household tested positive.

Colorado, 2023

In April 2023, the Colorado Department of Public Health and Environment learned of toxigenic C. ulcerans in a resident with upper respiratory symptoms that wouldn't clear up. The patient had close contact with a spouse, two dogs at home, and a visiting relative and visiting dog who stayed two nights; none had symptoms.

• The patient was advised to stay home from work and wear a mask outside the home.
• The bacterium was found in the patient and the visiting dog, but not in the visiting relative or the household's own two dogs. The patient's and the dog's isolates were the same type.
• The spouse declined testing but was treated with erythromycin. Tests showed both the human and dog isolates were susceptible to erythromycin, and after treatment neither the patient nor the dog tested positive.

What it means

• These are the first reported U.S. cases of toxigenic C. ulcerans in people with household pets colonized at the same time. The evidence suggests the bacterium passed between people and pets, though the direction couldn't be determined. Human-to-human spread has not been documented.
• Toxigenic C. diphtheriae infection is nationally notifiable, but C. ulcerans infection is not — yet it can mimic diphtheria and needs prompt identification, treatment and control.
• Penicillin or erythromycin are the recommended treatments, but these cases show that treatment of both people and animals should be guided by antibiotic susceptibility testing.
• Surveillance and routine vaccination with diphtheria toxoid-containing vaccines help protect people from severe toxin-caused illness from Corynebacterium species.
• A One Health approach — considering human, animal and environmental health together — can help control the bacterium's spread.

Sources

• Metz AR, White A, Ripplinger J, et al. "Notes from the Field: Toxigenic Corynebacterium ulcerans in Humans and Household Pets — Utah and Colorado, 2022–2023," MMWR Vol. 73, No. 23, CDC; a work of the United States government in the public domain.