While fleas are physically capable of jumping from dog to dog, they very rarely do so. As obligate parasites, newly hatched fleas must secure hosts or they will starve. To meet this daunting challenge, fleas have evolved legendary leaping abilities, flexible developmental timing and unique morphologies to help secure and retain their preferred hosts. Once fleas have completed the arduous task of securing a host, they are reluctant to abandon their food supply voluntarily.
Not All Fleas Are Created Equal
More than 2,000 flea species inhabit the world, each of which has a slightly different morphology, geographic range and preferred host species. Some specialize on a single species, such as the world’s largest flea, Hystrichopsylla schefferi, which feeds almost exclusively on mountain beavers (Aplodontia rufa). Others, such as the ubiquitous cat flea (Ctenocephalides felis) prefer sucking the blood of your pet dog or cat, but they will also feed on non-preferred hosts, such as opossums (Didelphis virginiana) and raccoons (Procyon lotor), when food is scarce. The cat flea is responsible for most dog infestations in the United States. Dog fleas (Ctenocephalides canis) primarily afflict only wildlife in the United States, but they are common among dogs in Europe.
Jumping for a Living
While fleas do not tend to hop from one dog to another, their incredible jumping adaptations are essential components of their natural history. Although their jumping ability may help fleas to move about on their hosts, more importantly, it helps the newly hatched adults to jump high, far and quick enough to land on a passing host. Additionally, they can jump to avoid the grooming habits of their hosts, a process that removes a significant number of fleas. Zoologists consider fleas to be some of the best leapers in the animal kingdom and have documented the tiny insects jumping up to 100 times the length of their bodies. A hungry flea may jump 10,000 times in a row to find a suitable host, according to the ASPCA website.
Spreading Their Eggs
Because adults are reluctant to leave their hosts, they primarily spread during the first three parts of the life cycle -- the egg, larval and pupal stages. Fleas deposit eggs on their hosts, but most of the eggs fall off. These eggs accumulate in places the hosts frequent, such as your dog's bed, and hatch into larvae, which feed on organic debris and adult flea feces. After molting a few times, the larvae enter a pupal stage to finish metamorphosis.
Flea pupae do not automatically hatch once they complete development -- without a suitable host, they may starve in as little as two days. Instead, they try to wait to hatch until a suitable host is close by. Sensitive to carbon dioxide and physical stimulation, the pupae can completely exit their cocoonlike encasements in a matter of seconds, when they sense that a suitable host is near. This flexibility of the pupal stage prevents them from hatching when the chances for survival are low and contributes to the species' viability. This phenomenon also explains some sudden flea infestations -- pupae lurking in the environment wait until your dog passes by to quickly hatch and jump onto him.
Though humans are not their preferred hosts, cat fleas may occasionally bite humans who have close contact with infected dogs or cats. While they will quickly move off in search of their preferred hosts, they can, rarely, transmit dog tapeworms (Diphylidium caninum) to small children. Your veterinarian can prescribe medications to kill adult fleas, but you must also keep your and your pets' living environment clean to reduce the number of eggs, larvae and pupae. It may take only a few flea bites to cause a dog irritation or allergic reactions, so treat flea infestations promptly.