Epidemiology of plague

Sources and reservoirs of infection

Plague is a typical natural-focal infection. Traditionally, two main types of epidemic foci are distinguished, which determine the circulation of the pathogen:

1. Natural ("wild") foci. In the wild, more than 250 different animal species serve as the reservoir for the microbe. Rodents (ground squirrels, marmots, voles, gerbils) and lagomorphs (pikas, hares) are of the greatest epidemiological importance. Ectoparasites continuously maintain epizootics among wild animals and can transmit the pathogen to synanthropic species.
2. Anthropurgic (urban, port) foci. These form when the infection is introduced into populated areas. The main reservoirs here are rats (brown, black, Egyptian), house mice, as well as domestic and farm animals (camels, cats, dogs). Animals pose the maximum danger during the period of pronounced bacteremia, immediately before their death.

A sick human can also become a source of infection if they develop the pneumonic or primary septic form of plague (especially with lung or intestinal involvement).

Mechanisms and transmission routes

The epidemiology of plague is characterized by multiple routes of transmission. The clinical form of the developing disease directly depends on how the microbe entered the body:

• Vector-borne (about 70% of cases). It is realized through the bites of infected fleas. Bacteria actively multiply in the flea's proventriculus, forming a specific clot (block). When attempting to feed, the flea regurgitates this clot along with bacteria into the wound. This route leads to the development of cutaneous and bubonic forms of plague.
• Contact (about 20% of cases). Infection occurs through damaged skin and mucous membranes during the dressing of carcasses of infected animals (camels, saigas, hares) or when caring for an infected person through household items. It also leads to bubonic or cutaneous forms.
• Aspiration (2–3% of cases). Airborne droplet and airborne dust routes. The infection is transmitted from a patient with the pneumonic form or as a result of accidents while working with Yersinia pestis cultures in laboratories. It causes an extremely severe primary pneumonic form.
• Alimentary. Associated with the consumption of infected food products (e.g., meat or milk from an infected camel). Leads to pharyngeal or intestinal manifestations of the septic form.

Note: With high strain virulence and a large infectious dose, any of the listed mechanisms can immediately lead to the development of the primary septic form of plague.

Microorganism persistence in the environment

The plague pathogen possesses paradoxical resistance to environmental factors. It tolerates competition with putrefactive microflora poorly, and therefore dies rather quickly in decomposing corpses (persisting slightly longer in bone marrow).

However, Yersinia pestis is a psychrophile (growth optimum 26–27°C) and tolerates low temperatures exceptionally well. In frozen animal carcasses, the bacterium can remain viable from 7 to 12 months. In the bodies of starving fleas at temperatures of 0–15°C, the microbe survives for more than one year, and in ticks—about two years.

At the same time, the pathogen is very sensitive to heat and disinfectants: it dies within 30 minutes at 58°C, and within 1–5 minutes upon boiling. Standard concentrations of chloramine, lysol, bleaching powder, or carbolic acid kill the bacterium in a few minutes. Among antibiotics, the microbe is sensitive to tetracycline, streptomycin, chloramphenicol, and rifampicin, but has natural resistance to penicillin due to penicillinase production.

Preventive measures

Plague prophylaxis is divided into general and specific. General measures are strictly regulated by the International Health Regulations and are aimed at preventing the importation of infection from abroad, as well as monitoring and reducing the activity of natural foci.

Specific prophylaxis (vaccination): A live or chemical vaccine is used. The route of administration depends on the patient's age and condition:

• Percutaneous: children, elderly individuals, pregnant women.
• Subcutaneous: individuals aged 14 to 60 years.

Routine revaccination is carried out after 1 year, and in an unfavorable epidemiological situation—after 6 months. It is important to remember that post-infection and post-vaccination immunity is not completely solid and does not protect against the disease in case of massive re-infection.

Emergency chemoprophylaxis: Prescribed to individuals who have been in contact with patients, animal carcasses, or infected items. The drug of choice is doxycycline, administered orally at 0.2 g per day for a 7-day course.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Genetic determination of virulence: pCad, pPst, pFra plasmids.
• Antigenic structure of Yersinia pestis and the phenomenon of antigenic mimicry.
• Pathogenesis of primary local fibrinolysis formation at the bite site.
• Functions of the FI capsular antigen and the FII "murine toxin".
• Biochemical properties and biovars of the pathogen (Antiqua, Mediaevalis, Orientalis).

Other topics in the section "Zoonotic infections — 2024 study guide"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.