Yersinia pestis and plague: Morphology and Identification, Antigenic Structure, Pathogenesis and Pathology, Clinical Findings, Diagnostic Laboratory Tests, Treatment,

Question 1

Yersinia pestis: morphology and identification

Answer 1

1. Yersinia pestis is a Gram-negative coccobacillus with bipolar (safety-pin) staining on Wayson or Giemsa stain.
2. It is nonmotile, non–spore-forming, and facultatively anaerobic.
3. It grows on ordinary media like blood agar and MacConkey agar, forming small, non-hemolytic colonies after 48 hours at 28°C.
4. It belongs to the Enterobacteriaceae family.

Question 2

Yersinia pestis: antigenic structure

Answer 2

1. Y. pestis expresses multiple important antigens: (1) F1 antigen – a capsular protein that is antiphagocytic;
2. (2) V and W antigens – required for intracellular survival and virulence;
3. (3) LPS – weakly endotoxic;
4. (4) Type III secretion system (T3SS) proteins – Yops (Yersinia outer proteins), which inhibit phagocytosis and inflammatory signaling.

Question 3

Yersinia pestis: pathogenesis and pathology

Answer 3

1. Y. pestis is transmitted primarily by flea bites or via direct contact with infected animals.
2. Inside the host, it survives in macrophages and later multiplies extracellularly.
3. It causes hemorrhagic necrosis in lymph nodes, lungs, and other organs.
4. Bacteremia may lead to septic shock and DIC (Disseminated Intravascular Coagulation).
5. Pneumonic plague occurs by inhalation or secondary spread from bubonic form.

Question 4

Yersinia pestis: clinical findings

Answer 4

1. Three major clinical forms: 1) Bubonic plague – most common; sudden fever, painful swollen lymph nodes (buboes), headache, chills; may progress to sepsis;
2. 2) Septicemic plague – no buboes, rapid bacteremia, DIC, purpura, gangrene (“black death”);
3. 3) Pneumonic plague – rapidly fatal pneumonia with hemoptysis, chest pain, and dyspnea; highly contagious by respiratory droplets; fatal without prompt treatment.

Question 5

Yersinia pestis: diagnostic laboratory tests

Answer 5

1. Diagnosis is by Gram stain and culture of bubo aspirate, blood, or sputum.
2. Bipolar staining is characteristic.
3. Culture grows in 48 hours.
4. Direct fluorescent antibody (DFA) and PCR can detect F1 antigen.
   *5. Serologic tests can confirm diagnosis retrospectively.
5. Notify public health authorities immediately, as plague is a reportable disease.*

Question 6

Yersinia pestis: treatment

Answer 6

1. Early antibiotic therapy is critical.
2. Streptomycin is the drug of choice.
3. Alternatives include gentamicin, doxycycline, and ciprofloxacin.
4. Supportive care is important for septicemic or pneumonic forms.
5. Without treatment, mortality exceeds 50% for bubonic and nearly 100% for pneumonic forms.

Question 7

Yersinia pestis: epidemiology

Answer 7

Plague is a zoonotic disease. Rodents are natural reservoirs; fleas are vectors. It is endemic in parts of Africa, Asia, and the western United States. Human cases occur after contact with infected animals or flea bites. Person-to-person spread occurs in pneumonic plague. It has caused three major pandemics in history.

Question 8

Yersinia pestis: control

Answer 8

Rodent control, flea eradication, and avoiding contact with wild animals are key measures. Patients with pneumonic plague require strict isolation. Prophylactic antibiotics (doxycycline or ciprofloxacin) are given to close contacts. A killed vaccine was used in the past but is no longer available in most places. A live attenuated vaccine is under development.