A campground at Yosemite National Park was temporarily closed on Friday, August 21 after a child fell ill with the plague while camping during the previous week. Officials have determined that infected squirrels at the Tuolumne Meadows Campground, abut 50 miles west of where the family was camping, were the vector of disease transmission. Fortunately, the infected child, a Los Angeles County resident, has since fully recovered.
Park officials will be treating this outbreak with flea insecticide to squirrel burrows, as well as chipmunks and other rodents in an “extremely precautionary public health measure,” according to park officials, to prevent further plague cases.
Transmission of plague still requires a vector—commonly fleas, which feed on blood and depend on hosts. In this case, the vector is squirrels. In an evolutionary adaption, the plague bacteria multiplies within the flea and blocks the passage to the flea’s stomach, causing the flea to feel continually starved. This mechanism forces the flea to relentlessly seek out hosts and feed until it vomits infected blood back into an organism’s wound, turning it into a host. The flea eventually dies from starvation as the plague propagates through the lymphatic system of the new host.
“Although the presence of plague has been confirmed at Crane Flat and Tuolumne Meadows campgrounds, the risk to human health remains low,” said the California Department of Public Health (CDPH). Generally, the highest risk of infection is in the areas surrounding foothills, mountains, and even along the coast of California.
The bubonic plague is a lethal infectious disease caused by the enterobacteria Yersinia pestis, named after bacteriologist Alexandre Yersin, who discovered the bacillus in 1894. There are three forms of plague: pneumonic, septicemic, and bubonic. All three have severely impacted human history, but the most commonly known epidemic is the Black Death, which led to high mortality rates in Europe, China, and Africa.
To this day, plague infects thousands of people worldwide, but medical advances have greatly decreased the mortality rate of the disease. Waldemar Haffkine, a Russian bacteriologist, developed the first plague vaccine to combat bubonic plague in 1897. Antibiotics can be used to effectively treat the plague, given a timely diagnosis.
The plague generally spreads to the lymph nodes of a human, where the bacteria causes haemorrhagic inflammation and subsequent expansion of lymph nodes—causing the bulbous swelling that led to the name “bubonic” plague. The lymph nodes are eventually overwhelmed and the infection enters the bloodstream to overtake any part of the body.
Fortunately, plague is relatively rare in this day and age. Around 42 cases have cropped up in California since 1970, not including this most recent case at Yosemite.
“Although this is a rare disease, and the current risk to humans is low, eliminating the fleas is the best way to protect the public from the disease,” said Dr. Karen Smith, CDPH director and state health officer. Campers are advised to avoid interactions with rodents and rodent burrows.