How did rats transmit plague? - briefly
Rats hosted fleas infected with Yersinia pestis, which transferred the bacteria to humans through bites. These fleas migrated between rodent populations and human habitats, allowing the disease to spread quickly.
How did rats transmit plague? - in detail
Rats served as the primary reservoir for Yersinia pestis during historic pandemics. The bacterium colonized the lymphatic tissue of infected rodents, producing a high concentration of pathogens in the bloodstream. Fleas that fed on these rodents ingested infected blood; within the flea’s foregut, the bacteria multiplied and formed a blockage that impeded normal feeding. When the flea attempted to bite a new host, the blockage forced it to regurgitate infectious material into the bite wound, directly introducing the pathogen to humans or other mammals.
Key elements of the transmission cycle include:
- Reservoir infection: Wild and commensal rats acquire plague through contact with other infected rodents or contaminated environments.
- Vector acquisition: Xenopsylla cheopis and other rat‑associated fleas become carriers after feeding on bacteremic rats.
- Bacterial blockage: Y. pestis produces a biofilm that blocks the flea’s proventriculus, altering feeding behavior.
- Regurgitation during feeding: Blocked fleas repeatedly attempt to ingest blood, ejecting bacteria into the host’s skin.
- Human exposure: Close proximity of humans to infested rodent habitats—sewers, granaries, dwellings—facilitates flea bites and subsequent infection.
Secondary pathways occasionally occurred when dead rats attracted scavengers, whose ectoparasites could also transmit the pathogen, or when humans handled contaminated carcasses, allowing direct contact with infected tissues. Nonetheless, the flea‑mediated route from rat to human remained the dominant mechanism driving plague outbreaks.