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Ectoparasites - Parasite Groups

Ectoparasites are divided into two main groups, arachnids and insects, classified by structural characteristics. The arachnid class includes ticks and mites. The insect class consists of flies, mosquitoes, fleas, and lice.

Arachnids

The arachnid class of ectoparasites includes ticks and mites. An arachnid has only two body segments: a fused head and thorax, and an abdomen. It has four pairs of legs as an adult and no wings or antennae.

The life cycle of arachnids involves incomplete metamorphosis: the eggs hatch into larvae then moult into nymphs which at least superficially resemble adults. On the other hand, complete metamorphosis is seen in insects, in which the immature nymph forms are totally unlike the adults.

Ticks

Ticks thrive on blood obtained from the host. They are subdivided into hard and soft ticks according to structural characteristics. In New Zealand only one type of tick is found Haemaphysalis longicornis.

Hard Ticks

The bodies of hard ticks are roughly oval and pointed at the front. The anterior segment is a false head, or capitulum; the shape of its base is characteristic of the particular tick genus. Structures on the capitulum may also help to identify the tick genus. Palpi are segmented structures used for probing the host. The hypostome anchors the tick to the host's skin, and mouth parts on the capitulum are adapted for sucking blood.

The abdomen, flattened top to bottom, can expand to several times its original size as a tick feeds on its host. This phenomenon, referred to as engorgement, is seen only in females. One part of the skin, the scutum, located on the back of the tick, does not expand during engorgement. In some species, the scutum may be decorated with coloured pigments. These patterns of pigmentation may help with identification. Male ticks are generally more colourful than females in ornate (coloured) species. The presence or absence of other structures may also help to identify ticks.

A further classification of hard ticks is made based on whether their life cycle involves one, two, or three hosts. This is described under the section on life cycles.


Ornate hard tick - Dermacentor

Soft Ticks

Soft ticks differ from hard ticks in many respects. They have a leathery outer skin rather than a hard cuticle, and both males and females engorge when feeding on the host. Their shapes vary among species. There is no scutum, and the capitulum is located on the bottom side of the tick near its front. Otobius megnini, the spinose ear tick, is an example of a soft tick. Only larvae and nymphs of this species are parasitic. Adults live in hidden areas in the environment, such as within cracks in the wood of barns.


Soft tick - Otobius

 

Mites

Mites are arachnids that can be seen clearly only with a microscope. Their bodies are usually round and flattened, although an exception to this general rule is Demodex, which is cigar-shaped. Some mites eat normal skin debris, such as scales. An alternate feeding habit in some species is puncturing the skin to suck lymph fluid. Sarcoptes scabiei mites actually burrow tunnels into the skin where they live, feeding and depositing eggs. Larvae that hatch may create side tunnels, or leave the area and migrate to undamaged skin to burrow new tunnels. Demodex bovis, the cause of demodectic mange, lives in hair follicles and the associated skin glands.

Insects

Insects are characterised by having three distinct body parts: head, thorax, and abdomen. Wings may or may not be present. Insects have one pair of antennae on the head and three pairs of legs on the thorax. Many insect parasites - including some species of flies, mosquitoes, and fleas - spend little time on the host. In contrast, the larval stages of the warble and botflies, and all stages of lice, remain on the bodies of hosts for significant periods of time.

 
Mite   Louse
Lice

Lice are common wingless, flattened insects with six legs adapted for clinging to hair. Two kinds of lice are common. The Anoplura or sucking lice have mouth parts adapted for puncturing skin and sucking tissue fluids and blood. The biting lice, Mallophaga, feed on skin debris and have mouth parts adapted for chewing.

 

Grubs

A particularly troublesome cause of myiasis in cattle is the cattle grub, the larva of the warble fly. These are not found in New Zealand. Larvae of these insects penetrate the skin of cattle and live in underlying tissue. Adult flies, which live only a few days and do not feed, lay eggs on the hair of cattle during persistent, diving attacks on the cattle, thus the common name "bomb flies". Once hatched, the larvae penetrate the skin of cattle. Although small at first, the larvae grow as they migrate under the skin from the legs to the back, eventually reaching a size of 2 to 3cm. A cyst forms around each larva once it has reached its resting site. In order to breathe, the larvae bore through the skin. The damage inflicted during larval migration makes the hides of infected animals less valuable. In addition, losses from trimming grub-damaged meat, which often includes the highest-priced cuts, can be considerable. Annual economic losses in the United States are estimated at $200 to $400 million. Ox warbles are a very costly problem.

There are two important genera of damaging grubs: Hypoderma (warbles) and Dermatobia. Two species of Hypoderma occur in temperate parts of the world, whereas Dermatobia occurs in tropical and subtropical regions of Central and South America.

Flies

Hornflies, buffalo flies, blowflies and screwworm flies spend most of their lives on cattle although of these species only blowflies are found in New Zealand. They lay their eggs in manure where the larvae (maggots) feed. Large numbers of these flies are a constant irritation to cattle, causing loss of condition.

Blowflies and screwworm flies lay their eggs in wounds (for example, from dehorning or castration), and the maggots feed on the flesh of the animal. Screwworms that attack cattle invariably cause fatal damage within 2 weeks.

 

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