When do fleas cause diseases? - briefly
Fleas become disease vectors when they acquire pathogens from infected animals during a blood meal and subsequently transmit them to new hosts, a risk that rises with large flea populations, warm climates, and seasonal peaks in host activity. The likelihood of transmission is greatest during outbreaks of rodent‑borne infections such as plague, murine typhus, and flea‑borne rickettsioses.
When do fleas cause diseases? - in detail
Fleas become vectors of illness when they acquire pathogens from infected animals and subsequently transmit them to new hosts. Transmission requires several conditions:
- The flea feeds on a host carrying a pathogen such as Yersinia pestis, Rickettsia typhi, or Bartonella species.
- The pathogen undergoes an extrinsic incubation period within the flea, often several days, during which it multiplies or migrates to the mouthparts or feces.
- The flea population reaches a density that increases contact rates with susceptible mammals or humans.
- Environmental factors—temperatures between 20 °C and 30 °C, relative humidity above 70 %—promote flea development and survival, extending the window for transmission.
- The infected flea bites a new host or contaminates skin wounds with infected feces, allowing the pathogen to enter the bloodstream or lymphatic system.
Specific disease scenarios illustrate these requirements:
Plague: Occurs when Y. pestis multiplies in the flea’s foregut, creating a blockage that forces repeated feeding attempts, thereby injecting bacteria into the host’s bloodstream. Outbreaks are linked to rodent epizootics and high flea loads in temperate regions.
Murine typhus: Transmitted by the feces of infected fleas after they feed on rodents. Human infection follows inoculation of contaminated feces into skin abrasions or mucous membranes, most common in warm, coastal areas with abundant rodent reservoirs.
Cat‑scratch disease: Fleas acquire Bartonella henselae from infected cats, then transmit the bacterium to other cats through flea bites. Humans become infected after a cat scratch or bite that introduces the bacteria from the cat’s contaminated claws.
Thus, fleas cause disease when they harbor viable pathogens, environmental conditions support their life cycle, and they interact with susceptible hosts through bites or fecal contamination.