Plague

Plague (pestis) is an acute zoonotic natural focal infectious disease with a predominantly transmissible mechanism of pathogen transmission, which is characterized by intoxication, damage to the lymph nodes, skin and lungs. It is classified as a particularly dangerous, conventional disease.

The symptoms of plague consist of either acute pneumonia or massive lymphadenopathy with high fever. The latter often progresses to septicemia. The diagnosis of plague is based on epidemiological data and clinical examination. Confirmation of the diagnosis is based on culture. Treatment of plague is with fluoroquinolone or doxycycline.

What causes the plague?

Plague is caused by Yersinia pestis. Yersinia (formerly Pasterella) pestis is a short bacillus that often stains bipolar (especially with Giemsa stain) and may resemble a safety pin.

Plague primarily occurs in wild rodents (e.g. rats, mice, squirrels, prairie dogs) and is transmitted from rodents to humans through the bites of infected fleas. Human-to-human transmission occurs through inhalation of airborne aerosols produced by patients with the pneumonic form of plague (primary pneumonic plague). The latter is a highly contagious disease. In endemic areas of the United States, a certain number of cases may be caused by domestic animals, particularly cats. Transmission from cats may occur through bites or by inhalation of airborne aerosols (if the animal is sick with the pneumonic form of plague).

In the past, there have been massive plague epidemics (such as the Black Death of the Middle Ages). More recently, plague has been reported as sporadic cases or limited outbreaks. In the United States, more than 90% of plague cases occur in the southwest, especially in New Mexico, Arizona, California, and Colorado. Yersinia is considered a possible bioterrorism weapon.

What are the symptoms of plague?

The most common form of the disease is bubonic plague, the incubation period of which is usually 2-5 days, but can vary from a few hours to 12 days. The onset of fever (39.5-41 °C) is acute and often accompanied by chills. The pulse may be rapid and thready; hypotension often develops. Enlarged lymph nodes (buboes) appear with or shortly before the onset of fever. The femoral or inguinal lymph nodes are the first to be involved in the pathological process, followed by the axillary, cervical, or multiple lymph nodes. The nodes are usually extremely tender and surrounded by an edematous zone. They may suppurate in the 2nd week of the disease. The underlying skin is smooth and reddened, without an increase in local temperature. In some cases, a primary skin lesion may occur at the site of the bite, which may be either a small vesicle with minor local lymphangitis or an eschar. The patient may be restless, delirious, confused, and have impaired coordination. The liver and spleen may be enlarged.

The incubation period of primary pneumonic plague lasts 2-3 days, after which the disease takes an acute course. High fever, chills, tachycardia and headache, which is often acute, are observed. Initially, the cough is insignificant, but develops within 24 hours. The sputum is initially mucous, but soon streaks of blood appear in it, after which it becomes pink or bright red (resembles raspberry syrup) and foamy. Tachypnea and dyspnea are present, but there is no pleurisy. Signs of compaction of lung tissue are rare, wheezing may be absent.

Septicemic plague usually occurs with the bubonic form as an acute, fulminant disease. Abdominal pain, most likely due to mesenteric lymphadenopathy, occurs in 40% of patients. Pharyngeal plague and plague meningitis are less common forms of the disease.

Minor plague is more benign than the bubonic form. It usually occurs only in endemic areas. Lymph node inflammation, fever, and prostration subside within a week. The mortality rate in untreated patients is about 60%. Most deaths occur as a result of sepsis within 3-5 days. Most untreated patients with pneumonic plague die within 48 hours of symptom onset. Septicemic plague may be fatal before signs of bubonic or pneumonic plague appear.

How is plague diagnosed?

Plague is diagnosed by staining and culture. Typically, specimens are obtained by needle aspiration of the bubo (surgical drainage may disseminate the PI). Blood and sputum cultures should also be done. Other tests include immunofluorescence staining and serologic studies. A titer greater than 1:16 or a 4-fold increase between the acute and convalescent titers is considered positive. PCR testing is diagnostic when available. Previous vaccination does not exclude plague; clinical disease may develop in vaccinated individuals. Patients with pulmonary symptoms or signs should have a chest radiograph, which shows rapid progression to pneumonia in pneumonic plague. The white blood cell count is usually 10-20 x 109 with large numbers of immature neutrophils.

How is plague treated?

Prompt treatment of plague reduces mortality to 5%. In septicemic or pneumonic plague, treatment should be started within 24 hours. Streptomycin is used at 7.5 mg/kg intramuscularly every 6 hours for 7 to 10 days. Many physicians use higher starting doses of up to 0.5 g intramuscularly every 3 hours for 48 hours. An alternative is doxycycline 100 mg intravenously or orally every 12 hours. Gentamicin may also be effective. In plague meningitis, chloramphenicol should be given at a loading dose of 25 mg/kg intravenously, followed by 12.5 mg/kg intravenously or orally every 6 hours.

Routine isolation is sufficient for patients with bubonic plague. Patients with primary or secondary pneumonic plague require strict respiratory isolation. All persons who have been in contact with a plague patient should be kept under medical observation. Their body temperature should be measured every 4 hours for 6 days. If this is not possible, tetracycline 1 g once daily for 6 days can be administered, but this may lead to the emergence of antibiotic-resistant strains.

Plague can be prevented by rodent control, the use of insect repellents to minimize flea bites, and the plague vaccination. Travelers should receive drug prophylaxis with doxycycline 100 mg orally every 12 hours during periods of exposure.