Plague: a veterinary perspective.

F or most people, mention of plague conjures up images of an antiquated threat. Infection by the bacterium Yersinia pestis is most often associated with the infamous Black Death of the Middle Ages, a pandemic that cost Europe a third of its population in the 14th and 15th centuries. 1 Far from being a historic medical curiosity, this zoonotic disease continues to be a threat to the health of humans and animals in the western United States and throughout the world, including Eurasia, Africa, and North and South America. 2 In addition to rodents, a number of mammalian species have been found to be naturally infected with Y pestis, including lagomorphs, felids, canids, mustelids, and some ungulates. Between 5 and 15 human plague cases are reported each year in the United States. Although most humans with plague were exposed through a bite from an infected flea, others have become infected as a result of contact with infected wild animals such as rabbits and wild rodents and infected domestic cats. 3 By quickly recognizing and treating plague in domestic animals, veterinarians protect the health of companion animals, the people who care for them, and the surrounding community , playing a vital role in public health. Organism Yersinia pestis, the causative agent of plague, is a gram-negative, nonmotile member of the family Enterobacteriacae, which resembles a safety pin when stained with Wright, Giemsa, or Wayson stains. 2 The organism is sensitive to high temperatures and dry environments , but grows slowly even at an optimum temperatures (28 o C [82 o F]). 4 If cultures are discarded prior to 48 hours, a diagnosis of plague may be missed. Several types of media will support Y pestis growth, including nutrient broth, blood agar, and unenriched agar. The small (1 to 2 mm) gray colonies are nonmucoid and have a characteristic hammered copper appearance. 4 The differential expression of transmission and virulence factors, moderated by temperature and environment, allows Y pestis to survive in flea vectors, transmit readily to mammalian hosts, and propagate in the new host. 5 This adaptability has resulted in Y pestis enzootics in rodents on every populated continent except Australia. 4 Ecology Yersinia pestis is maintained in the environment by susceptible rodent species and their associated fleas. Dog and cat fleas (Ctenocephalides spp) may become infected with Y pestis but are not efficient vectors for the organism. In the southwestern …