Plague has surfaced several times in major, world-wide epidemics, one of the most important of which was the so-called "black death" that devastated Europe in the 14th century, when an estimated 25,000,000 people died. Historically, plague outbreaks have started slowly with transmission of the disease agent, a bacterium, via the bite of infected rodent fleas, primarily the Oriental rat flea, but then developed into rapidly growing epidemics involving human-to-human transmission (see below).
The initial infection following feeding by an infected flea is marked by painful, swollen lymph nodes, initially in the groin region. This form of plague is known as "bubonic plague," which, if untreated, can develop into a form of plague known as "septicemic plague." In septicemic plague, bacteria invade the blood and infect major body organs. In addition to damaging organs and causing internal hemorrhage, septicemic plague can develop rapidly into a highly infectious disease condition known as "pneumonic plague." In pneumonic plague, bacteria infect a patient's lungs and may be spread via coughing. If untreated, fatality rates of septicemic and pneumonic plague are as high as 50-90%, and they approach 15% even when treated promptly with antibiotics.
Today, plague exists as a disease of wild rodents in most of the arid regions of the world, including the western U. S. and southwestern Canada. Every year, a small number of people (approximately 10-15 per year) in the southwestern U.S. become infected with plague, typically after venturing into or living in or adjacent to areas in which they are bitten by infected fleas that are parasites of wild rodents. One of the most important vectors is a flea species with the scientific name Oropsylla montana. This flea is a parasite of the rock squirrel, Citellus variegatus, which is a common reservoir animal, from which fleas acquire plague bacteria when they take a blood meal.
- A bacterium with the scientific name Yersinia pestis (Pasteurella pestis in older literature). It is a highly infectious, invasive, and dangerous bacterium.
- World wide, 1,000-3,000 cases per year, primarily in arid regions of Asia, Africa, and the Americas.
- In the U.S. from 1971-1995, 1-40 cases per year (average of 13), nearly all in southwestern states.
- Bubonic plague: high fever, chills, and painful, swollen lymph nodes.
- Septicemic plague: rapid rise of high fever, abdominal pain, and internal hemorrhage.
- Pneumonic plague: high fever, breathing difficulties, coughing, and bloody sputum.
- Numerous species of wild rodents and several species of rodents that live in or near humans, including the rock squirrel and certain species of ground squirrels and chipmunks.
- NOTE: Prairie dogs typically die too quickly of the infection to be considered reservoir hosts.
- In rural regions: numerous species of fleas that are parasites of wild rodents.
- In urban areas: the primary vector is the Oriental rat flea Xenopsylla cheopis.
- Bubonic plague: via the bite of infected fleas.
- Pneumonic plague: human-to-human via air-borne droplets associated with coughing.
- NOTE: Cats also can develop pneumonic plague, and cat to human transmission has been reported.
- NOTE: Unlike cats, dogs do not appear to develop pneumonic plague.
- Bubonic plague: swollen and painful lymph nodes, severe headache, fever, and shaking chills.
- Pneumonic plague: pneumonia-like symptoms such as coughs, bloody sputum, chills, and high fever.
- Septicemic plague: difficult to diagnose (except by blood analysis) and progresses very rapidly.
- Physician prescribed antibiotics administered within hours of the onset of symptoms of bubonic plague.
- Antibiotic treatment of pneumonic and septicemic plague often is unsuccessful.
- There is no vaccine available in the U.S.
- Restrict human access into areas in which plague transmission is occurring in wild rodents.
- Use an insect repellent such as DEET when entering plague-infested areas.
- Be aware of the possibility of pneumonic plague, especially in cats that exhibit signs of pneumonia.
- Avoid landscaping that encourages wild rodents in and adjacent to residential and recreational areas.
- NOTE: Large boulders used in landscaping often encourage nesting by the rock squirrel.
- Monitor pets that venture into wild rodent habitat for the presence of rodent fleas.
- Consult a veterinarian regarding flea control on pets.
- NOTE: The cat flea is not a vector of plague.