The first instances of a diphtheria-like illness being transmitted between people and their pets in the United States have been reported in Utah and Colorado. These cases, which occurred in 2022 and 2023, involved the bacterium Corynebacterium ulceran, a close relative of the diphtheria-causing germ.

In Utah, the first case was documented in 2022. A diabetic patient developed a non-healing leg wound, which testing revealed was colonized by C. ulceran. Further tests found that the patient's spouse and two of their three cats also carried the bacterium. Everyone in the household, including the pets, was treated with antibiotics, and the infections were eventually cleared.

The second case, reported in 2023, involved a Colorado resident with persistent upper respiratory symptoms. This patient lived with a spouse and two dogs and had a visitor with another dog. Testing revealed C. ulceran in the patient and the visitor’s dog. All household members received erythromycin treatment, and the patient recovered.

C. ulceran, although harmful to livestock and wildlife, rarely passes between humans and pets. These cases highlight the need for increased awareness among doctors. The symptoms of C. ulceran are very similar to diphtheria, and routine vaccination with diphtheria toxoid-containing vaccines may help protect against it.

Researchers, led by Amanda Metz of the Colorado Department of Public Health and Environment, emphasized that it is still unclear whether the humans contracted the bacterium from their pets or vice versa. Notably, C. ulceran does not spread from person to person.

The findings were published on June 13 in the Morbidity and Mortality Weekly Report by the U.S. Centers for Disease Control and Prevention.