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Ectoparasites - Life Cycles

Life cycles of arthropods involve a series of structural changes known as metamorphoses, the actual sequence of which varies with different parasite groups. Complete metamorphosis begins when adults lay eggs from which larvae hatch. The larval forms grow and shed their skins (moult) several times, each time to accommodate their increases in size. Larvae may either live freely or be dependent on their hosts for obtaining nourishment. Eventually a hard cased structure called a pupa is formed, which may have the capacity to survive winter. The pupa hatches into the adult parasite, the final stage of metamorphosis. Thus, there are four distinct stages in the life cycle: egg, larva, pupa, and adult. Incomplete metamorphosis involves a larva that grows and moults one or more times to become an adult-like form known as a nymph, which in turn grows and moults one or more times to become an adult. In this case there are only three distinct stages: eggs, larvae, and immature adults (nymphs) which grow to maturity without further change in body type.

Arachnids

Ticks

Ticks have a life cycle incorporating incomplete metamorphosis. Adult ticks feed and mate on mammals. Engorged females drop to the ground and lay eggs. The eggs hatch, producing six-legged larvae, or seed ticks. The seed tick moults twice, passing through an eight-legged nymphal stage before reaching maturity. A blood meal must be taken before each moult can occur. Ticks are classified as one-, two-, or three-host ticks, depending on how many times they drop off, moult, and seek a new animal. A one-host tick remains on the animal from the seed-tick stage to maturity. A two-host tick drops off the initial host to moult from nymph to adult. The adult seeks a second animal for the final blood meal and copulation. The three- host tick drops to the ground for each moult, after which a new host is sought. Each nymphal stage of the multi-host ticks may occur on the same species of host, i.e., several different species are not required. The only tick in NZ is a 3 host tick Haemophysalus longicornis.

A knowledge of the life cycles of ticks is of great practical importance in planning a control program.

Mites

The life cycles of the common equine mites take about 3 weeks and involve incomplete metamorphosis. The scabies mite (Sarcoptes) spends its entire life cycle in skin tunnels on a single host. Sarcoptes have not been recorded in horses in NZ. Scab mites (Psoroptes) and itchy leg mites (Chorioptes) live, feed, and deposit eggs at the edge of crusty, oozing lesions, which gradually spread owing to the mite activity.

The only parasitic stage of chigger mites (Trombicula) is the larval stage, which drops off the host for the nymphal and adult stages. Mites are host specific. Trombicula mites are not found in New  Zealand.

       

Insects

Lice

All stages of the louse life cycle, which takes about 3 to 4 weeks, occur on the host. Lice can live for only several days in the absence of the host. Louse eggs, cemented to animal hair, hatch as nymphs, which are small, immature adults. The nymph moults three times before becoming an adult.

Botflies

 

Botflies lay their eggs on hairs of the head, neck or legs of horses. The horse becomes infected when either the eggs are licked off the hairs (G. intestinalis) or the larvae hatch and migrate into the mouth. After developing in the tissues of the mouth and tongue, the larvae are swallowed and attach to the stomach walls, where they overwinter. After several months the "bots" pass out in the faeces, pupate in the ground, and mature to adults in summer (see illustration).

       

 

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