How Fleas Spread Disease

How fleas survive

Most fleas have four life stages: egg, larva, pupa (in a cocoon), and adult.

Diagram showing the four-step life cycle of fleas. Adult fleas lay eggs in the fur and environment of an animal or human host. After the eggs hatch, fleas enter their free moving, larval stage. The larva will feed and then enter the pupa. In the pupal stage, fleas are protected with a cocoon until they are ready to emerge as an adult. Adult female fleas will seek a blood meal and lay eggs, completing the life cycle.

This diagram shows the typical lifecycle of fleas. The lifecycle of fleas can be very quick or last many months to years depending on the environmental conditions throughout the life stages.

  1. After finding an animal or human host and taking a blood meal, adult fleas will mate and begin laying eggs in the fur and surroundings of the host. Eggs will hatch in one to ten days depending on environmental conditions such as temperature and humidity.
  2. After hatching from an egg, fleas enter their larval stage. Larvae are free moving and feed on blood and flea feces (poop; also called “flea dirt”), in order to continue their development.
  3. Within 5-20 days of feeding on flea dirt, the larvae will spin a cocoon, and enter the pupa stage. The cocoon protects pupa from environmental conditions and insecticides/repellents for several days or weeks until adult fleas are ready to emerge.
  4. Adult fleas will not emerge from the cocoon until there is a clear presence of a host, such as movement or body heat, which will signal that there is a blood meal readily available.
  5. Adult females begin to feed from a host within a few hours of emerging from the cocoon and soon after will mate and begin laying eggs.

How fleas find their hosts

Fleas prefer animal hosts but will resort to biting people when animals are unavailable. Adult fleas find hosts by detecting body heat, movement, the vibrations caused by movement, and breathing.

How fleas spread disease

Fleas transmit germs that cause disease primarily through the processes of feeding on hosts or through fecal contamination, when infected flea feces (poop; also called “flea dirt”) are scratched into an open wound.