The U.S. Centers for Disease Control and Prevention (CDC) reported that a plague-infected dog spread its disease to four Colorado residents, the first report of a dog infecting a human with the plague in the U.S.

The 2-year-old American pit bull terrier fell ill last summer with a fever and jaw rigidity along with other symptoms, and was euthanized the following day at a local veterinarian’s office due to rapid decline in health. The dog’s owner was admitted to the hospital four days later with a fever and a bloody cough. The first blood culture performed was misidentified, but when redone, the disease was identified as pneumonic plague, according to the CDC report. The remains of the dog also tested positive for the plague bacteria. The owner recovered from the potentially fatal disease after 23 days. A close contact of the owner as well as two veterinarian employees that treated the dog were infected as well but have fully recovered.

“Frankly one of the biggest surprises of this outbreak is the source,” said John Douglas, of Tri-County Health Department in Colorado to ABC News. “Primarily … dogs don’t get sick at all or they get a minor illness [after being infected with the disease].”

Janine Runfola, of the Tri-County Health Department in Colorado noted that cats are more likely to infect humans than dogs because they exhibit more symptoms after being infected. The plague is known to kill prairie dogs in the American Southwest in large numbers, which can then cause small outbreaks in domestic animals or humans. With only eight estimated infections each year, the disease is very rare in the U.S. and pneumonic plague accounts for only 3 to 5 percent of these cases.

Information for this article taken from abcnews.go.com and nwpr.org