Medical Entomology

Order: Siphonaptera(Fleas)

• There are 2500 species and subspecies of fleas belonging to 239 genera, but only relatively few are important pests to human.

• About 94% of known species bite mammals and the reminder are parasitic to birds.

• Fleas are found throughout most of the world, but many genera and species have a more restricted distribution, for example the genus Xenopsylla, which contains important vectors of plague, is confined to the tropics and warmer parts of some temperate countries.

General characteristics• Mouth parts points downwards and

are adapted for piercing and sucking. Both males and females pierce the skin and suck the blood of the host.

• Wings are absent, due to parasitism.

• Hind legs enlarged and modified for jumping.

• The abdomen consists of ten segments, the last three of which are greatly modified for sexual purposes, particularly in the male.

• Male fleas are easily distinguished by the rakish upward tip of the abdomen, while in females it is rounded. In males, the terminal abdominal segments are modified into claspers for holding the female.

• While in females, the most distinguished organ of the genitalia is the spermatheca.

• A flat or convex plate called sensilium or pygidium, and having spicules and dome shaped structures is present dorsally on segment eight of both sexes. This structure is characteristic of fleas, but its exact function is unknown, although it is clearly sensory.

Holometabolous insects.

Taxonomic features

1) The mesopleuron (a sternite located above the middle pair of legs) in several genera it is clearly divided into two parts by a thick vertical rod like structure termed the mesopleural rod

Ex, Xenopsylla cheopisRat flea

Taxonomic features

2) In some species a row of coarse well developed tooth like spines, collectively known as comb or ctenidium, may present along the bottom margin of the head capsule (Genal comb) and/ or along the posterior margin of the pronotum (Pronotal comb)

Ctenocephalides canis(Dog flea)

Pronotal comb

Genal comb

Nosopsyllus henlyei

• Only Pronotal comb present.

Pulex irritans(Human flea)

• Neither pronotal nor genal comb present.

• Pleural rod of mesopleuron absent.

Human flea Vs. Rat flea

Medical importance

Biological disease transmission.• 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 rat fleas, and humans may be infected by fleas that have fed on infected animals.

• When rodents infected with plague die the fleas (rat fleas) leave their hosts and are then likely to attack and infect people (Zootic plague).

• Other fleas, such as the human flea, may subsequently transmit the disease from person to person (Demic plague).

Tunga penetrans (Sand 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.

External morphology

• Adults of both sexes are exceedingly small, only about 1mm. long.

• They have neither genal nor pronotal combs.

• They are easily separated from other fleas of medical importance by their very compressed three thoracic segments, and the scarcity of spines and bristles on the body.

Life cycle

• Eggs are shed by the gravid female into the environment . Eggs hatch into larvae in about 3-4 days and feed on organic debris in the environment. Tunga penetrans has two larval stages before forming pupae .

• The pupae are in cocoons that are often covered with debris from the environment (sand, pebbles, etc).

• The larval and pupal stages take about 3-4 weeks to complete. Afterwards, adults hatch from pupae and seek out a warm-blooded host for blood meals.

• Both males and females feed intermittently on their host, but only mated females burrow into the skin (epidermis) of the host, where they cause a nodular swelling . Females do not have any specialized burrowing organs, and simply claw into the epidermis after attaching with their mouthparts.

• After penetrating the stratum corneum, they burrow into the stratum granulosum, with only their posterior ends exposed to the environment . The female fleas continue to feed and their abdomens extend up to about 1 cm.

• Females shed about 100 eggs over a two-week period, after which they die and are sloughed by the host’s skin. Secondary bacterial infections are not uncommon with tungiasis.

Treatment

• 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. 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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