Flea-Borne Diseases

Fleas are known to transmit tapeworms as well as bacterial and viral pathogens to both dogs and humans. Moreover, the presence and blood-feeding of these parasites often results in allergic reactions and can cause a substantial blood loss.

Figure 1: Female flea depositing an egg (scanning electron micrograph).


Flea-related dermatitis

Flea-related dermatitis is the most common dermatological reason of veterinary consultation by pet owners, making appropriate flea control indispensable. It may also be the most common overall disease problem of domestic animals.

Figure 2: Caudal dorsum of a dog showing flea-associated alopecia.


In literature it is often distinguished between a dermatitis resulting from flea bites without the presence of hypersensitivity reaction – and the typical flea bite hypersensitivity, synonym Flea Allergy Dermatitis (FAD). Newer research and diagnostic techniques suggest a hypersensitivity component in any dermal reaction onto flea bites, and that dermatitis without allergy is rare.

In contrast to typical FAD, flea bites can cause a local reaction in form of irritation of the skin, pruritus, and an insignificant, transient invasion of lymphocytes in the close area of the flea bite. This reaction is caused by the direct contact of the skin with the ectoparasite and its bite. The main clinical sign is pruritus. Alopecia, crustiness and the flea bite itself as well as pustules and papules can further be observed.


Blood-sucking fleas can also cause an unbalance in the circulatory system, leading to anaemia in severe cases. Iron deficiency anaemia due to heavy infestations can be a particular problem in young animals.

Flea control

Regular application of parasiticides to prevent flea infestation is a common strategy in veterinarian practice. The cosmopolitan distribution of the insects and the fact that fleas are major nuisance pests, a matter of public health and the source of FAD, make control definitely necessary. A lot of different flea control products are provided by the pharmaceutical world, with many of them exhibiting almost 100% efficacy. Major differences regarding the speed of action and the issue of resistance are due to the active substances.

If a substance possesses a fast mode of action, the probability decreases that transmission of pathogens occurs within the remaining feeding time. With imidacloprid, Bayer provides a proven active ingredient, which effectively stops flea feeding within minutes.

Resistance in fleas against frequently used flea control products is an issue of major importance. Flea control measures should be designed to minimise the risk of resistance developing with incorrect dosing. But often, biological factors like reinfestation from refugia on domestic and wild animals, or the failure of adjusting insecticide application to variations in humidity and temperature – or cultural conditions such as substrates or even carpet types – may cause control failures. In 1999, Bayer has started a global flea susceptibility program to monitor possible resistance developments against imidacloprid in fleas, so far with negative results.

Figure 3: Cranial view of an adult flea (scanning electron micrograph).


Further information

  • Krämer F, Mencke N: Flea Biology and Control. The Biology of the Cat Flea – Control and Prevention with Imidacloprid in Small Animals. 2001, Springer Verlag, Heidelberg

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