Yersinia pestis and Plague
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Direct answer
Yersinia pestis is a Gram-negative coccobacillus, non-motile, that causes plague, a zoonosis maintained in rodents and transmitted to people mainly by fleas. Its three clinical forms are bubonic, septicaemic and pneumonic plague, and the pneumonic form can spread from person to person through respiratory droplets. Plague is treated with streptomycin or gentamicin, and early therapy is the main determinant of survival.
What you must remember
- Morphology: short Gram-negative coccobacillus with bipolar safety-pin staining on Wayson or Giemsa stain.
- Physiology: non-motile at every temperature (unlike other Yersinia species), non-lactose fermenting, and grows on blood agar.
- Virulence: plasmid-encoded genes include the F1 capsular antigen, which resists phagocytosis, and the Pla plasminogen activator.
- Vector: the Oriental rat flea, Xenopsylla cheopis, is the principal vector between rodents and humans.
- Clinical forms: bubonic plague is commonest, with a painful inguinal or axillary bubo; septicaemic plague causes shock; pneumonic plague is the most transmissible.
- Diagnosis: Wayson-stained smear of bubo aspirate, culture on blood agar, F1 antigen detection and PCR.
- Treatment: streptomycin or gentamicin, with doxycycline or ciprofloxacin as alternatives, started as early as possible.
- Prophylaxis: antibiotic prophylaxis for close contacts of pneumonic cases, with isolation of those patients.
From flea bite to bubo
Plague moves from a rodent reservoir to humans through a flea, and the bubo is the hallmark of the bubonic form. The steps trace that sequence and show where clinicians can interrupt it.
- Reservoir: wild rodents carry the bacillus in their blood, and fleas feed on the infected animals. When a rodent dies, the fleas seek a new host, which is how the cycle reaches people living near rodent colonies.
- Transmission: an infected flea bites a person and deposits the bacteria into the skin, starting the bubonic form after a short incubation period.
- Lymphatic spread: the organisms travel to the regional lymph node, which swells into a painful, tender bubo that is typically inguinal or axillary.
- Dissemination: untreated bacteria enter the bloodstream, causing septicaemic plague with shock and disseminated intravascular coagulation.
- Pneumonic spread: a primary lung infection produces pneumonic plague, which passes between people in respiratory droplets and can start an outbreak among close contacts.
- Intervention: early antibiotics, isolation for pneumonic cases and prophylaxis for close contacts are the three priorities, and surveillance of rodent and flea activity limits new cases.
Where the exam tries to catch you
Plague questions turn on the commonest form, the transmission route and the drug of choice.
- Trap: calling pneumonic plague the commonest form Correct: bubonic plague is the commonest form, while pneumonic plague is the most dangerous for person-to-person spread.
- Trap: treating bipolar staining as proof of plague Correct: staining is only suggestive, and Yersinia pestis is confirmed by culture, F1 antigen detection or PCR.
- Trap: choosing penicillin as the first-line drug Correct: streptomycin or gentamicin is the standard treatment, with penicillin not used as the first choice.
Frequently asked questions
Why is pneumonic plague the most feared form of Yersinia pestis infection?
Pneumonic plague is the most feared form because it spreads from person to person through respiratory droplets and can cause rapidly fatal pneumonia. Untreated pneumonic plague has a very high mortality, which makes early antibiotics and isolation essential.
Which drugs are used to treat plague caused by Yersinia pestis?
Streptomycin or gentamicin is the standard treatment for plague, with doxycycline or ciprofloxacin used as alternatives. Treatment should start as early as possible, because delay increases mortality in Yersinia pestis infection.
Which vector transmits plague in India?
The Oriental rat flea, Xenopsylla cheopis, is the principal vector of plague between rodents and humans in India. Fleas feed on infected rodents and then transmit Yersinia pestis to people during later bites.
What is the role of the F1 antigen in Yersinia pestis?
The F1 antigen is a capsular protein that helps Yersinia pestis resist phagocytosis, and it is encoded by a plasmid. The F1 antigen is detected in clinical samples by rapid tests, which gives a useful diagnostic clue during an outbreak.
Is a vaccine routinely available against plague?
No plague vaccine is routinely available for general use in most countries at present. Prevention therefore relies on rodent and flea control, surveillance and antibiotic prophylaxis for close contacts of plague cases.