Definition
Flea allergy dermatitis or flea bite hypersensitivity is a very common pruritic skin disease caused by hypersensitivity to flea saliva. Dogs with flea allergy dermatitis may display intense pruritus despite only a few fleas being present. Dogs become infested from their environment; therefore in heavy flea infestations, flea bites may also be seen on people.
Cause
The majority of flea infestations are due to Ctenocephalides felis (the cat flea) although pets may become infested with other species.
The adult flea is an obligate parasite, requiring blood from its host to reproduce. The flea saliva, passed into the host while the flea is feeding, contains multiple substances that can be irritating or allergenic. It is not known why individual dogs become hypersensitised. Dogs that are not hypersensitised to flea bites may show very few signs of flea infestation despite the presence of large numbers of fleas.
Approximately 5% of any flea infestation is present as adult fleas on the pet; the rest (egg/larval/pupal stages) live in the environment (usually in the carpet or soil). Completion of the flea life cycle is dependent on ambient temperature and humidity, and can take as little as 12 days or as long as 6 months.
Signalment and history
Flea allergy dermatitis can affect dogs of any age or type, although it is rare in dogs under 6 months of age. Most dogs first present between 10 months and 5 years of age.
Some hypersensitised dogs have multiple allergies so flea allergy dermatitis should be ruled out in dogs suspected of having atopic dermatitis or other allergic diseases.
Flea infestations are more common and severe in warm humid weather (ideal conditions for rapid flea development).
A history of use of flea control does not rule out flea allergy dermatitis. A thorough history should include the flea control products used, frequency and dose of application, presence (and treatment histories) of other animals in the household, frequency of swimming or bathing, and other household flea control methods employed.
In heavy infestations people from the household may experience flea bites. The bites appear as single or multiple small, raised papules and occur most commonly below knee level.
Evidence of anaemia or diagnosis of infectious micro-organisms (for example Rickettsia spp., Bartonella spp.) for which fleas can act as vectors may be indicative of recent flea infestation.
Fleas are the intermediate host for Dipylidium caninum, or flea tapeworm. Dogs are infected by swallowing infected fleas and may pass tapeworm segments in the faeces within 14 to 21 days of infection. A diagnosis of flea tapeworm indicates prior flea exposure.
Clinical signs
The typical area affected by flea allergy dermatitis is the dorsal lumbosacral area (the top and back half of the dog), around the tail base and on the flanks and inner thighs.
Flea bites cause papules or weals that last for up to 72 hours. These then develop crusts on the surface. As the disease becomes more chronic, self-trauma leads to further lesions such as alopecia, lichenification and hyperpigmentation. Secondary infection may result in ‘hot spots’, bacterial pyoderma and secondary seborrhoea. Dogs with chronic flea infestations may have worn incisors and/or hair caught in their teeth due to excessive chewing and grooming.
Involvement of feet or ears suggests another underlying disease (for example atopic dermatitis or food allergy).
Diagnosis
Differentials
Major differentials include atopic dermatitis, food allergy, other parasite infestations (for example Cheyletiella, lice), Malassezia (yeast) dermatitis, or bacterial folliculitis.
Diagnosis is generally based on lesion distribution, visible evidence of fleas (fleas, flea faeces, or eggs), and response to trial treatment.
Diagnostic approach
- Distribution: the distribution of lesions on the caudodorsal (back and top) area of the dog’s body is strongly suggestive of flea allergy dermatitis.
- Coat brushings to look for evidence of adult fleas, flea eggs, or flea faeces in the coat. Note: it may be hard to find fleas if a patient has been recently bathed or if it is severely allergic and has groomed them off itself.
- Confirm hypersensitivity
- Trial treatment is the most commonly used diagnostic test: (i) strict flea control implemented over 8 weeks with an appropriate flea adulticide in combination with environmental therapy; (ii) due to its speed of action, trial treatment with CAPSTAR® (nitenpyram) administered daily for 4–6 weeks can be particularly useful to rule out flea allergy dermatitis
- Serology can be carried out to assess levels of flea antigen-specific IgE
- Intradermal testing involves injection of flea antigen into the skin and measuring the inflammatory response, to support a diagnosis of flea allergy.
Treatment and management
Short term
In the short term, antipruritic treatment such as corticosteroids may be required to provide relief.
Long term
Strict flea control offers long-term management. While elimination of flea infestation is ideal, long-term flea control is aimed at minimising the amount of flea bites received by the dog.
An ideal flea control protocol
- Treatment of all dogs and cats in the household.
- Control of adult fleas on the animals (flea adulticides.)
- Control of environmental stages of fleas (eggs, larvae, pupae): (i) insect growth regulators/insect development inhibitors; (ii) chemical treatment of environment (e.g. pest controllers); (iii) physical treatment of the environment (vacuuming, mopping floors, washing pet bedding at temperatures >60°C, reducing organic debris in yard or garden, and fencing off heavily infested areas.)
- A plan for monitoring.
Treatment should be selected to match the individual needs of the patient and owner.
Considerations in selecting a flea control protocol
- Climatic conditions (warm humid climates may require strict year-round flea control measures.)
- Frequency of swimming or bathing (possible effect on treatments.)
- Number of animals in the household or immediate environment.
- Other ectoparasites found in the dog’s environment (mites, ticks, etc.)
- Compliance issues (including health concerns for owner, ease of administration, budget.)
- Individual pet, household and owner considerations (pregnancy, allergies, tolerance of medication by other species in the home.)
Owners should be advised that flea pupae are resistant to insecticides and can survive for up to 6 months, depending on environmental conditions, and may continue to hatch out during this time.
To prevent future episodes, flea treatment should be carried out rigorously, starting before the flea season. In many climates this means all-year-round treatment.
Managing relapses
- Check to ensure recommended products are being applied properly and consistently.
- Check whether all in-contact animals are being treated (including visitors) or whether the dog is visiting infested households.
- Check environmental management strategies.
- Consider concurrent allergy or other skin disease (for example atopic dermatitis.)
