An Introduction to Medical Entomology
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GENERAL CHARACTERISTICS OF SIPHONAPTERA
[Also See: Siphonaptera Key ]
Siphonaptera (or Suctoria) are wingless insects that are ectoparasitic on warm-blooded animals. They are compressed laterally with short antennae positioned in grooves. They have piercing and sucking mouthparts, maxillary and labial palps are present, coxae are large and the tarsus has five joints. Metamorphosis is holometabolous. The larvae are legless and the exarate pupae are enclosed in a cocoon.
There are about 221 genera and over 2,205 species and subspecies of fleas in the world. The order has five families with species of medical importance: Hectopsyllidae, Dolichopsyllidae, Pulicidae, Hystrichopsyllidae and Ischnopsyllidae. Service (2005) reported that about 94 percent of species attack mammals while the remaining species are parasites of birds. Fleas are also widely distributed all over the world, but the most important vectors of plague (Yersinia pestis) occur in the tropics and subtropics. Medically the most important genera are Ctenocephalides. Leptopsylila. Nosopsyllus. Pulex. Tunga smfXenopsylla. Their combs and meral rod can identify them:
Thirteen species which are of medical importance include: Ctenocephalides canis (Curtis) [dog flea], Ctenocephalides felis (Bouche) [cat flea], Cediopsylla simplex (Baker) [rabbit flea], Ceratophyllus gallinae (Schrank) [chicken or hen flea], Ctenophthalmus pseudargyrtes Baker [Small mammal flea], Echidnophaga gallinacea (Westwood) [stick tight flea], Hoplopsyllus anomalus Baker [rodent flea], Leptopsylla segnis [European mouse flea], Nosopsyllus fasciatus (Bosc.) [rat flea], Oropsylla montana (Baker) [ground squirrel flea], Pulex irritans L. [flea of humans], Tunga penetrans L. [jigger flea] and Xenopsylla cheopis (Roth.) [Oriental rat flea]. The common names of fleas (e.g. "dog flea") are misleading as humans may also be attacked by any of these species especially when in close proximity of the preferred host. New discoveries of medically important species are being made in South America; e.g., Ectinorus insignis (Beaucournu et al 2013) and Ctenidiosomus sp. (Lopez-Berrizbeitia et al. 2015).
to one other order, the Diptera, by certain aspects of their metamorphosis and somewhat by their mouthparts.
The mouthparts are made up of a pair of long serrated mandibles, a pair of short triangular maxillae with palps, and a reduced labium with palps. There is a short hypopharynx and a larger labrum-epipharynx similar to that of the Diptera. The labial palps, held together, serve to support the other parts, a function which is performed by the labium in the Diptera. In piercing the host, the mandibles are most important and blood is drawn up a channel formed by the two mandibles and the labrum-epipharynx (Borradaile & Potts, 1958).
The thoracic segments are free and wings are absent. Although the eggs are laid on the host they soon fall off and are afterwards found in little-disturbed parts of the host's habitat. Therefore, in houses they reside in dusty carpets and unswept corners of rooms. In a few days the larvae hatch and feed on organic debris. The legless and eyeless larvae possess a well-developed head and a 13-segmented body. At the end of the third larval instar a cocoon is spun and the flea changes into an exarate pupa from which the adult emerges. The whole life cycle takes about a month in the case of Pulex irritans..
Pulex irritans is the common flea of European houses, but by far the most important economically is the oriental rat flea, Xenopsylla cheopis, which transmits Bacillus pestis, the bacillus of plague, from the rat to humans. This bacillus lives in the gut of the flea and the faeces deposited on the skin of the host are rubbed into the wound by the scratching which follows the irritation from the bite. Ceratophyllus fasciatus, the European rat flea, also transmits the plague organism as can also Pulex irritans, but since the latter does not live successfully on rats, it is a less dangerous vector (Borradaile & Potts, 1958).
Detailed Habits & Morphology
All members of the Siphonaptera feed exclusively on warm-blooded animals. Their mouthparts lack mandibles and a siphon is formed of structures of the labrum, labium and maxillae. The labium is an elongated and fleshy covering mechanism. The maxillae are interlocking and a maxillary sheath is present but not obvious. A labrum is also present.
Fleas are apterous but their extinct ancestors are known to have possessed wings, which was deduced from pleural plates on the thorax. Hair-like structures called geocomb and corolla comb, are present on the head. The antennae have three segments and lie in a groove on the head (see ent159).
The larvae are eruciform (wormlike) with a distinct head capsule. They do not possess legs but leg-like setae instead. Larvae are not parasitic. There is an exarate pupa formed in a cocoon (see ent160).
Their general pest status of humans and domestic animals and their ability to vector diseases makes them of great economic importance.
Role as Parasites. -- Fleas are well adapted to the parasitic habit by being laterally compressed. They also have a very hard exoskeleton, their legs are developed for leaping and the hairs on their body are directed backward.
Service (2008) pointed out that the role fleas have in transmitting plague involves certain important characteristics as follows:
Saliva, with anticoagulants, is passed to the host during feeding, and the blood enters the pharynx, esophagus and proventriculus, where there are a lot of spines that are pointed to the rear. These spines may prevent blood regurgitation of t blood into the esophagus. The proventriculus is important in the mechanism of plague transmission." Then the blood passes into a large stomach for digestion. The posterior intestines form a small widened rectum with rectal glands that remove water so that the faeces are dry. Both male and female fleas suck blood and can serve as vectors of plague.
Diseases Transmitted by Siphonaptera
Bubonic Plague. -- The vector of Pasteurilla pestis is the rat flea. This bacillus wiped out one quarter of the population of London, England. The fleas search out other hosts as soon as the rat dies. Transfer is accomplished by (1) defecation on the body and the inoculum is scratched into the wounds, and (2) the flea cannot digest the bacillus, so it regurgitates into the wound made by its mouthparts.
Cat-Scratch Disease. -- Bartonella henselae occurs in cat fleas and can infect humans through a cat's claws if contaminated.
Chigger Infection. -- The fertilized female flea burrows under the skin and becomes extremely distended. A serious tropical form is known as Tunga penetrans.
Murine Typhus. -- This typhus is caused by Rickettsia typhi. It is transmitted when infected faeces come into contact with abrasions or mucous membranes. Faeces retain infectivity of months or years. The disease primarily affects rodents, especially rats. It is spread by species of Xenopsylla, Nosopsyllus and Leptopsylla. Different species of Rickettsia may also cause typhus in humans.
Sylvanic Plague. -- The vectors are fleas that live on rodent hosts. This is actually mild type of bubonic plague, which is found in Western North America. However, humans may also die from infection.
Tapeworms. -- The tapeworm, Dipylidium caninum, affecting dogs and cats, can also be transmitted by fleas to humans. Transmission from animals can occur through handling.
Tularaemia. -- Sometimes Francisella tularensis may be transmitted to humans by fleas even though ticks are the principal vectors.
CONTROL OF SIPHONAPTERA
The problem of controlling fleas involves several distinct measures as advised by Matheson (1950): (1) control on domestic pets and in the home; (2) control of fleas on poultry and domestic animals and in their living areas; (3) control of fleas on rats and other wild rodents that are sources of plague; (4) prevention of the spread of plague by restricting the movement of flea carriers.
In buildings cleanliness is very important. Unclean carpets, crevices, kitchens, bathrooms, closets, cellars, etc. are all places where fleas may breed. The commercial treatment with insecticides may be required to reduce flea numbers, or even the fumigation of an entire building could be necessary. (See Hinkle et al. in References for new control approaches).
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