Fleas are contagious. However, adult fleas don’t spread from host to host. Flea infestations spread through eggs. Outdoors, infested wildlife enter into yards and drop flea eggs. After developing, the fleas emerge from cocoons and jump on dogs and cats. Indoors, eggs fall and develop in carpeting. Once one pet is infested, all untreated animals in a home will likely acquire fleas.
Cat Fleas Don’t Leave their Hosts
Cat fleas are permanent parasites. Once on a host, the adults become permanent residents, rarely leaving of their own volition. They feed, mate and lay eggs on the host. Fleas typically only leave when they’re dislodged or die. When cats aren’t allowed to groom, 85% of female and 58% of male fleas will remain on-host for at least 50 days. Fleas won’t leave even when populations reach high levels.
There’s one exception: Fleas will quickly abandon a dead or dying host. Once the host’s body temperature begins dropping, they’ll begin jumping off the animal. As a result, new infestations may start on a predatory animal when it attacks and kills its flea-infested prey.
Fleas Rarely Transfer Between Hosts
Dogs and cats seldom get infested with fleas from other animals. Fleas aren’t “contagious” in this sense. It’s a misconception that infestations begin this way. Adult cat fleas rarely transfer hosts. When two cats live together but are separated, only 3% to 8% of fleas transfer between hosts. When the cats remain together, 2% to 15% of the fleas move between cats.
Fleas Rarely Surviving Host Grooming
It’s a myth that fleas spread from a host grooming them off into the environment. The vast majority of fleas get ingested by the host during grooming. Only a small percentage gets dislodged into the environment. And most dislodged fleas get damaged during the process, dying soon afterwards. They’re commonly missing their heads or legs. However, some fleas may survive scratching and grooming. To prevent starvation, these undamaged fleas must return to a host within a couple of days.
Infestations Spread through Eggs
The majority of infestations begin as eggs, with dogs and cats first acquiring fleas as they emerge from cocoons. Fleas continually lay eggs on infested animals, and the eggs continually drop from their fur. The eggs fall into the environment wherever the animal has access. Most accumulate in areas where hosts regularly feed, groom, rest or sleep.
Infestations Begin Outdoors
In most cases, flea infestations originate outdoors. Many urban animals can carry cat fleas, for instance opossums, raccoons, foxes, coyotes, skunks and feral cats. As infested wildlife travel through neighborhoods, they drop flea eggs into yards. Both feral and domestic animals seek shelter in similar locations, such as below decks. Flea eggs that drop in shady, protected habitats are likely to survive. When pets enter these zones, they can quickly acquire fleas.
Img 1 Wild animals that can be hosts to cat fleas — raccoon, skunk, feral cat, fox, opossum and coyote.
It’s not uncommon for feral and domestic animal territories to overlap. Fleas can be spread from pets to feral animals, and from feral animals to pets. This perpetuates infestations throughout neighborhoods. There is a continual cycle of infestation where animal territories are shared.