Pestis

Plague is a disease that affects humans and other mammals. It is caused by the bacterium, Yersinia pestis. Humans usually get plague after being bitten by a rodent flea that is carrying the plague bacterium or by handling an animal infected with plague. Plague is infamous for killing millions of people in Europe during the Middle Ages. Today, modern antibiotics are effective in treating plague. Without prompt treatment, the disease can cause serious illness or death. Presently, human plague infections continue to occur in the western United States, but significantly more cases occur in parts of Africa and Asia.

Plague has a remarkable place in history and has had enormous effects on the development of modern civilization. Some scholars have even suggested that the collapse of the Roman Empire may be linked to the spread of plague by Roman soldiers returning home from battle in the Persian Gulf in 165 AD. For centuries, plague represented disaster for people living in Asia, Africa and Europe and because the cause of plague was unknown, plague outbreaks contributed to massive panic in cities and countries where it appeared.

Numerous references in art, literature and monuments attest to the horrors and devastation of past plague epidemics. We now know that plague is caused by a bacterium called Yersinia pestis that often infects small rodents (like rats, mice, and squirrels) and is usually transmitted to humans through the bite of an infected flea.  In the past, black rats were the most commonly infected animals and hungry rat fleas would jump from their recently-dead rat hosts to humans, looking for a blood meal. Pneumonic plague, a particular form of plague infection, is instead transmitted through infected droplets in a sick person's cough.

Three major historical plague pandemics are known:

• The Justinian Plague started in 541.
• "Black Death" or the Great Plague in the 14th century.
• Modern Plague began in China in the 1860s and appeared in Hong Kong by 1894.

However, as a bacterial disease, plague can be treated with antibiotics, and can be prevented from spreading by prompt identification, treatment and management of human cases. Applications of effective insecticides to control the flea vectors also provide assistance in controlling plague.

Recent outbreaks: the most recent plague epidemics have been reported in India during the first half of the 20^th century, and in Vietnam during wartime in the 1960s and 1970s. Plague is now commonly found in sub-Saharan Africa and Madagascar, areas which now account for over 95% of reported cases.

The bacteria that cause plague, Yersinia pestis, maintain their existence in a cycle involving rodents and their fleas. In urban areas or places with dense rat infestations, the plague bacteria can cycle between rats and their fleas. The last urban outbreak of rat-associated plague in the United States occurred in Los Angeles in 1924-1925.

Since that time, plague has occurred in rural and semi-rural areas of the western United States, primarily in semi-arid upland forests and grasslands where many types of rodent species can be involved. Many types of animals, such as rock squirrels, wood rats, ground squirrels, prairie dogs, chipmunks, mice, voles, and rabbits can be affected by plague. Wild carnivores can become infected by eating other infected animals.

Scientists think that plague bacteria circulate at low rates within populations of certain rodents without causing excessive rodent die-off. These infected animals and their fleas serve as long-term reservoirs for the bacteria. This is called the enzootic cycle.

Occasionally, other species become infected, causing an outbreak among animals, called an epizootic. Humans are usually more at risk during, or shortly after, a plague epizootic. Scientific studies have suggested that epizootics in the southwestern United States are more likely during cooler summers that follow wet winters. Epizootics are most likely in areas with multiple types of rodents living in high densities and in diverse habitats.

The plague bacteria can be transmitted to humans in the following ways:

• Flea bites. Plague bacteria are most often transmitted by the bite of an infected flea. Flea bite exposure may result in primary bubonic plague or septicemic plague.
• Contact with contaminated fluid or tissue. Humans can become infected when handling tissue or body fluids of a plague-infected animal. This form of exposure most commonly results in bubonic plague or septicemic plague.
• Infectious droplets. When a person has plague pneumonia, they may cough droplets containing the plague bacteria into air.  Transmission of these droplets is the only way that plague can spread between people. 

Plague symptoms depend on how the patient was exposed to the plague bacteria. Plague can take different clinical forms, but the most common are bubonic, pneumonic and septicemic.

• Bubonic plague: Patients develop sudden onset of fever, headache, chills, and weakness and one or more swollen, tender and painful lymph nodes (called buboes). This form usually results from the bite of an infected flea. The bacteria multiply in the lymph node closest to where the bacteria entered the human body. If the patient is not treated with the appropriate antibiotics, the bacteria can spread to other parts of the body.
• Septicemic plague: Patients develop fever, chills, extreme weakness, abdominal pain, shock, and possibly bleeding into the skin and other organs. Skin and other tissues may turn black and die, especially on fingers, toes, and the nose. Septicemic plague can occur as the first symptom of plague, or may develop from untreated bubonic plague. This form results from bites of infected fleas or from handling an infected animal.
• Pneumonic plague: Patients develop fever, headache, weakness, and a rapidly developing pneumonia with shortness of breath, chest pain, cough, and sometimes bloody or watery mucous. Pneumonic plague may develop from inhaling infectious droplets or may develop from untreated bubonic or septicemic plague after the bacteria spread to the lungs. The pneumonia may cause respiratory failure and shock. Pneumonic plague is the most serious form of the disease and is the only form of plague that can be spread from person to person (by infectious droplets).

Plague is a serious illness. If you are experiencing symptoms like those listed here, seek immediate medical attention. Prompt treatment with the correct medications is critical to prevent complications or death.

Plague is a plausible diagnosis for people who are sick and live in, or have recently traveled to, the western United States or any other plague-endemic area. The most common sign of bubonic plague is the rapid development of a swollen and painful lymph gland called a bubo. A known flea bite or the presence of a bubo may help a doctor to consider plague as a cause of the illness.

In many cases, particularly in septicemic and pneumonic plague, there are no obvious signs that indicate plague. Diagnosis is made by taking samples from the patient, especially blood or part of a swollen lymph gland, and submitting them for laboratory testing. Once plague has been identified as a possible cause of the illness, appropriate treatment should begin immediately.

Plague is a very serious illness, but is treatable with commonly available antibiotics. The earlier a patient seeks medical care and receives treatment that is appropriate for plague, the better their chances are of a full recovery.

People in close contact with very sick pneumonic plague patients may be evaluated and possibly placed under observation. Preventive antibiotic therapy may also be given, depending on the type and timing of personal contact.