Plague

• Plague is a serious infectious disease transmitted to humans through bites of rodent fleas and is caused by Yersinia pestis.

It can spread through contact with infected fleas.

• Plague is a disease that affects humans and other mammals.

Bubonic plague

Cause

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 rural areas in the western United States, but significantly more cases occur in parts of Africa and Asia.

Epidemiology and ecology

• The bacteria that cause plague, Yersinia pestis, maintain their existence in a cycle involving rodents and their fleas.
• Plague occurs 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.

World distribution of plague 1998

Enzootic cycle

• 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.

Epizootic infection

• 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.

Urban outbreaks

• In parts of the developing world, plague can sometimes occur in urban areas with dense rat infestations. The last urban outbreak of rat-associated plague in the United States occurred in Los Angeles in 1924-1925.

Transmission

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. During plague epizootics, many rodents die, causing hungry fleas to seek other sources of blood. People and animals that visit places where rodents have recently died from plague are at risk of being infected from flea bites. Dogs and cats may also bring plague-infected fleas into the home. Flea bite exposure may result in primary bubonic plague or septicemic plague.

Acral gangrene due to plague

• Contact with contaminated fluid or tissue. Humans can become infected when handling tissue or body fluids of a plague-infected animal. For example, a hunter skinning a rabbit or other infected animal without using proper precautions could become infected with plague bacteria. 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. If these bacteria-containing droplets are breathed in by another person they can cause pneumonic plague. Typically this requires direct and close contact with the person with pneumonic plague. Transmission of these droplets is the only way that plague can spread between people. This type of spread has not been documented in the United States since 1924, but still occurs with some frequency in developing countries. Cats are particularly susceptible to plague, and can be infected by eating infected rodents. Sick cats pose a risk of transmitting infectious plague droplets to their owners or to veterinarians. Several cases of human plague have occurred in the United States in recent decades as a result of contact with infected cats.

Symptoms

• 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.

Types of plague

• 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

• 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).

Diagnosis

• Plague should be considered in any patient with clinical signs of plague and a recent history of travel to the western United States or any other plague endemic area. Bubonic plague is the most common primary manifestation, with a bubo usually occurring in the groin, axilla or cervical nodes. Buboes are often so painful that patients are generally guarded and have restricted movement in the affected region. The incubation period for bubonic plague is usually 2 to 6 days.

• If bubonic plague is untreated, plague bacteria invade the bloodstream and spread rapidly, causing septicemic plague, and if the lungs are seeded, secondary pneumonic plague. Septicemic and pneumonic plague may also be primary manifestations. A person with pneumonic plague may experience high fever, chills, cough, and breathing difficulty and may expel bloody sputum. If pneumonic plague patients are not given specific antibiotic therapy, the disease can progress rapidly to death.

• Although the majority of patients with plague present with a bubo, some may have nonspecific symptoms. For example, septicemic plague can present with prominent gastrointestinal symptoms such as nausea, vomiting, diarrhea, and abdominal pain (MMWR, 2006). Additional rare forms of plague include pharyngeal, meningeal, and cutaneous.

• Appropriate diagnostic samples include blood cultures, lymph node aspirates if possible, and/or sputum, if indicated. Drug therapy should begin as soon as possible after the laboratory specimens are taken. If plague is suspected, local and state health departments should be notified immediately. If the patient has pneumonic signs, he/she should also be isolated and placed on droplet precautions.

Rat flea - Xenopsylla brasiliensis

Diagnostic Testing

• If plague is suspected, pre-treatment specimens should be taken if possible, but treatment should not be delayed. Specimens should be obtained from appropriate sites for isolating the bacteria, and depend on the clinical presentation:

• Lymph node aspirate: An affected bubo should contain numerous organisms that can be evaluated microscopically and by culture.
• Blood cultures: Organisms may be seen in blood smears if the patient is septicemic. Blood smears taken from suspected bubonic plague patients early in the course of illness are usually negative for bacteria by microscopic examination but may be positive by culture.
• Sputum: Culture is possible from sputum of very ill pneumonic patients; however, blood is usually culture-positive at this time as well.
• Bronchial/tracheal washing may be taken from suspected pneumonic plague patients; throat specimens are not ideal for isolation of plague since they often contain many other bacteria that can mask the presence of plague.
• In cases where live organisms are unculturable (such as postmortem), lymphoid, spleen, lung, and liver tissue or bone marrow samples may yield evidence of plague infection by direct detection methods such as direct fluorescent antibody (DFA) or PCR.
• Y. pestis may be identified microscopically by examination of Gram, Wright, Giemsa, or Wayson’s stained smears of peripheral blood, sputum, or lymph node specimen.Visualization of bipolar-staining, ovoid, Gram-negative organisms with a “safety pin” appearance permits a rapid presumptive diagnosis of plague.

Carriers of the New Black Plague

• If cultures yield negative results, and plague is still suspected, serologic testing is possible to confirm the diagnosis. One serum specimen should be taken as early in the illness as possible, followed by a convalescent sample 4-6 weeks or more after disease onset.

Treatment

• Begin appropriate IV therapy as soon as plague is suspected. Gentamicin and fluoroquinolones are typically first-line treatments in the United States. Duration of treatment is 10 to 14 days, or until 2 days after fever subsides. Oral therapy may be substituted once the patient improves.

Prevention

• Reduce rodent habitat around your home, work place, and recreational areas. Remove brush, rock piles, junk, cluttered firewood, and possible rodent food supplies, such as pet and wild animal food. Make your home and outbuildings rodent-proof.
• Wear gloves if you are handling or skinning potentially infected animals to prevent contact between your skin and the plague bacteria. Contact your local health department if you have questions about disposal of dead animals.
• Use repellent if you think you could be exposed to rodent fleas during activities such as camping, hiking, or working outdoors. Products containing DEET can be applied to the skin as well as clothing and products containing permethrin can be applied to clothing (always follow instructions on the label).
• Keep fleas off of your pets by applying flea control products. Animals that roam freely are more likely to come in contact with plague infected animals or fleas and could bring them into homes. If your pet becomes sick, seek care from a veterinarian as soon as possible.
• Do not allow dogs or cats that roam free in endemic areas to sleep on your bed.

• The regimens listed below are guidelines only and may need to be adjusted depending on a patient’s age, medical history, underlying health conditions, or allergies. Please use clinical judgment.

Plague Resources
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