Section III. CLASS ARACHNIDA
1-9.
TICKS
Ticks are found throughout the world, in tropic and temperate zones. They are
divided into two groups: the soft ticks of the famiIy Argasidae (100 species) and the
hard ticks of the famiIy Ixodiae (700 species). Hard ticks are capable of transmitting
such diseases as tularemia, Rocky Mountain spotted fever, Lyme disease, Q fever, and
tick-borne encephalitis. They can cause a direct injury resulting in a condition known as
tick paralysis. The soft tick may transmit spirochetes that cause tick-borne relapsing
fever. The tick gets spirochetes when ingesting the blood of an infected animal. The
spirochetes then multiply within the body of the tick, invading the tissues and the body
cavity of the tick. Spirochetes are transmitted to the tick's eggs by transovarian
transmission, even to the third generation.
a. Hard Ticks (FamiIy Ixodidae). The mouthparts can be seen from above;
therefore the hard tick has a shield on the back and (see figure 1-22). The life cycle of
the hard-tick is a type of gradual metamorphosis, consisting of four stages: egg, larva
(not worm-like), nymph, and adult (see figure 1-23). The completion of this Iife cycle
may take from 6 weeks to 2 years. All stages past the egg feed on the blood of
vertebrates, mostly mammals. The female becomes greatly distended whiIe feeding, a
period of usually 5 to 10 days. Copulation takes place on the host whiIe the female is
feeding. After copulation, the female takes more blood, drops to the ground, finds a
sheltered place, and in a few days deposits a gelatinous mass of eggs that may number
into the thousands. This oviposition may take several days, after which the female dies.
Under favorable conditions, the eggs hatch in about a month; but during cold weather,
they may not hatch for several months. Some days after hatching, the six-legged larvae
(also called "seed ticks") climb weeds, stems, or twigs or walk over the ground to find a
suitable host such as a small mammal. They engorge on the blood of the host, drop to
the ground, and molt into the nymphal stage. The nymph then awaits an animal, feeds,
drops to the ground, and molts into the adult which then repeats the cycle.
Figure 1-22. Adult hard tick.
MD0170
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