Taking a break from the latest pandemic microbe, there have been a couple of recent items about another very old pandemic bug that’s never really completely gone away – Yersina pestis, known commonly as plague, and the cause of the Black Death of the mid 1300s, aka the deadliest pandemic recorded in human history.

Even though we now know what causes plague (a bacterium) and how its transmitted (primarily by fleas, but also some routes of direct transmission), its various forms (bubonic, pneumonic and septicemic) and even how to treat it (antibiotics), this disease still rears its ugly head periodically in various parts of the world, including North America.  Make no mistake, it is still a serious and potentially fatal disease, particularly when its signs are not recognized and appropriate treatment is delayed.  This was the case in two dogs from Colorado that were described in a recent report in BMC Vet Research (Schaffer et al. 2020).  Both dogs presented with fever and vague signs of illness, but within 24 hours they were coughing up blood (hemoptysis).  In the first dog they suspected a possible pulmonary contusion or rodenticide toxicity, and the dog was euthanized the next day.  The true cause wasn’t confirmed until 9 days later when the dog’s owner was also hospitalized with signs of pneumonia.  The second dog underwent surgery to have part of its lung removed, thinking it may have had a foreign body/aspiration pneumonia.  When the removed piece of lung yielded Y. pestis five days later, that dog was also euthanized.

There are a few noteworthy points about these two cases:

  • The dogs were both from an endemic area where plague is known to circulate in the wildlife population, but clinical infection in dogs is uncommon, and infection with the pneumonic form in dogs is exceedingly rare.
  • The pneumonic form of the disease is particularly dangerous for others, as infected animals (or people) can expel the bacteria in their sputum when they cough.  Four cases of plague in people were detected due to exposure to one of these dogs, but the public health investigation involved contact tracing of over 100 people in each case.
  • The lobar pattern seen on radiographs in both dogs is also atypical, so these animals had an uncommon presentation of an uncommon disease that is even more uncommon in this species, and in one case it was also outside of the typical transmission season for plague – not an easy diagnosis to make right away. The automated bacterial identification systems used in these cases also delayed the diagnoses.
  • It is really important to rule out plague in animals with compatible signs, even if they’re not “typical” or at the typical time of year, when they live in (or have visited) an endemic area – for the health of the animal and the people handling it.

More recently, on the other side of the world in Mongolia where this same bacterium is also endemic, an outbreak of bubonic plague in people has been linked to marmots (a kind of large rodent, similar to a groundhog).  Marmots are considered a local delicacy, and in the process of hunting, butchering, preparing (but likely not thoroughly cooking) and eating marmots, there are lots of opportunities for transmission of Y. pestis when it is present, but the highest risk is still from the fleas on the animal.  At least 4 cases of bubonic plague have been reported in the area so far, including one fatal infection in a 15-year-old boy, all associated with consumption of marmot meat.  Several areas in the region are now under quarantine, contact tracing and testing is being conducted, and authorities are discouraging hunting and consumption of marmots.

Plague is a classic example of one of the many pathogens with which we coexist, and which is unbounded by time, space or species.

Image: A female Oriental rat flea, Xenopsylla cheopis. (CDC Public Health Image Library 22259)