The BTHG would like to thank Merial Animal Health Ltd, the makers of Frontline, for giving permission to use photographs from their publication “Under the Microscope – A Guide to Canine & Feline Ectoparasites”.


Fleas are the most common external parasite to be found on dogs.  They are dark brown wingless insects, measuring 2-3 mm in length.

There are two species of fleas that are commonly found on dogs in the UK, namely, the Cat Flea and the Dog Flea. Surprisingly, of these it is the Cat Flea that occurs most frequently on dogs.

The Flea Life Cycle.

Fleas have a typical insect 4 stage life cycle (egg > larva > pupa > adult).  This life cycle may be as short as 2-3 weeks but can be as long as 6+ months depending on ‘environmental’ conditions.  An ambient temperature of 65-90º F and a relative humidity of more than 75% are necessary for development to take place.

Figure 1 - The Life Cycle of a Flea

It is important to note that it is only the adult fleas that live on the dog and that all the other stages in the life cycle exist in the environment, often in or near the dogs bedding/sleeping areas.  Furthermore, adult fleas only account for about 1% of the potential flea population. These points are important when considering strategies for the control of fleas.

Figure 2 - Drawing of an Adult Flea

Adult fleas mate on the host dog and each female will lay about 25-40 eggs per day and can lay up to 1500-2000 eggs before dying.  The eggs are ‘pearly-white’ in colour, oval and about 0.5 mm long, i.e. just visible to the naked eye.  These eggs are readily dislodged and fall from the dog onto the bedding, carpet, etc.

Figure 3 - Flea Eggs            Figure 4 – Flea Larvae

In 1-10 days the eggs hatch into 1st stage larvae.  These larvae are 1-2 mm long, whitish in colour and have segmented bodies covered with short hairs, i.e. like very small maggots.  The larval stage can last from 5-14 days and during this time the larvae feed on organic material such as skin debris and, importantly, adult flea faecal material which consists of dried but undigested blood from the dog.  As a consequence, as they grow, the larvae become darker in colour. During this growth phase the larvae moult twice and increase in size prior to pupating. The 2nd stage larvae are 2-3 mm long and the 3rd stage larvae are 3-5 mm long.

Each larva develops into a pupa about 5 mm long by spinning a white silken cocoon in which it metamorphoses i.e. changes into an adult.  This cocoon is sticky and quickly attracts dust/dirt which acts as camouflage, i.e. it has the appearance of a ‘crumb’. Pupae are typically found in places that are protected from light and other environmental hazards.

Figure 5 - Flea Pupae

The time spent as pupae varies considerably – the adult fleas may emerge in as little as 1-2 weeks or emergence may be delayed for up to 4-6 months.  Emergence of the adult fleas is usually triggered by the warmth and pressure from a sleeping dog or by vibrations as the dog moves.

As soon as they emerge the adult fleas seek a host.  This does not present a problem because it is usually the presence of a host that stimulates emergence from the cocoon.  Female adult fleas are somewhat larger than the male fleas – about 4 mm long as opposed to 2-3 mm long. Within minutes of arriving on the dog both males and females proceed to feed by biting the host and sucking blood.  They will take blood feeds several times each day whilst on the host.  Mating takes place on the dog and the female starts to lay eggs.  Under suitable conditions adult female fleas can live for several months.

How do you know if a dog has fleas?

Although fleas are big enough to be seen with the naked eye, they move very quickly and are rarely seen unless the infestation is severe or the owner is very lucky.

However, the dog may show signs of mild irritation, i.e. scratching and rubbing, and an examination may reveal reddening of the skin.  Excessive scratching may result in the loss of hair and broken skin.

In some cases the dog may develop an allergic reaction to the flea bites and show signs of dermatitis.  This is often referred to as Flea Allergy Dermatitis (FAD).

The standard diagnostic test for FAD is to thoroughly comb the dog, catching any debris etc on a sheet of a damp kitchen towel or damp cotton wool.  The kitchen towel/cotton wool is then examined for small dark specks of ‘dirt’ that change into a reddish brown stain.  This is indicative of active flea presence - the discolouration is due to blood from the flea faeces dissolving in the water.  

Remember that it is this faecal material that forms a considerable part of the diet of the flea larvae.

Prevention and Treatment.

The old maxim is certainly true – prevention of infestation is certainly better than trying to treat an existing problem.

There is a wide range of different products available for the control of fleas but basically they may be divided into 2 broad groups:

It is essential to consider the whole life cycle of the flea when devising a control strategy – remember that adult fleas only constitute about 1% of the potential population and that the other 99% are away from the dog.  For total control it is probably advisable to initiate a programme using both types of product, particularly if you have a dog which is hypersensitive to flea bites, i.e. an integrated control strategy should be adopted – just killing the adult flea population by treating the dog will not resolve the problem.

There are a number of products for flea control available ‘over the counter’ at pet shops, supermarkets, etc, and obtainable without the need for a veterinary prescription. Unfortunately, these generally seem to be less effective.

Points to consider:

The Pets at Home Flea Survey.

The following “article” appeared in the Veterinary Record (Volume 172, Issue 19) and acts as a reminder of the problems that may be encountered if the problem of fleas is allowed to get out of hand.

The results of a recent survey suggest that many pet owners only treat their pets for fleas two or three times each year.  The survey of 1000 pet owners was conducted by Pets at Home in March of this year (2013) to mark the start of “National Flea Month”.  This event takes place in May of each year with the aim of raising awareness about fleas, the problems that they may cause and how owners can protect their pets and their homes.

Half of the pet owners responding to the survey said that they had experienced fleas in the past two years; more than half said that they or a member of their family may have been bitten by a flea.  One in eight respondents said that they had been been confronted by a guest after being bitten in their home, and a fifth said that they had refused to let friends into their home because they were worried about the flea situation inside it.  Significantly,  a third of the pet owners surveyed admitted that they were finding it “nearly impossible” to deal with the flea problem.  One third of respondents said that they had found fleas on the furniture, clothes or their body, and one in ten had replaced carpets and furniture to try to control the problem.

However, the survey also suggested that half of pet owners treated only their pets for fleas, and ignored their carpets and soft furnishings.  Only a third of respondents treated both the home and the pet to deal with a flea problem and one in 10 believed that treatment was only necessary when “a flea” was seen in the home.

Veterinary surgeon Maeve Moorcroft, Head of Pets at Pets at Home, said: “For every flea that you see on your pet, there will be roughly 95 in your home at various stages of development, hidden in carpets, soft furnishings and even in clothes.  Rather worryingly, one flea can become 1000 in just 21 days.

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