Dermatology Pocket Guide April 2025

FAD Facts:
A Comprehensive Guide
to Flea Allergy
Dermatitis in Cats
and Dogs
Dermatology
Pocket Guide
1

Flea Allergy Dermatitis
Flea Allergy Dermatitis (FAD)
is a common hypersensitivity reaction
to flea salivary antigens, leading
to severe pruritus and dermatological
lesions in affected animals. It is one
of the most prevalent dermatological
conditions in cats and dogs. Below is
a concise reference guide covering
clinical signs, diagnosis, treatment,
and prevention strategies.
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FLEA ALLERGY DERMATITIS
Pathophysiology
` Flea saliva contains histamine-like
compounds, proteolytic enzymes, and
polypeptides. When the flea feeds on
the host, these proteins are released
and can act as antigenic stimuli
in sensitive animals that provoke
intense pruritus and inflammation.
` FAD involves Type I (immediate),
Type IV (delayed), and late-phase IgE
mediated hypersensitivity reactions.
` Dogs exposed intermittently to
flea bites develop more severe and
persistent skin lesions compared to
dogs continuously exposed (Wilkerson
et al., 2004). In cats, intermittent or
continuous exposure has not been
found to have an effect on lesion
severity (Colombini et al., 2001).
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What are the
Clinical Signs?
• Cats
` Papulocrustous dermatitis (“miliary
dermatitis”) commonly affecting
the dorsal trunk with variable
pruritus and hair loss
` Symmetrical alopecia, particularly
affecting the caudal thighs, ventral
abdomen and flanks secondary
to overgrooming
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WHAT ARE THE CLINICAL SIGNS
` Self-induced excoriations, often
on the head and neck
` Eosinophilic granuloma complex (may
manifest as eosinophilic plaques,
linear granulomas, or indolent ulcers)
` Severe infestations may cause fleainduced
anaemia, particularly in
young or debilitated animals.
• Dogs
` Erythematous papules, progressing
to crusting, excoriations and patchy
regional alopecia
` Pruritus may be intense (although not
in all cases), leading to self-trauma
and secondary infection
` Hair loss (alopecia)
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WHAT ARE THE CLINICAL SIGNS
` Lesions concentrated across the
caudal, dorsal trunk, extending to the
perineum and caudal thighs in more
severe cases
` Fibropruritic nodules may develop
in severe chronic cases
` Severe infestations may cause
flea-induced anaemia, particularly
in young or debilitated animals
` Note that atopic dogs are at increased
risk of flea bite hypersensitivity
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Causes
The most common flea found on
both dogs and cats is the cat flea,
Ctenocephalides felis. Even a single flea
bite can trigger an allergic response in
affected animals.
2
Flea Lifecycle
Overview
3
1
4
1. Eggs
2. Larvae
3. Pupae
4. Adults
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Facts
` The female flea can lay 25-50 eggs
per day, 24-48 hours after finding
a suitable host to take her first
blood meal
` The eggs fall off the host with flea
faeces into the environment (carpets,
bedding, soil)
` Flea larvae hatch and feed on organic
debris and flea faeces
` There are three larval development
stages before they pupate
` Pupae can remain dormant for weeks
to months, hatching in response
to vibrations, warmth and shadow
` Newly hatched fleas seek a host
and immediately start to feed
` “Flea dirt” is then passed by the flea
and falls off the host to become
a food source for developing larvae
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Only 1–5%
of the flea population
resides on the host;
the remaining
95% exist in
the environment.
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DIAGNOSIS:
How to Confirm FAD
Diagnosis is based on clinical signs,
exclusion of differentials, and response
to flea control, rather than a single
diagnostic test.
Recommended Diagnostic Approaches:
` Physical Examination: Identify
characteristic lesions and distribution
patterns and look for fleas and flea
faeces (“flea dirt”)
` Flea Comb Testing: Use a fine-tooth
comb to collect fur/material from
the coat for examination. Place the
collected material on moistened wet
paper (flea faeces turn a reddishbrown
on moist white paper)
` History: Identify if the owner or
in-contact humans have developed
any suspicious skin lesions
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DIAGNOSIS: HOW TO CONFIRM FAD
` Cytology: Can be used to assess for
any secondary skin infections often
associated with FAD
` Therapeutic Trial: The most reliable
diagnostic tool – strict flea control
for at least 2 months
Other available tests such as
intradermal skin testing by injecting
flea allergens or serologic testing
are unlikely to be necessary and are
controversial because of poor reliability.
Improvement of symptoms after
implementing strict flea control is
considered definitive for FAD diagnosis.
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TREATMENT:
Flea Control
Treatment for Flea Allergy Dermatitis
(FAD) in cats and dogs involves
a multi-faceted approach targeting
flea elimination, symptom relief,
and prevention of future infestations.
There are three main elements that
need to be addressed when managing
the fleas themselves:
1. Eliminate the fleas on the host
2. Reduce the number of eggs laid
3. Eliminate the pre-emerged larva pool
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TREATMENT: FLEA CONTROL
Eliminate the fleas on the host
1. Isoxazoline products
→ Examples include Fluralaner
(Bravecto®), Afoxolaner (NexGard®),
Lotilaner (Credelio®), Sarolaner
(Simparica®)
→ These are potent inhibitors of
y-aminobutyric acid (GABA)-gated
channels and are highly specific
products
→ In addition to killing fleas, they kill
lice, ticks, mites, mosquitoes, biting
flies and sea lice
2. Or use another veterinary licensed
oral or topical flea product
Ensure the owner is complying with
the frequency of administration and
continuing the treatment ongoing to
break the flea life cycle.
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TREATMENT: FLEA CONTROL
Reduce Egg Laying and Eliminate
Larve in the Environment
→ Recommend regular vacuuming
of carpets, furniture, and cracks
to remove eggs and larvae
→ Advise washing pet bedding at high
temperatures and tumble drying it
afterwards if available to destroy flea
eggs and larvae
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TREATMENT: FLEA CONTROL
→ Use long lasting environmental
household flea sprays to treat the
inside of the house (always follow
the instructions and note the product
precautions)
→ Consider recommending professional
extermination for severe infestations
Treatment of in Contact Animals
Ensure all in contact pets and animals
are treated with flea control products
and this is also administered ongoing
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TREATMENT:
Symptomatic Relief
1. Anti-inflammatories
→ Oral prednisolone at a tapering dose
for a short time period (days rather
than weeks)
→ Methylprednisolone injection
could be considered in cats
2. Topical treatments:
→ Medicated shampoos to soothe skin,
enhance the skin barrier and aid
in flea removal
Treatment of Secondary Infections
Secondary bacterial and fungal
infections can be identified through
cytology. Often topical products are
sufficient to manage these infections,
however if required, appropriate
antibiotics or antifungal medications
can be administered).
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TREATMENT: SYMPTOMATIC RELIEF
Nutritional Support
1. Omega-3 fatty acid supplements:
→ To help reduce inflammation and
manage allergic skin reactions
Ongoing Management
1. Regular follow-ups:
→ Schedule check-ups to monitor
progress and adjust treatment as
necessary
2. Consistent flea prevention:
→ Emphasise the importance of yearround
flea control for all household
pets
3. Client education:
→ Stress the significance of
environmental management to
prevent reinfestation
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Additional resources
Take a Read
How to set up a peripatetic
veterinary dermatology service
Canine Dermatology Update:
What We Know About
the Microbiome and Skin
by Lisa Coder (MS, DVM)
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