Plague is a disease caused by a bacterial infection. The bacteria that causes plague, Yersinia pestis, maintains its existence in a cycle involving rodents and their fleas. Many types of animals such as rock squirrels, woodrats, ground squirrels, prairie dogs, chipmunks, mice, voles and rabbits can be infected with plague. Dogs and cats have been found to carry and spread plague-infested fleas. Wild carnivores can become infected by eating other infected animals. Plague bacteria passes between these mammals and their fleas in what is called an enzootic cycle. Fleas pick up the bacteria when they bite and feed on animals infected with plague. The infected fleas can then bite other animals and people, spreading the disease. Improper handling of tissue or body fluids of plague-infected animals is another way people can become infected. For example, a hunter skinning a rabbit or other infected animal without using proper precautions could become infected. This type of exposure commonly results in bubonic plague or septicemic plague, which is the most common form. Pneumonic plague, another form of plague, is spread when a person with pneumonic plague coughs plague infected droplets into the air. If these droplets are breathed by another person they can become ill with pneumonic plague. Cats usually become very ill and can directly infect humans when they cough infectious droplets into the air. Dogs are less likely to become ill, but can still bring plague infected fleas into homes. Scientists think that plague bacteria are always circulating at low rates within populations of certain rodents without causing excessive rodent die-off. Occasionally the disease moves from these rodents to other animals. This can cause an epizootic, during which many wild animals may die of plague. In this situation, people are more at risk, especially if they occur in areas with multiple types of rodents living in high densities.
Symptoms of bubonic plague include sudden onset of fever, headache, chills, weakness and one or more swollen, tender and painful lymph nodes (called buboes). This form usually results from the bite of an infected flea. Without treatment, the bacteria can spread to other parts of the body. Septicemic plague causes fever, chills, extreme weakness, abdominal pain, shock and sometimes bleeding into the skin and other organs. It results from infected flea bites or from handling an infected animal. Pneumonic plague also causes fever, headache and weakness and a rapidly developing pneumonia with shortness of breath, chest pain, cough and sometimes bloody or watery mucous. It develops from inhaling infectious airborne droplets or from bacteria reaching the lungs because of untreated bubonic or septicemic plague. This is the most serious form of the disease, as it may cause respiratory failure and shock. This is the only form of the disease that can be spread from person to person. Plague is a serious illness and should be treated promptly with the correct medications to prevent complications or death.
Recorded history tells us that plague has appeared in three major epidemics which occurred in the 6th, 14th and 19th centuries. They are known as the Justinian Plague, the Black Death and the Third Pandemic, respectively. By the middle of the 6th century, the Byzantine Emperor Justinian I had spread his empire throughout the Mediterranean and Europe. Anticipating a long-lived dynasty, his dreams were shattered in AD 540 when flea infested rats brought by ships from Egypt reached the harbor town of Pelusium. The rats and their plague-infected fleas jumped ship and spread the disease into the city. From there, plague spread to Alexandria and Constantinople, Justinian's capital, and continued throughout the empire. After the plague's devastation ended in AD 590, it had killed as many as 100 million people - half the population of Europe.
The plague of the 14th century is believed to have originated in China in the early 1340's. From there it spread along trade routes to India, Egypt, and Asia Minor. Word of this horrible disease reached Europe long before the disease arrived. It entered Europe by way of Italy in 1347. It took a staggering toll in the Italian peninsula then began its sweep throughout Europe. It reached Europe in 1348 and spread into Russia in 1351. The people of the middle ages suffered tremendously with death tolls between 20% and 40% in their cities and communities. In some areas, as many as 60% and higher died, often forcing people to abandon their towns and villages. Adding to the devastation of these kinds of losses was the mystery as to the cause of the almost certain death in those who contracted plague. Common beliefs about plague and its origin varied. Some believed it crept out of cracks in the earth left by earthquakes. Others said it was a judgment of God, and some blamed other groups of people for conspiring to poison their wells. One third of Europe is believed to have perished during this time.
The Third Pandemic began in China in 1855, spreading bubonic plague to all inhabited continents, and ultimately killing more than 12 million people in India and China alone. According to the World Health Organization, the pandemic was considered active until 1959, when worldwide casualties dropped to 200 per year. The Third Pandemic lasted for a period of 104 years. Before recorded history, plague was certainly present, but became more apparent as trade and travel to many areas of the Old World increased.
In 1894, bacteriologists Alexandre Yersin and Shibasaburo Kitasato independently isolated the responsible bacterium that causes plague. Yersin further determined that rodents were most likely involved in its transmission. Today plague is well understood. Modern antibiotics are effective treatments for plague. Vaccines are available but are not given routinely. Proper precautions to minimize exposures to infected animals and their fleas are more effective overall.
Outbreaks of human plague cases still occur in rural and urban settings around the world as well as in the United States. Plague was first introduced into the U.S. in 1900 in San Francisco by rat-infested steamships that had sailed from affected areas, mostly from Asia. Epidemics occurred in port cities including Los Angeles in 1908. Plague then spread from urban rats in port cities to rural rodent species and became enzootic in many areas of the western U.S. The last urban plague epidemic in the U.S. occurred in Los Angeles from 1924 to 1925. To date, plague is still found only in the western half of the U.S. In recent decades, an average of seven human plague cases have been reported yearly. Since 1900, there have been almost 500 human cases of plague in California. Plague bacteria have been isolated from animal or human sources in 49 of the 58 counties in California since 1900. Worldwide, plague epidemics have occurred in Africa, Asia and South America, but most human cases since the 1990's have occurred in Africa. Globally, the World Health Organization reports 1,000 to 2,000 cases per year, though the true number is likely much higher.
Plague control is accomplished most successfully by keeping rodents out of indoor areas inhabited by humans and managing them around buildings and recreational areas. This will reduce the risk of human contact with rodents and their fleas. To be effective, the most important control measure involves eliminating rodents' access to sources of food, water and shelter (harborage). Stored and fresh foods, dog food, livestock feed and even garbage must be kept in rodent proof containers or areas that eliminate access by rodents. Snap traps and poisons may be effective at killing rats and mice, but fleas quickly abandon a dead host in search of another. By eliminating the rodent you could become the next host of a plague infected flea. Treatment with chemicals to kill rodent fleas is effective but should be conducted by professionals.
Surveillance and control measures are conducted in California by trained personnel from state or local health departments and vector control agencies. Many recreational and camping areas in the western U.S. are in areas where plague can can occur. If plague is found, control measures are implemented to reduce or stop the spread of plague bacteria between rodents and fleas. Control is accomplished primarily by encouraging sanitation methods such as eliminating food sources, garbage, brush or trash piles and abandoned structures. By manipulating rodent habitat, food and shelter, plague cycles can be stopped. These activities are usually conducted only where people may come into contact with disease infested rodents and fleas.
Plague is considered rare in the U.S., but it is still a good idea to educate yourself about rodents, small mammals and fleas before visiting areas where plague is known to exist. When camping, you may see "Plague Caution" signs, which you should read. Sometimes recreational areas or campgrounds must be closed to prevent plague transmission to humans. Applying repellent containing the active ingredient D.E.E.T. to you and your family, putting flea and tick collars on pets, staying on trails and out of grassy areas and taping pants to your shoes or boots will help reduce the risk of flea bites and potential plague transmission.