Control on the host animal

A.Washing the dog or cat with insecticides (for example shampoos containing pyrethrin I and II and piperonyl bubutoxid) kills the adult fleas that happen to be on the dog or cat while it is being washed. There is no residual effect that protects against the fleas that jump on to the pet after the treatment. Therefore, these methods only functions a supplements to another control methods.

B. Flea collars and pour-on medications work by depositing the insecticide into the fur of the pet. From there, it sprinkles onto the places where the animal is, and thereby also where the flea eggs, larvae, etc. are located. These control methods can often kill a small population of fleas, but they are also used as support other control methods of the pet’s surrounding environment. Flea collars and pour-on products are also used preventively.

C. Methoprene is an insect hormone. When injected into the fur of the dog or cat, it will prevent the newly laid eggs from hatching. If used on its own, this control method best serves to prevent flea infestation or to control very small flea infestations. This can also be used as a supplement to other control methods.


An English zoologist has estimated that a typical cat flea population includes about 5000 fleas at various stages of development: 25 adult fleas are on the dog or cat. In the surroundings – mainly in carpets and in the animal’s sleeping place – there are 500 adult fleas, 500 cocoons, 3000 larvae and 1000 eggs. Therefore, the surrounding environment must also be treated.

There are several different ways to control fleas on cats. Some are quite effective if you are thorough, while others work best in combination with others. Most flea control methods are combinations of the following methods.


Is your dog or cat scratching itself more than usual? Is it biting itself with its front teeth? Have you seen any fleas? Do the people who are in contact with the pets get itchy red bumps – especially at the ankles? Check for fleas in the pet.

Regular flea inspection. Look closely in the groin of the pet where fleas like to sit. Go through the fur by moving your hand against the direction of the hairs, so that the hairs separate, and you have a clear view of the skin. Look for black irregular pieces of dirt (flea droppings) in the fur. Also check the place where the pet sleeps.

Thorough flea inspection. Collect material from the skin and examine it under a microscope. The material can be obtained by combing and brushing the dog or cat or by vacuuming the fur with a handkerchief on the mouth piece of the vacuum cleaner. Look for adult fleas, flea larvae, eggs and excrement.

The flea excrement. Small, black particles that dissolve and turn red in water can be flea excrement. Flea excrement contains blood from the host animal. The blood can be determined by means of one of the test strips used for the detection of blood in urine. Shake a little dust from the pet’s fur or sleeping place into a glass of water. Dip the test strip in the water and read the results. Positive result for blood (hemoglobin) can be caused by several different things, but flea excrement is the most common cause. More importantly, a negative result almost certainly proves that the pet is not infested by fleas and that it has not been infested by fleas recently.

ِTraps can also be used for the detection of fleas in the home. However, the traps only attract adult fleas. Despite this, they are occasionally used to keep the flea infestation level down. Lighted candles in water dishes on the floor of houses have been used to trap fleas. The fleas are attracted by the warmth of the light and are then trapped on the surface of the water. During the middle Ages, people wore small patches of fur underneath their clothes. The fleas liked to crawl around in the fur. Occasionally, they shook the patch of fur vigorously, and the fleas fell off. Another personal flea trap was a hollow stick with holes in it. It was placed under clothing in warm places and the fleas crawled through the holes. Inside the hollow stick, there was another stick which was covered in honey. The fleas then stuck to the honey-covered stick.

Today, it is still o you can buy flea traps. It is basically a light source placed on the floor and the fleas jump onto a tray with adhesive paper. By examination of the adhesive paper, it is possible get an idea if there is a need for flea control, or if control of a flea control treatment has worked satisfactorily. Since only adult fleas are attracted to the trap, it is not as effective as the control methods, which work on all stages of fleas, but it may work as a supplement other control measures.

Flea bite allergies

About 10% of dogs or cats that have had fleas for a long time develop an allergy to flea saliva. You can see that something is wrong from the skin and the fur. There may be dermatitis or hair-loss. In other cases, there is an increased production of dandruff. The itching is severe and troublesome for the animals. The symptoms are non-specific and it is a good idea to take the pet to a veterinarian who can determine, if it suffers from flea bite allergies (or other allergies, irritation or disease). This can be done by shaving a piece of skin and exposing it to flea saliva and examine the skin’s reaction.

If the causes of the skin symptoms are flea bite allergies, flea control is necessary. Conditions, which resemble flea bite allergies, can be seen in the dog if the owner – in an attempt to eradicate the fleas – has treated the animal’s fur with excessive amounts of pesticides.

Cat fleas bite people

How a cat flea sucks blood

Fig. 64. Cat fleas are almost upside down when feeding. (Wenk)

You can get infested by cat fleas by being in contact with infested pets, or by being in places where infested pets are or have been. Cat fleas, however, prefer their natural host animals such as dogs or cats. Consequently, the family pet can have fleas without the other family members noticing. For this reason, there is no risk of infecting other people at work or school with fleas, even if there is a pet with fleas in your home. The risk of being bitten is high if the dogs or cats are removed, and the fleas only have access to human blood – or if you are in places where dogs or cats are kept, and where the hungry fleas are now sitting and waiting for something to bite.

The may be in an empty apartment where the previous resident had pets, or in cellars and outbuildings where semi-feral cats have lived. After a vacation, it is also typical that many fleas when returning to a house where a large number of fleas have hatched while you were away attack you. In a house with cat fleas, flea bites are often seen equal frequent in children. On adult humans, flea bites are mostly common with women (2/3) and not as common with men (1/3). The cause of this phenomenon is not known.

Cat flea biology

A female cat flea can begin laying eggs when it has mated and have been an adult flea for a few days. Usually, a cat flea lives for about a month and can lay about 200 eggs. The eggs are laid in dog or cat fur. They fall off the host, wherever it is. On a dark background, the eggs look like spilled sugar. The hatching time of the eggs is 2-3 days at room temperature. The larvae then crawl into cracks in the floor or in the carpet, where they feed on organic material as well as bloody excrements from the adult fleas. During this period, which lasts a few weeks, they cannot tolerate low humidity. A relative humidity of less than 45% will kill them. This is one of the reasons why flea problems are not as common during the heating season.

At the end of the last larval stage, the larva spins a cocoon around itself. At the beginning, the cocoon is sticky. It is camouflaged by sand and dust, and is virtually impossible to vacuum up off a carpet. The larvae often stay in the cocoon for about a week. The adult flea now decides when to emerge from the cocoon. If the air is too dry, too cold, or if there is no host animal in the vicinity, the flea might stay inside the cocoon for months.

Dog fleas and cat fleas

Season for cat fleas in august

Fig. 62. Season for cat fleas in august.

The cat flea will happily bite people

The cat flea will happily bite people

The dog flea, Ctenocephalides canis and the cat flea, Ctenocephalides felis, belong to the flea species that bite humans in the absence of a more suitable host. The two flea species are similar.

Dog Flea was once fairly common, but today, the cat flea is more common. Cat fleas can live on both dogs and cats, while dogs’ fleas are never seen on cats. In actual practice, the pets are nearly almost infested by cat fleas, but if a dog has dog fleas, their biology and control are the same. Cat fleas are a relatively new problem. The increase in the number of inquiries about the species to the Danish National Pest Infestation Laboratory began in the late 1960s. Today, it is estimated that one third of all Danish dogs and cats are infested by fleas each year. It is not known why cat fleas have become so common in our parts of the world.

The number of pets has increased and at the same time, it has become increasingly common for the animals to have access to all rooms of the house. Another contributing factor is the increasingly common wall-to-wall carpeting. It is difficult to remove flea eggs and larva from the carpets just by cleaning them. From this perspective, the flea problem is a self-made problem.

Human fleas

Woodcut by Plinius of human flea

Fig. 60. Pliny the Elder wrote a book of natural history nearly 2000 years ago. The woodcut of the overweight human fleas, are from the 1492 edition, which states the following about human fleas: “They are called pulices because they prey on girls” (lat: puluere).

The human flea, Pulex irritans, was previously the most common flea inside houses. Nowadays, the chances of finding a population of human fleas inside a house are very small. In rare cases, human fleas can survive in damp houses where the larvae mainly live in creeks in the floor, in upholstered furniture or in damp mattresses.

When human fleas are so rare today it is because the climate inside houses have become too dry for the human flea larvae, which need high humidity. Human fleas do not only live on people. They are also found on pigs in stalls, as well as on foxes, badgers and rats, but not in Denmark.

In Southern and Central Europe, the human flea is still common inside houses. Human fleas are controlled by spraying the house with one of the remedies used to control cat fleas.

General treatment

You cannot eliminate a flea problem by treating the person which has been bitten. Neither powder, nor sprays or ointment has effect in this regard. The only rational thing to do is to find the hot bed and eradicate the fleas, where they are found in the house. If the itching is so bad that there is a danger that you might scratch a hole, it may help some to apply a local anesthetic on the bite. The remedies that pharmacies sell for relief of sunburn are excellent for this situation.

Flea bites

Flea breaking free from cocoon

Fig. 58. The adult flea breaks out of the cocoon by scratching a hole with its thorn on the head. The last larval skin and the pupa skin are left in the cocoon. (Humphries)

Fleas are mostly found on the ground or on the floor, and they will first and foremost attack ankles and lower legs.

In contrast to bed bugs, fleas also crawl under clothing. Often they even bite in places where the clothes are tight. Bites during the night, especially on the thighs, buttocks and shoulders are also often caused by fleas, which like to crawl under the body and bite where there is contact with the bed. One of the characteristics of flea bites is that they are often grouped. Each group is caused by bites of the same flea that had trouble finding a vein offering enough blood.