Hard ticks are responsible for transmission of several different kinds of organisms including rickettsiae, Borrelia and arbo-viruses. The genera of medical importance are Amblyomma, Dermacentor, Haemaphy-salis, Hyalomma, Ixodes and Rhipicepha-lus. A female Dermacentor, to characterize the group of hard ticks, is illustrated in Fig. 16.5. The feature that distinguishes hard from soft ticks is the presence of a scutum (shield) and protruding mouthparts. Care has to be taken in identifying the engorged specimen, for the body is so greatly distended as to obscure the head and mouth-parts (Fig. 16.5). The female has a smaller scutum than the male, but since both males and females take blood meals, there is no need to distinguish between them.
Eggs are laid in a large mass on the ground, hatching after weeks or months into six-legged larvae. These larvae resemble mites, but are differentiated from them by prominent mouthparts and a scutum. The larvae climb on to grass or prominent vegetation to await a passing mammal on to which they cling. Once attached, they crawl around to find an area of soft skin, such as in the ears, eyelids or belly of an animal. On humans, they may surreptitiously climb up the leg and attach themselves to the scrotum or between the buttocks. Once in a favourable site, they pierce the skin with their powerful mouthparts, inject saliva and feed on the host's blood. Larvae will remain attached for 3-7 days, after which they drop to the ground and seek a place to moult. Developing into an eight-legged nymph, the nymph repeats the feeding pattern, being attached for 5-10 days and then falling to the ground once more for the final moult.
From the nymph develops a male or female adult, which subsequently quests for a new host on which it remains for a considerable period of time (up to 1 month) becoming greatly engorged with blood. Finally dropping off, the female digests her blood meal and begins egg laying, after which she dies.
The life cycle of ticks is modified by temperature and humidity, such that if it becomes too cold, the cycle of development will be delayed until more favourable conditions return. Larvae and nymphs tend to feed on small mammals and humans, whereas adults prefer larger animals, such as cattle and game animals.
Control of ticks is mainly through the use of insecticides and repellents. Permethrin, malathion and propoxur are suitable insecti cides, administered as dusting powders or solutions to infested animals. Cattle are commonly treated by making them swim through an insecticidal bath or dip. This should be carried out on a regular basis, with monitoring of ticks for insecticidal resistance. Dogs are important carriers of ticks and should be similarly treated by insecti-cidal baths, making sure that they are totally immersed as the ears are a common site for ticks to attach. Dogs should wear tick-repellent collars.
Repellents, such as diethyltoluamide or dimethylphthalate smeared on skin or as a solution to impregnate clothing, are effective in preventing ticks from becoming attached. Items, such as socks, can be treated with insecticides in the same way as mosquito nets (see Sections 3.4.1 and 3.4.4).
Ticks take nearly 2 h to attach themselves and start feeding, so a careful search of the body, paying particular attention to the upper legs, groin and buttocks, should be made after passing through tick-infested country. Larval and nymph stages can be very small. Ticks should not be pulled off directly as the mouthparts may be left behind, which will continue to cause irritation. Applying methylated spirit, ether, benzene or similar solutions will kill the tick and sterilize the wound.
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