Fleas

Fleas

Fleas are small, wingless bloodsucking insects (order Siphonaptera) with a characteristic jumping movement. They feed mainly on mammals but also on birds. Of the 3000 species only a dozen commonly attack humans. The most important species are the rat flea, the human flea and the cat flea (Fig. 4.6). Their bites can cause irritation, serious discomfort and loss of blood. The rat flea is important as a vector of bubonic plague and flea-borne typhus. Cat fleas incidentally transmit tapeworms. The sand flea or jigger burrows into the skin of humans and may cause infections. Fleas that bite people occur in most parts of the world.

Biology

The life cycle of fleas has four stages: egg, larva, pupa and adult (Fig. 4.7). Adult fleas are 1-4 mm long and have a flat narrow body. They are wingless with well developed legs adapted for jumping. They vary in colour from light to dark brown. The larvae are 4-10 mm long and white; they have no legs but are very mobile. The cocoon (pupal stage) is well camouflaged because it is sticky and soon becomes covered with dust, sand and other fine particles.

Both female and male fleas take blood-meals. Fleas breed close to the resting and sleeping places of the host, in dust, dirt, rubbish, cracks in floors or walls, carpets, animal burrows and birds’ nests. High humidity is required for development. The larvae feed on organic matter such as the faeces of the host, small dead insects and undigested blood expelled by adult fleas. At the end of the larval period the larva spins a loose whitish cocoon within which it develops into a pupa.

Fig. 4.6. A cat flea (Ctenocephalides felis felis) (by courtesy of the Natural History Museum, London).

Fig. 4.7. The life cycle of the flea (by courtesy of the Natural Natural History Museum, London).

Fig. 4.8. A scratching cat is an indication of a flea infestation.

The adult fleas are fully developed within 1-2 weeks but only emerge from the cocoons after receiving a stimulus, such as the vibrations caused by movement of the host. In vacant houses they may survive in the cocoons for up to a year. People moving into a vacant house can cause many fleas to emerge simultaneously from the cocoons and attack people or animals in large numbers. Under optimal conditions the development from egg to adult takes 2-3 weeks.

Behaviour

Fleas avoid light and are mostly found among the hairs (Fig. 4.8) or feathers of animals or in beds and in people’s clothing. If possible, a flea will feed several times during the day or night. Heavy infestations with fleas are recognized by marks on clothing and bedding of undigested blood ejected by the fleas. Most flea species feed on one or two host species, but in the absence of their normal host they feed on humans or other animals. Adult fleas can survive several months without food. Fleas move around by jumping; some species can jump as high as 30 cm.

Public health importance

Nuisance

Humans are most commonly bitten by the cat flea, Ctenocephalides felis and, less commonly, the dog flea, C. canis. The so-called human flea (Pulex irritans) is, in spite of its name, less important. Fleas jump up from the ground and most frequently attack people on the ankles and legs, the easiest parts to reach, although sleeping people can be attacked anywhere on the body. Flea bites cause irritation and sometimes extreme discomfort. Heavy infestations may cause allergic reactions and dermatitis.

Plague

Plague is a disease caused by the bacterium Yersinia pestis. It occurs primarily in wild animals, such as rats and other rodents. Plague bacteria are transmitted by fleas, and humans may be infected by fleas that have fed on infected animals. In the past, plague was called the black death and caused disastrous epidemics.

Plague is still dangerous because it occurs widely in rodent populations. Rural or sylvatic plague may be contracted in the western USA, South America, Africa, the former USSR, parts of the eastern Mediterranean area, and central and southeast Asia. Human plague frequently occurs in several countries in Africa, Bolivia/north-eastern Brazil, Ecuador, Myanmar, Peru and Viet Nam (9).

Rural plague is acquired by people entering rural areas and handling wild animals. Most at risk are hunters who may be bitten by infected fleas while handling recently killed animals.

Urban plague may occur when rats living in and around human dwellings are infected. Rat fleas (Xenopsylla species) that normally feed on rats may occasionally feed on humans and thus spread the disease to them. When rodents infected with plague die the fleas leave their hosts and are then likely to attack and infect people. Other fleas, such as the human flea, may subsequently transmit the disease from person to person.

There are three clinical types of plague:

· Bubonic plague. Swellings (buboes) filled with bacteria develop in the lymph nodes, especially in the armpits and groin. This form is normally transmitted to humans by infected fleas. If left untreated, it causes death in about 50% of cases.

· Pneumonic plague. This is a secondary form in which the lungs become affected. It is highly contagious, the plague bacillus easily spreading from person to person in sputum or droplets coughed up or sneezed by sick people. Pneumonic plague occurred in epidemics in past centuries, killing millions of people. If left untreated it very often results in death.

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· Septicaemic plague. The bloodstream is invaded by the plague bacillus, resulting in death before one of the above two forms can develop.

Prevention and control

Partial immunity is acquired after an infection. A vaccine is available which provides protection for a period of only a few months. Treatment with streptomycin, tetracycline or its derivatives or chloramphenicol is highly effective if used within a day after the onset of symptoms.

Urban plague is controlled by rapidly applying insecticide dusts in rodent burrows and on to rodent runways where it will be taken up by the animals on their fur, thus killing the vector fleas. Dusting against fleas should be followed by measures to control rodents.

People working in the field may protect themselves by dusting their clothing with insecticidal powder, using impregnated clothing, and using repellents on a daily basis.

Flea-borne typhus

Flea-borne typhus, also called murine typhus fever, is caused by Rickettsia typhi and occurs sporadically in populations of rats and mice. It is transmitted mainly by rat fleas and cat fleas, and humans can become infected as a result of contamination from the dried faeces and crushed bodies of the fleas. The disease occurs worldwide and is found in areas where people and rats live in the same building. Its symptoms are similar to those of louse-borne typhus (see p. 257) but milder.

Prevention and control

Immunity is acquired after the first infection. The treatment of sick people is similar to that for louse-borne typhus (see p. 257). Control is carried out by applying residual insecticides to the runs, burrows and hiding places of rats. If these measures are successful in killing fleas, rodent control measures can be taken (see p. 250, box).

Other diseases

Fleas occasionally transmit other diseases and parasites from animals to humans, for instance tularaemia caused by the bacillus Francisella tularensis, and the parasitic tapeworms that occur in dogs and cats. Children playing with domestic pets may become infected by swallowing fleas that carry the infective stage of the worms.

Control measures

The recommended control methods depend on whether the intention is to deal with fleas as a biting nuisance or as vectors of disease.

Fleas as a nuisance

An effective repellent, such as deet, applied to skin and clothing, prevents fleas from attacking. A disadvantage is that repellents applied to the skin last only a few hours (see Chapter 1). Longer-lasting protection is obtained by dusting clothing with insecticide powder (see p. 262) or by using insecticide-impregnated clothing (see Chapter 1).

Simple hygienic measures

Fleas and their eggs, larvae and cocoons can be effectively removed by keeping houses well swept and floors washed. Removal with a vacuum cleaner is also effective. When people enter an infested house that has been vacant for some time, large numbers of newly emerged fleas may attack. The treatment of floors with detergents, insecticides or a solution of naphthalene in benzene is recommended; care should be taken to avoid inhaling benzene fumes.

Application of insecticides

Heavy infestations can be controlled by spraying or dusting insecticides into cracks and crevices, corners of rooms and areas where fleas and their larvae are likely to occur. Insecticides can also be applied to clothing and the fur of animals. Fumigant canisters that produce aerosols of quick-acting insecticides (e.g. the pyre-throids, propoxur and bendiocarb) kill fleas directly and are convenient to use (see p. 240 and Chapter 3). However, the insecticidal effect is brief and reinfestations may appear quickly.

Cat and dog fleas

Fleas can be detected in the hair around the neck or on the belly of cats and dogs. Treatment involves applying insecticidal dusts, sprays, dips or shampoos to the fur. Dusts are safer to use than sprays because the insecticides are less likely to be absorbed through the skin in the dry form. Dusts also produce less odour and do not affect the skin as much as sprays. Carbaryl and malathion should not be used on kittens and puppies under four weeks of age. Pets can be provided with plastic flea collars impregnated with an insecticide. Flea collars are effective for 3-5 months, whereas other treatments give only short-term control.

Recently, lufenuron tablets have been used to control fleas in cats and dogs. The tablets are administered once monthly at a dose of 30 mg per kg of body weight to cats and 10 mg per kg of body weight to dogs and are safe for use in pregnant and nursing animals. Lufenuron is taken up by the female flea during feeding and acts by inhibiting egg development (10).

Dusts must be rubbed thoroughly into the hair and can be applied by means of a shaker (Fig. 4.9). They must not be allowed to get into the eyes, nostrils and mouths of animals. Heavy applications should not be made to the abdomen as the material will be licked off. Application should begin above the eyes and all the areas backward to the tail and haunches should be covered, ensuring thorough treatment around the ears and underneath the forelegs. A small animal can be treated with one tablespoonful of dust, while 30g may be required for a large dog. Sprays must wet the hair completely and can be applied with a hand-compression sprayer. It is also possible to spray with an insecticide aerosol from a pressurized spray can.

Fig. 4.9. Dusting a dog with insecticide powder to control fleas.

Re-treatment may be necessary if reinfestation occurs. Important sources of reinfestation are the places where animals or humans sleep or spend much time, such as beds, bedding and kennels. Where possible, animal bedding should be burned or laundered in hot soapy water. A vacuum cleaner may be used to remove accumulations of dust that contain flea larvae and pupae, and infested premises can then be treated with a residual insecticide. Treatment with insecticidal powders or solutions is possible (11). Because flea cocoons are much less susceptible to insecticides than the larvae and adults, treatments should be repeated every two weeks over a period of six weeks to ensure that all emerging fleas are killed (12).

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This flea species does not usually remain on the person after feeding and by day it rests in cracks, crevices, carpets and bedding. Regular cleaning of houses, and of bedrooms in particular, should prevent large infestations.

More effective control is achieved by dusting or spraying insecticides on to mattresses and cracks and crevices in floors and beds. Bedding left untreated should be washed and cleaned during insecticide application. Fleas in many parts of the world have developed resistance to DDT, lindane and dieldrin (1315). Suitable insecticides for spraying or dusting are indicated in Table 4.2.

Table 4.2 Insecticides and application methods effective against fleas

Type of application

Pesticide and formulation

malathion (2%), diazinon (0.5%), propoxur (1.0%), dichlorvos (0.5-1.0%), fenchlorvos (2%), bendiocarb(0.24%), natural pyrethrins (0.2%), permethrin (0.125%), deltamethrin (0.025%), cyfluthrin (0.04%), pirimiphos methyl(1%)

Pesticide power (dust)

malathion (2-5%), carbaryl (2-5%), propoxur (1%), bendiocarb (1%), permethrin (0.5-1.0%), cyfluthrin (0.1%), deltamethrin (0.05%), temephos (2%), pirimiphos methyl(2%), diazinon (2%), fenthion (2%), fenitrothion (2%), jodfenphos (5%), (+)-phenothrin (0.3-0.4%)

propoxur (0.1%), (+)-phenothrin (0.4%)

propoxur, dichlorvos, cyfluthrin, permethrin, deltamethrin, (+)-phenothrin

Flea collar for dog or cat

dichlorvos (20%), propoxur (10%), propetamphos, diazinon

diethyl-toluamide (deet), dimethyl phthalate, benzyl benzoate

Retreatment is probably not needed if all infested places in a house are treated or cleaned. Infants’ bedding should not be treated but should be thoroughly washed.

Fleas that transmit diseases

Control measures during epidemics of plague or typhus must be effected in two stages:

(1) insecticidal dusting of rat habitats to kill rat fleas;
(2) rat control.

A control campaign with the sole aim of killing rodents could result in increased disease transmission to humans: the deaths of many rodents could cause large numbers of fleas to leave the dead hosts and seek alternative sources of blood.

The most common and effective method of controlling rodent fleas has been to use DDT in a 10% dust formulation. Alternative insecticides in dust formulation are increasingly used (see Table 4.2) because of the resistance of fleas in many areas to DDT and also because of environmental concerns.

Dust is applied to burrows, runways and other sites where rodents are likely to pick it up. When the rodents groom themselves they spread the dust on their fur, thus killing the fleas.

Before control is begun, it is important to know where rodent burrows and runways are. To save insecticide, the burrows should first be closed off; only those that are subsequently reopened should be treated. Insecticidal dust should be blown into each burrow with a duster. A patch of dusting powder, 1 cm in depth, should be left around the opening. Patches of dust 15-30 cm wide should be placed along runways. Dust should be applied only where it will remain undisturbed by humans and the wind. Care must be taken not to apply insecticides to areas where they can contaminate food. Many insecticidal dusts remain effective for 2-4 months if used indoors in undisturbed places.

Fig. 4.10. Equipment for applying anti-flea dusts (© WHO).

A plunger-type duster is suitable for fast applications of dust to rodent burrows and runways, in attics and spaces under buildings. It consists of an air pump like a bicycle pump to which a container for the dust is attached. The air from the pump is led into this container, agitating the contents and expelling them from an orifice (Fig. 4.10).

Alternatively, a hand shaker can easily be made from a can by fitting a 16-mesh screen at one end. A can with nail-holes punched in the top can also be used. Insecticidal dust of low toxicity can be applied to human clothing or the fur of animals with such equipment.

Integrated rat and flea control

To control urban outbreaks of plague or typhus, insecticides to kill rat fleas are applied at the same time as or a few days earlier than rat poisons. Suitable rat poisons are warfarin, coumafuryl, difenacoum, brodifacoum, coumatetralyl, bromadialone, chlorophacinone and zinc phosphide (16, 17).

In places where food for human consumption is stored and in crowded areas, such as markets, it is safer to use bait boxes (Fig. 4.11) in which the rodents contaminate themselves with the antiflea dust before they die from eating the toxic bait. Bait boxes can be placed along rodent runs at intervals of 60 metres. A suitable bait consists of 100g of rolled oats mixed with rat poison.

Fig. 4.11. Models of bait boxes.

(a) Boxes made of bamboo.

(b) A bait box made of floorboard (30 x 20 cm) covered by a metal roof (© WHO).

Sand fleas or jigger fleas

The sand flea, chigoe or jigger flea (Tunga penetrans) is not known to transmit disease to humans but, unique among the fleas, it is a nuisance because the females burrow into the skin. Sand fleas occur in the tropics and subtropics in Central and South America, the West Indies and Africa.

Biology

The larvae of sand fleas are free-living and develop in dusty or sandy soil. The adults are initially also free-living but, after copulation, the fertilized females attach themselves under the skin of humans, pigs, dogs, poultry and other animals, penetrating soft areas of skin, for instance cracks in the soles of the feet, between the toes, and under the toenails. Other parts of the body may also be affected.

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Public health importance

Usually a person is infested by only one or two jiggers at a time but infestation with hundreds is possible. People who do not wear shoes, such as children, are most commonly affected. The flea burrows entirely into the skin with the exception of the tip of the abdomen. It feeds on body fluids and swells up to the size and shape of a small pea in 8-12 days (Fig. 4.12). The body of the female flea is completely filled with thousands of eggs which are expelled in the next weeks (Fig. 4.13). Most of the eggs fall to the ground where they hatch after a few days.

Fig. 4.12. The female sand flea attacks bare-footed persons by burrowing into soft skin on the feet (18).

Fig. 4.13. Detail of foot with jigger infections. Eggs are expelled through the dark opening in the centre (by courtesy of the Natural History Museum, London).

An infestation begins to irritate and itch when the female is almost fully developed. Sometimes it causes severe inflammation and ulceration. If the female flea dies in the skin it may cause a secondary infection which, if ignored, could lead to tetanus, gangrene and even the loss of a toe.

Natural extrusion of the egg sac or removal of the jigger with a dirty pin or needle leaves a tiny pit in the skin which may develop into a sore. The sore may extend and develop into a septic ulcer. An infection under a toenail may cause pus to form.

Prevention, control and treatment

Jigger populations often maintain themselves in the domestic environment by breeding on livestock and domestic animals. Efforts should be made to remove the jiggers from these animals. Infections in dogs can be controlled by the administration of ivermectin (0.2 mg/kg of body weight) or by bathing the feet with dichlorvos (0.2%) (19). The former treatment may kill other parasites, such as Dermatobia larvae, which causè skin infections. In infested areas, people should inspect their feet daily for freshly burrowing jiggers, which are visible as minute black spots and cause an itchy sensation.

Wearing shoes prevents attacks. The fleas may also be deterred by a repellent applied to the skin, although walking bare-footed in dirt quickly removes it. If it is possible to locate the area of soil where the jiggers originate it could be burnt off or sprayed with a suitable insecticide in an effort to kill the fleas.

With some skill it is possible to remove the jigger with forceps or with a sharp object, such as a needle, a thorn or the tip of a knife (Fig. 4.14). The object and the site of infection should be cleaned, if possible with alcohol, to reduce the risk of infection. Removal can be done in a painless way but care should be taken not to rupture the egg sac. Infection may result if eggs or parts of the flea’s body are left in the wound. After removal, the wound should be dressed antiseptically (with alcohol or iodine) and protected until healed.

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Fleas

The flea species present in New Zealand are mostly a public health nuisance, but have the potential to be a public health risk due to their ability to transmit infectious diseases.

Fleas range from about 1–5 mm in size (usually around 3 mm). Their bodies are flat, shiny and have a tough surface. These features mean the flea can move through the hair of their hosts without being dislodged.

Where fleas live

Fleas are very common in New Zealand, particularly in association with cats and dogs.

Several species are found on a range of warm-blooded hosts, including humans (eg, the cat flea, the dog flea, the bird flea, and the northern rat flea). Adult fleas are found on the hosts themselves, whereas the larvae and pupae live in places like the burrows or nests of hosts.

Dog fleas, photo by Pest and Diseases Image Library, Bugwood.org.

When fleas have not fed for some time they are likely to be less specific about their choice of host and this may involve having a human blood meal. While the human flea is rare in New Zealand, cat and bird fleas are very common.

Health risks from fleas

Fleas can transmit infectious diseases from one host to another and are historically known as carriers of the plague. However, today fleas are better known as pests and for the irritation they cause.

Fleas can carry tapeworms

Fleas can play host to tapeworms, which can also cause infection in humans. You can get tapeworms if you accidentally swallow an infected flea. This can be treated with antibiotics.

If you get flea bites

Treatments for flea bites are limited – however, if a rash develops then you should see your doctor.

Getting rid of fleas

There are things you can do to reduce the chances of having fleas.

  • If you have pets, ensure they are treated for fleas.
  • Vacuum the carpets and furnishings your pets use to remove fleas.
  • Clean or remove bedding and nests.

Use insecticide to kill fleas if you find them.

www.health.govt.nz

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