Dr. Arnold Plotnick, D.V.M.,
V.P. Bergh Memorial Animal Hospital, ASPCA
In the year 2000, it seems strange to be writing about plague, a disease of antiquity that has periodically decimated the world’s human population. There are no statistics on how many animals have died from the plague. A worldwide epidemic of plague in 542 killed an estimated 100 million people, including one-quarter of the population of Europe. Another epidemic, commonly referred to as the “Black Death,” occurred in 1346 and lasted 300 years, resulting in 25 million deaths. A third epidemic has extended into this century. Between 1965 and 1970, more than 25,000 cases of plague were reported in South Vietnam; the actual number of cases could be as high as 250,000.
“A plague on both your houses…”
The plague is and will remain a threat to human and animal health. Caused by Yersinia pestis bacteria, plague is maintained in wild rodent populations in semiarid areas all over the world. Of the 383 confirmed cases of human plague in the United States between 1944 and 1995, more than 50 percent occurred in New Mexico, and a majority of the rest in Arizona, California and Colorado. In most of these cases, the disease was transmitted to people via the bite of a flea.
Typically a flea will bite a plague-infected animal and consume its blood. Following that, the Yersinia organism multiplies in the stomach of the flea and is transmitted to the next animal that the flea bites. In the United States, prairie dogs, rock squirrels and ground squirrels are the most commonly infected wild hosts, while dogs and cats may transport the flea from wildlife to humans.
In addition to flea bites, cats may contract the plague by consuming infected rodents, and humans by handling infected rodents brought home by their dog or cat. Depending on the form of plague they have (see below), pets, particularly cats, may transmit Yersinia pestis through bites and scratches or by sneezing and coughing. The first report of a domestic cat transmitting plague was published in 1977, and a large percentage of proven cases of plague have occurred in veterinarians and their assistants. Why cats are more prone to develop plague than other carnivores who consume rodents is not known.
Three faces of plague
Plague presents itself in three forms: bubonic, pneumonic or septicemic. A typical scenario might develop as follows: A cat eats a plague-infected rodent. As bones from the carcass scrape or pierce the oral cavity or throat, infection occurs. If the organism can be contained at the level of the lymph nodes of the mouth and throat, bubonic plague results. If the organism breaks through the lymph node barrier and enters the blood stream, it may be filtered by and confined to the lungs, resulting in pneumonic plague. If, however, the organism overwhelms the cat’s defense mechanisms, a devastating blood infection occurs, resulting in septicemic plague. This form of the disease often impairs the ability of the blood to clot. As widespread hemorrhage and bleeding occurs, tissues take on a reddish black discoloration; hence the name, “Black Death.”
Analysis of 119 documented feline cases of plague showed that 53 percent were bubonic, 10 percent were pneumonic, 8 percent were septicemic, and 29 percent were unclassified (but presumed to be septicemic). No particular age, breed or sex of cat is more susceptible than others. The nonspecific signs of lethargy, fever and poor appetite were commonly seen. Many cats had abscesses of the lymph nodes beneath the jaw. Overall mortality in this study was 33 percent, with the pneumonic form having the greatest risk of death.
Today, with appropriate antibiotic therapy, mortality is about 5 percent to 20 percent. The tetracyclines seem to be the most effective antibiotics in uncomplicated cases. So far, the organism has not developed resistance to antibiotics. Efficacy of antibiotic therapy, however, for both humans and animals, depends on timing. If given more than 24 hours after onset of the pneumonic or septicemic form of the disease, antibiotics are of little benefit, and the prognosis becomes grave.
Cat owners who live in plague-endemic areas should prevent their cats from hunting and eliminate outdoor sites that might harbor rodents, such as piles of wood or brush. Meticulous flea control with the new once-a-month topical insecticides will reduce the risk of transporting plague-infected fleas into the home. Better yet, keep cats indoors. With care, diligence and luck, the 21st century will not see millions of its citizens, or their animal companions, succumb to this ancient fate.
Dr. Plotnick was former vice president of ASPCA Bergh Memorial Animal Hospital.