Fleas

FLEAS, Aphaniptera, a clearly distinct order of parasitic insects, all species of which are blood-sucking in the adult state. Wings are absent. The body is strongly flattened from the sides; the head is connected to the prothorax by a broad base and bears a pair of simple eyes (see Figure 1 d), if not reduced, a pair of short club-shaped antennae (see Figure 1 f) and oral organs (see Figure 1 jl and Fig. 3), consisting of a long thin labrum (see Figure 3 Lb), a pair of serrated, file-like mandibles, a pair of flat, pointed maxillae (with palpi; see Figure 3 Mx, Px) and a labial proboscis (with a pair of palpi; see Figure 3 I, P). The oral organs (see Figure 3) serve for piercing the host's skin; blood is pumped into the stomach by the action of the muscles of the anterior intestine's cephalic part. The thorax is three-segmented, bearing three pairs of legs, of which the posterior pair is the longest (jumping legs). The abdomen is ten-segmented. On the back of the ninth segment lies a rigidium-scutellum with sensory organs. The body is covered with hairs or denticles; the latter are arranged in rows and form ctenidia (or combs, see Figure 2). These external features are important for flea taxonomy. Males are smaller than females; the abdomen of the former is bent upward at the tip and bears a complex copulatory apparatus. Fleas are external parasites of warm-blooded hosts and inhabit the fur of mammals or the plumage of birds. Under suitable conditions, they can live for a long time without their host. In the Old World, there are 35 genera of fleas. The species composition of fleas parasitizing a given host species is not always and not everywhere the same. For example, in the Kirghiz steppes, the human flea (Pulex irritans) has transferred to dogs, from which it has displaced the dog flea, Ctenocephalus canis. Cases of transfer of fleas of various rodents (rats, ground squirrels, marmots, etc.) to humans are known. In the examination of 3,293 fleas from rats in England, 15 species of these insects were identified, the number of specimens of which varied from 1,986 to 1 for a separate species. Fleas can drink the blood not only of their natural host but also of hosts of other species. The detailed establishment of such facts has great significance (in plague epidemiology). Fleas are insects with complete metamorphosis. They do not attach the laid eggs to the substrate. P. irritans lays up to 448 eggs in its lifetime. They develop in dry refuse (floor cracks, animal lairs, etc.). From the egg emerges a white vermiform, legless larva covered with long bristles (see Figure 5). Its mouthparts are adapted for scraping dry excrement of adult fleas (the best food!) or decomposing plant residues. It lives freely and only in rare cases can be on the host. The larva molts three times and, donning a silken cocoon, turns into an immobile pupa, from which an adult flea hatches. The periods of the various stages of flea metamorphosis vary strongly depending on temperature, degree of humidity, and nutrition. (The limits of variation in days are given in the table on p. 579.) At +6°, eggs of Ceratophyllus fasciatus give larvae, whereas for the human flea and Xenopsylla cheopis a minimum of +13° is required for the same purpose. At +22° and 40% humidity, larvae of Cer. fasciatus die. Less than 50% humidity harms fleas at temperatures above +16°. Under these conditions, larvae of P. irritans do not hatch at all from the eggs, and for Cer. fasciatus about 70% of all eggs develop completely. Starving fleas tolerate temperature fluctuations better than a decrease in humidity. In connection with weather changes, the total number of fleas of a given species varies strongly throughout the year. For southern Europe, the flea species, egg, larval period, pupa, cycle length, Imago, total life (average) are: Pulex irritans, human flea, 4-12, 5-14, 10, 8-100 (202), 12-114, 14-84, 6-220 (239), 3-450, 9-191, 19-264, 20-467, 31-256, 106, 100. The "flea season" lasts from June to the end of summer. In London, the maximum of Pulex irritans is in August-September, and rat fleas abound in February-March. Pathogenic significance of fleas. 1. When sucking the blood of a host, the flea injects its saliva (see Figure 6), under the influence of which roseolaceous spots with an intensely colored center appear on the skin. Subjective sensations are usually weak; in more sensitive subjects, blisters or nettle rash appear. Flea saliva causes local inflammation of the skin; vessels dilate, stasis of formed elements and diapedesis are observed. Lymphocytes accumulate in nests around vessels; edema of tissues is also observed (E. Pavlovskii and A. Stein). Fleas of the family Sarcopsyllidae - sand flea, Dermatophylus - after fertilization pierces the skin and penetrates into the thickness of the epidermis, forming nodules the size of a pea on the legs or in other parts of the body. Thus, this flea spends the greatest part of its life in the position of an "intradermal" parasite. In pure cases, the flea with matured eggs in it falls out to the outside for egg laying. Secondary phenomena are frequent - suppuration, necrosis, phlegmon, etc. (tropical America, Africa, Persia, China). 2. Fleas are intermediate hosts of some helminths, namely: the tapeworm of the pumpkin-shaped - Dipylidium caninum (dog, cat, human), the tapeworm of the rat - Hymenolepis diminuta (rats, mice, human) and the nematode - Dirofilaria immitis (dog). In the first two cases, flea larvae swallow the eggs of tapeworms, from which cysticercoids develop (see Figure 7), located in the body cavity of the adult flea. When infected fleas are swallowed by dogs, rats, humans, etc., from the cysticercoids in the intestine of the final host, the respective species of tapeworms develop. The presence of the latter in humans belongs to the phenomena of guest-parasitism (see Guest-Parasites), testifying in this case to an unfavorable hygienic environment of life of the infected persons with the respective helminths. 3. The debated question in the literature about the transmission of fleas of leishmania - the causative agents of kala-azar, or visceral leishmaniasis, is resolved in the present time in the negative. 4. Judgments about the supposed role of fleas in the spread of poliomyelitis and rheumatism are even more hypothetical (Brues, 1923; Clarke, 1920). 5. The unquestionable significance of fleas (first of all, of rodents - ground squirrels, marmots, rats) in the transmission of plague is indisputable. Before the death of a plague-infected rodent, its blood is filled with plague bacteria. Fleas suck the infected blood and, upon the cooling of the corpse of the dead animal, leave it and transfer to some new host, whom by a coincidence of circumstances can also be a human. Plague bacilli do not only not die in the intestine of the flea, but even multiply and under laboratory conditions can remain in the flea virulent for up to 8 months. (Golov and Ioff). When feeding on a new host, the flea contaminates its skin with its excrement containing plague bacilli. The latter penetrate into skin scratches or into the puncture sites of the oral organs, and in this way conditions for the infection of a given subject with plague through the mediation of fleas are created. Transmission of plague bacilli is also possible through the mediation of the oral organs during the reverse current of absorbed blood, which encounters in the flea's stomach an obstacle in the form of a plug of crowded bacteria (see Figure 8). Measures against fleas. Caulking cracks in floors. Regular sweeping of floors and washing them with salty, soapy water, carbolic or cresolic solutions. Pay attention to corners where garbage accumulates. Floor humidity hinders the development of larvae. General cleaning of the premises is achieved by fumigation with sulfur anhydride, formaldehyde vapors. Do not keep animals in the house or frequently change bedding, remove garbage, wash animals with a cresolic emulsion or powder the fur (of cats) with naphthalene. When sleeping in a stranger's premises to repel fleas, the underwear and bed linen are moistened with a few drops of strong carbolic acid in different places. See Disinsection, Deratization.