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Flea Infestations - Health Issues

Biting or scratching is usually the first reaction of an animal with fleas. Some animals may begin excessive grooming to try to rid themselves of the irritation, eating many of the fleas in the process. Light-haired dogs or cats who do this may develop an orange-brown discoloration due to salivary staining.

These general symptoms are usually referred to as pruritus.


As the flea feeds on a cat or dog, it releases saliva to stop blood from coagulating. This saliva contains chemicals that cause an irritant reaction and pruritus (itching) in the host.


Recent research indicates that FAD may be caused by intermittent exposure to large numbers of fleas. Animals sensitized in this way may subsequently become intensely reactive to flea saliva. This is important as it suggests the way to prevent FAD may be to prevent repeated flea infestations.

The initial reaction is usually a reddened wheal, which forms a papule or swollen nodule and crusts over. After that, several secondary changes are possible: 

  • superficial pyoderma (skin infections affecting the skin surface)
  • seborrhea (scaling, crusting, yellowish patches on the skin)
  • diffuse erythema (reddening of the skin over various parts of the body)
  • hair loss
  • "hot spots" - bare, eroded, oozing patches (a severe localized skin infection or pyoderma)

The itching that occurs in dogs with FAD is intense, and results in self-mutilation. Generally, clinical signs are distributed over the inner thigh and abdomen and along the spine and hindquarters. Medications are sometimes needed to relieve the clinical signs temporarily.


One species of tapeworm uses the flea as an intermediate host in its own life cycle. Eggs deposited by the adult tapeworm are shed into the environment where they are consumed by the flea larvae. If a pet then ingests an adult flea that consumed tapeworm eggs as a larva, the tapeworm parasite is passed on.

Although tapeworm doesn't usually cause serious disease in pets, they are disgusting all the same. Tapeworm segments can usually be seen wriggling near the hind end of the animal near the base of the tail.

People can also become infected with tapeworm if they accidentally ingest an infected flea.


Because fleas are blood sucking insects, a heavy infestation can produce parasitic anaemia, particularly in young animals. Fleas have been reported to produce anaemia in dogs, cats, goats, cattle and sheep. Severe flea infestations in young pups and kittens can cause anaemia to the point of death.


The most common species of flea on both cats and dogs is the cat flea. Cat fleas will not live on humans, but they certainly will bite us!

Disease transmission due to fleas in man is not a common occurrence, but any suspected reactions should be discussed with a doctor.

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Flea control products for cats and dogs from Novartis Animal Health