Investigators from Colorado and the US Centers for Disease Control and Prevention today reported a pneumonic plague outbreak that they say is the largest one to date and that involves dog-to-human transmission, which is very rare, as well as possible human-to-human transmission, which hasn't been reported in the United States since 1924.
The outbreak occurred in July 2014, killing a 2-year-old American pit bull terrier and sickening four people—the dog's owner, two veterinary workers who handled the sick dog, and a woman who was exposed to both the dog and the index patient. The authors published their findings today in the latest issue of Morbidity and Mortality Weekly Report.
The report highlights the unusual presentation in the dog as well as misdiagnosis of the initial human case (the ill dog's owner) when an automated test gave inaccurate results, posing a risk to health workers who cared for him.
Man's illness triggers search for more cases
The first plague infection in the outbreak came to health officials' attention after Yersinia pestis turned up in a health department lab sample from a man hospitalized with pneumonia whose condition continued to deteriorate after an automated hospital lab test initially identified Pseudomonas luteola.
The resulting investigation found that the man's sick dog had had symptoms including breathing difficulty and hemoptysis and was recently put to sleep at a veterinary clinic. Following the man's diagnosis, samples from the dog were tested and were found to be positive for Y pestis.
Investigators found that three other people had been in close contact with the sick dog and became sick with plague: two veterinary clinic employees and a woman who had contact with both the index patient and the dog. All of them, like the dog's owner, recovered from their infections.
Before their diagnoses, one of the veterinary clinic employees sought emergency department treatment when her bronchitis-like symptoms didn't improve after she treated herself with amoxicillin. A few days later she was given azithromycin at an emergency department, but after she learned she had been exposed to plague, she saw a health provider who treated her with levofloxacin.
Another veterinary clinic worker started taking doxycycline on the day the dog died. A few days later she started having pneumonia-like symptoms and learned she had been exposed to plague, which prompted her to start another course of doxycycline. Medical evaluation of her lungs revealed crackles, but chest radiographs were normal. Her symptoms resolved after a full course of doxycycline.
The fourth patient, a woman who was a close contact of the dog's owner, got sick 4 days after he started having symptoms. She had handled the dog after it died, getting blood on her hands. After the man's infection was identified as plague, she was hospitalized and treated for the disease.
In total, 114 people were identified as contacts of the dog or one of the sick patients: 36 in the veterinary setting, 58 in human medical settings, and 20 others. Antibiotic prophylaxis was recommended for 88 who were interviewed within 7 days of exposure, and the remaining 26 were asked to self-monitor for fever for 7 days.
Unusual features, useful lessons
The authors said the short incubation period for human plague (5 to 6 days) favors the index patient as the source of his close contact's infection, although because animal incubation periods of up to 10 days have been reported, the dog as source can't be ruled out.
Another unusual feature of the outbreak is the presentation of the infection in the dog; plague in dogs is usually asymptomatic or mild. The authors emphasized that although rare, plague should be considered in sick dogs that have had wildlife exposure in areas where the disease is endemic.
They also pointed out that the delay in diagnosis in the index case due to misidentification by the automated blood culture system has been seen before with plague and has resulted in the exposure of health workers. The team said this outbreak reinforces the need to critically evaluate the results of automated systems.
In addition, the group observed that the clinical course was milder and hospitalization wasn't required for the vet clinic workers. They say this could have been a result of self-medication or, in one, the eventual medical prescription of azithromycin, although that antibiotic isn't recommended for plague.
Apr 30 MMWR report