Dog infects four humans with the plague: Researchers fear outbreak could involve human-to-human transmission

The outbreak of the pneumonic plague is the largest in the US in over 90 years

A bubonic plague smear, prepared from a lymph removed from an adenopathic lymph node, or bubo, of a plague patient (Getty Images)

A dog has spread the plague to four people in Colorado, in an outbreak officials say could involve the first person-to-person transmission of the infection in the US in 90 years.

A study published by the US Centers for Disease Control and Prevention found the infection was spread when a pitbull terrier was taken ill and eventually put down by vets. The two vets who treated the animal, his owner and a friend of the owner developed similar symptoms as the dog shortly afterwards and tested positive for Yersinia pestis, which causes the plague.

All four were treated with antibiotics and made a full recovery, NBC reports.

Dr John Douglas, director of Colorado's Tri-County Health Department, said there are eight cases of the plague in humans each year on average. "Plague is virtually always confined in this day and age to rural regions in the West," he told the network. "That is because the vector of plague is typically the prairie dog although there are other rodents that can transmit as well."

Tri-County's Janine Runfola said the dog was coughing up blood, which is likely how his owner was infected.

The two vets treated themselves with antibiotics before being taken in for monitoring and did not become seriously ill, something the researchers believe suggests the plague could sometimes infect humans without their knowledge.

However, the owner’s female friend was a more troubling case for the team. "She also had contact with the dog and also had more intensive contact with the patient when he got sick," said Dr Douglas. The timeline of events suggest she is more likely to have contracted it from the owner, not the dog. "[But] there's no way to be sure that she also didn't get it from the dog," he added.