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The lifecycle of a Flea

Most people are only too aware of the existence of the humble flea, and even more aware of the trouble that it can cause them and their pets. However, despite the modern day advances in treatments available, we still find that many people have great difficulty in controlling fleas, and the associated diseases which can come with flea infestation.

Life Cycle of a Flea

The flea is an insect - a factor that a lot of people often miss. The flea life cycle is typical of a number of insects (eg butterflies) having four stages of development.




The eggs are laid by the adult flea, one egg at a time, while it is ´riding´ on your pet. The eggs which are produced fall off the animal and drop into the surroundings. This could be outside the supermarket, or in the park, but the vast majority will be in the place where your pet spends most of its time. This could be in its bed, on your sofa, on your bed (if you allow your pets such habits), or any other place where your pet spends a significant part of its life.

These eggs are tiny, but visible to the naked eye if you know what to look for. They are soon lost in the surroundings.



In normal surroundings eggs hatch in a couple of days, particularly in a nice centrally heated house. If the weather is particularly cold it can take up to a couple of weeks.

The flea grub or larva hatches by hacking a hole in the eggshell with a specially developed area of the mouth, usually refered to as a ´milk tooth´. This is important as we will see later when we discuss control.

The larva is sticky and covered in hairs. As a result it quickly becomes coated in any form of dust or debris in its surroundings which camoflage it nicely. It feeds on food particles present in the dust and debris arround it.

Flea larvae rapidly move away from light, so congregate anywhere where it is warm dark and dusty. The spaces between the cushions of the armchair are ideal, as is the gap between the fitted carpet and the skirting board.

The larva will live and feed here for on average about 2-3 weeks, but up to 3 months if necessary. it will grow to be about 6mm long, but hardly visiblebecause of its camoflage.



Once the larva has grown sufficiently, it will spin itself a silk cocoon, just like a caterpillar.

Having covered itself in sticky silk it is again disguised by the dust and debris in its surroundings. Safely inside it will metamorphose into the adult flea.


The Adult

The pupa can stay alive for up to 2 years before hatching out. The trigger seems to be vibration. A room can be empty for several months, but walking across the floorboards can cause enough disturbance that a host of fleas will be rampant within minutes. If anything in the vicinity is at the same temperature as blood, the fleas are rapidly attracted to it - you, the dog, the cat, or anything else which may be a source of food.

Although fleas have species which they prefer to feed from, it is a case of any port in a storm. Probably 80% or more of the fleas we see from dogs are technically cat fleas, and most of the fleas biting people are from their dog and cat. We have however on more than one occasion found human fleas from a cat or dog, so it is not all a one way journey.

Adult fleas are about 2-4mm long depending on the species. They are flattened from side to side to allow them to run through the hair on their host. Jumping helps escape and access to a new host, but is useless otherwise.

Adults can live for several weeks. They feed on the blood of their host. They push a fine tube made from their mouthparts into the skin and into the soft tissues below. To stop blood clotting while they feed they spit saliva into the bite which acts as an anticoagulant. They then suck the blood.

Having fed, they are then able to lay an egg. They can repeat this process up to once every half hour when at their peak, and can lay more than 300 eggs in a lifetime. These move into the surroundings and off we go again! The whole cycle in good Summer weather or efficient central heating can be comfortably less than 4 weeks.

Dispite popular belief, most fleas do not pass animal to animal, but stay with the host that they first attack. Most infections are picked up directly from the environment. Of course other infected animals will be contributing to the pool of new infestation by shedding eggs. Hence most animals in fact set up their own private supply of pet fleas. This is important to consider when we look at control later.


The Problem

Dog with FleasFleas are irritating creatures, as anyone will know who has ever been bitten. When a flea bites, it first blows a little bleb of saliva under the skin of the host to stop the blood clotting while it feeds. It is the response of the body to this saliva which causes the local reaction which we understand as a flea bite.

With the occasional flea bite, just as we would immediately pay attention to the area and attempt to remove the source of the irritation, so do our pets. Their defense is usually to swallow the flea, hence removing it from the site. At this stage the flea problem is minimal.

If the attack is repeated and by significant numbers however, problems of a much more severe nature develop. These include

  • Flea allergic dermatitis
  • Tapeworm infestation
  • Bacterial skin disease and seborrhoea
  • Anaemia from blood loss, particularly in very small puppies and kittens which are heavily infested.

These problems are detailed elsewhere in our clinical notes section.



If we understand the life cycle of the flea, we can work to efficiently break the cycle and keep our pets almost totally free. Any persons pet can get the occasional flea, but they should never need to be troubled by a severe outbreak with correct use of modern products from their Veterinary Surgeon.

As pet´s lifestyles vary, so do the treatments which are used to treat infestations with fleas, and to prevent reoccurence. Your Veterinary Surgeon or one of their trained staff, eg a qualified Veterinary Nurse, will be the best person to guide you to the most effective treatment and prevention for your situation. The decision may be based on the potential exposure of the pet to other external parasites - ticks, lice or mange mites - as well as just protection against fleas.

Products are chosen to have an effect on all stages of the flea life cycle. Recent television advertising of products has misled some people into believing that one product can produce a miracle cure. However in practice we find that we are only truly successful with a combination of products, particularly in the early treatment stages of an outbreak. It cannot be stressed too often that the treatment will need tailoring to the individual situation.

Treatment is aimed at two main areas.

  1. Treating the Adult Fleas on the pet. This is the most obvious part of treatment, but even the best products can fail to give total control without...
  2. Treating the Environment to remove the developing fleas to stop future infection.

The ´Pros and Cons´ of the various types of treatments are outlined below in generalisation. None of the products can approach their efficiency if the manufacturers instructions are not read and followed carefully.


Treating the Animal

Cat with Fleas


Not very effective. May remove perhaps 10% of the burden on a pet after considerable time grooming!



These are now accepted by the majority of people to be virtually obsolete in the effective control of fleas. There is difficulty in obtaining an even distribution through the coat. The powder is breathed in by the person treating the pet as well as the pet which can aggravate respiratory problems in either, and the products used are not as effective as more modern drugs, many fleas being resistant to the drugs in the powder.



Bathing is a good way to remove the fleas in an animals coat, to remove the flea faeces and debris that accumulate, and generally to stimulate a healthier environment for the skin. The vast majority of products stop working, however, once the coat is rinsed and dried. There is no lasting residue of drug in the coat to resist reinfection. New infestation will begin literally within the hour in an infected environment.

Bathing may be a good first measure to begin a course of treatment which uses an alternative method to maintain freedom.



Collars have in the past played a useful role in flea control. Most of the effective collars contain OPs, so should be used with care. The instructions should be read carefully before use. They should not be handled or sucked by children, particularly when freshly applied.

Most collars are active for about 3-4 months, when they should be replaced. Collars are available which act for longer. Many contain coat conditioners too. Some are ´Organic´ using natural pyrethrins, but in our experience they are not quite so effective as the OPs.

Collars used for cats MUST have an easily breakable fastening or a safety elasticated strap. We regularly see cats which have collars of all types dangerously tight.

Collars can cause local skin reactions and sensitivities, and also will cause breaking and loss of the hair locally, as will any collar.

Always read the instructions fully before buying a product of this type.

Useful products include.

  • ´Preventef´ collars ( Virbac) for dogs over 3 months and cats over 6 months of age. Contain OPs and skin coditioners. Last about 4 months. Cannot be used with pregnant animals.
  • ´Natura´ flea collar ( Virbac) for dogs over 3 months. Contains Permethrin (not OP)
  • ´Prevender´ dog collar ( Virbac) Lasts for 300 days, and will break when due for replacement. Contains OPs. For dogs over 3 months. Not for pregnant or nursing bitches.



There are a number of sprays available with a variety of drugs in them. The effectiveness varies. There are two main types - aerosols and ´pump action´ sprays.

Most sprays have some residual effect in the coat. This varies with the drug, some being as little as one week, others being up to 2 months or more. If looking at price, then calculate how long a spray will last. A cheaper spray is not so econimical if you need four cans to treat your pet for the same length of time as one of the more expensive products costing twice as much.

Some of the older sprays commonly used contain Organo-phosphorus compounds (OPs). These should be used with care, particularly following the instructions for use carefully. Special care should be taken when used in the vicinity of people with asthmatic and respiratory problems. If you are worried about this, ask before you buy. As OPs are absorbed through the skin they can build up in the body and show side effects in the pet also. Although they are extremely effective in killing skin parasites, and have done a tremendous job in reducing flea related skin disease, these drugs are rapidly being phased out of use.

Many animals, particularly cats, react quite violently to the use of aerosols. Once you may succeed with your cat, but next time there is a blood bath (your blood!), and the treatment is going to be ineffective. Your veterinary surgeon or nurse may be able to show you how to restrain your cat safely to spray it. This problem has been somewhat reduced by the use of pump action sprays, but care is always needed.

Similarly the use of foams to treat dogs and cats in recent years has helped. The aerosol is sprayed onto the (preferably rubber gloved) hand of the operator and produces a foam ball which is massaged into the coat. The pet is much more tolerant of the lack of direct application from the can.

Useful Products:

  • ´Frontline´ ( Meriel Animal Health Ltd). A pump action spray. Will last up to 4 weeks in dogs and 6 weeks in cats with one treatment. Can be used on puppies and kittens when just a few days old. This is currently the front runner in this group. See also Spot On preparations.

NB it is essential that aerosol products produced to treat the environment are never used to treat infestations on pets.


Spot-On preparations

These are very much the products of choice for many situations.

Spot on preparations are small quantities of concentrated drug which are applied to the skin of the animal in an area they cannot be licked off, usually high on the back of the neck.

OP spot-ons received bad press a couple of years ago when a manufacturing fault led to a series of overdose reactions. As a result OP spot-ons were withdrawn from the veterinary market and are not now used. Take care however as they are reportedly used still in some ´pet shop´ preparations.

Modern spot-on preparations are by far the most convenient and effective of the drugs used to treat fleas today. The simplicity of use by applying a few drops of liquid to the skin causes no stress to the pet, and hence none to the operator either. The modern products are very effective flea killers and are very safe to use.

Useful Products include

  • ´Advantage´ ( Bayer plc). A spot on applied monthly to dogs and cats over 8 weeks of age. Very safe. Very effective but only against fleas.
  • ´Frontline´ ( Meriel Animal Health Ltd). Applied every 2 months to dogs above 10 weeks of age and every 5 weeks to cats above 12 weeks of age. Very safe. Very effective not only against fleas but also against ticks.
  • ´Revolution´ ( Pfizer Ltd). A new spoton which not only gives good protection against fleas, but also is extremely effective aginst Sarcoptic Mange mites. It also kills the more common roundworms. It needs to be applied monthly, and is the most expensive of the products of this type.



Treating the Environment.

As stated above the prevention of reinfection is essential in the control of flea outbreaks. The ´treatment´ of the environment is a very important part of control by killing eggs, larvae, and young adults before they attack their host. It is important to realise that most animals will build up their own private breeding colony of fleas just for themselves, and failed treatment regimens ar usually associated with failure of the owner to appreciate this fact.

There are two main ways to treat the environmental developement of fleas. The traditional method is the use of insecticidal chemicals which stay active and destroy any developing insect life. These are usually available now as Aerosol preparations which are applied according to the directions in the areas where fleas are liable to develop - ie anywhere warm, dark and dusty.

Most of these chemicals are toxic and must be used with care. Always read the directions fully before purchase to ensure that the product will do what is required of it and that it is safe to use in your particular environment.

Modern products specifically for flea control contain hormonal drugs which can prevent the hatching and developement of the flea larvae, making them much more specific for this job.

Products of this type which are available are

  • ´Indorex´ ( Virbac) A household spray with developemental inhibitors which will stay active for up to 1 year.

There is no doubt that over the years many of us in the veterinary profession have met a number of situations where the environment is heavily contaminated and the slightest move brings clouds of starving fleas clambering up the legs looking for a decent meal. In these circumstances, or where it is not possible for health or safety reasons for an owner to treat the infestation alone, we have always found a confidential and sympathetic response from our local Environmental Health Department. Their charges are about the same as a couple of environmental sprays in our area, and the treatments which they use are extremely effective, and their operators quick and efficient. There should never be any stigma or loss of face in contacting these people for help.



A popular recent developement has been the use of ´Program´ ( Novartis Animal Health Ltd) tablets in the dog or Program liquid or Program injection in the cat. Program is a chemical effective only in the developing flea. It has no function in the pet at all, and is totally inert. The result is that the milk tooth (see above) of the larva in the flea egg cannot develop, the flea cannot hatch, and so dies in the shell. The chemical is picked up by the flea as it feeds prior to laying eggs. It is a dramatically effective way of breaking a flea cycle of reinfection. It stops fleas getting to the places where the sprays above cannot be used.

However it is important to note that many people expect Program to kill adult fleas also - IT WON´T. They will die naturally of old age and hopefully they will not be replaced. Program is a superb drug however to stop reinfection once the initial infestation on the pet is clear and in combination with one of the good adult flea killers such as ´Frontline´ (see above) the effect is outstandingly effective in even the worst outbreaks.

Program on its own fails when contact with environments which are shared with other animals which are not using the product, as repeated reinfection can occur. However, the animal´s own home environment will still stay clear.

Similarly, as the flea must feed for the drug to be effective, then it is of limited use in flea allergic skin disease, as the bite is the initiation of the skin´s response.


In Conclusion.

Any cat or dog which goes through its life having not had a flea bite at sometime along the way is pretty near a walking miracle. Fleas are a fact of life. However with a little knowledge and correct advised use of modern products the problems of fleas and their control can be almost totally eliminated.


Flea Facts

All You Wanted to Know About Fleas and More

Everyone knows fleas aren´t fun. However, here are some flea facts that will amaze you!


  • Flea fossils date back to the Lower Cretaceous period, meaning fleas have been around for about 100 million years. At that time, fleas may have infested a Tyrannosaurus Rex or Triceratops!


  • Fleas can jump up to 150 times their own length. To put that into perspective, if a human competed in the Olympic long jump with that ability, that athlete would certainly win the gold medal with a gravity-defying 1,000 foot long jump. So they can easily jump onto your pet from the ground, or from another pet.


  • On average, a flea´s lifespan is two to three months. However, pre-emerged fleas (not living on a pet) can survive undisturbed and without a blood meal for more than 100 days.


  • The female flea can lay 2,000 eggs in her lifetime.


  • The female flea consumes 15 times her own body weight in blood daily.


  • The largest recorded flea is the North American Hystrichopsylla schefferi. Found in beavers, it can measure 12mm in length - almost 1/2-inch!


  • A flea can bite 400 times a day1. That´s a rate of 4,000 bites a day if your pet has just 10 fleas.