AU.Merial.com : Disease Information : Sheep : Ectoparasites

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.

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.

Ticks thrive on blood obtained from the host. They are subdivided into hard and soft ticks according to structural characteristics. Ticks are a problem in sheep, but not to the extent that they are in cattle.

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 if 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 and 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 made based on whether their life cycle involves one, two, or three hosts. This is described under the section on life cycles.

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.


Adult ornate tick		Larval tick

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. 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 ovis, the cause of demodectic mange, lives in hair follicles and the associated skin glands.

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, bugs, and fleas - spend little time on the host. In contrast, others, such as blowfly larvae and lice, remain on the bodies of hosts for significant periods of time.

Lice are common wingless, flattened insects with six legs adapted for clinging to hair. Two kinds of lice are common. The Siphonaptera 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.

Sheep keds (Melophagus ovinus) are wingless flies. They irritate sheep as do sucking lice, but, in addition, keds suck significant amounts of blood. These insects stain the fleece with their faeces.

Blowflies are a most important parasite of the sheep industry. Losses are estimated at more than $50 million yearly. These losses are caused by reduced growth of the sheep, reduced and inferior wool production, and extremely high labour costs expended in attempts to control the parasite.

Under normal conditions, blowflies do no attack live healthy sheep. If animals suffer open wounds, for example from branding or castration, some species of blowflies, such as the screwworm fly, will deposit eggs in the wounds. These will hatch into maggots which eat the flesh of the animal.

The seriousness of the blowfly problem in Australia is due to a very complex association between different species of blowflies and a breeding practice that was introduced to Australia in the 1890's. In attempts to grow more and denser merino wool a strain of sheep from Vermont (USA) was introduced to Australia. This strain of merino has very wrinkled skin (thus a larger area on which to grow wool) and a particularly heavy wool grease. The wrinkles were soon bred into the entire population of merino sheep.

The wrinkles retain moisture, particularly urine, around the tail of ewes. Urine-soaked fleece combined with bacteria sets up a skin irritation which is very attractive to the blowfly Lucilia. These blowflies are referred to as "primary sheep blowflies" because they are the first (and in most cases the only) blowflies to lay eggs on live sheep. Once the Lucilia eggs hatch, the larvae feed on the sheep's flesh and produce putrefaction products which attract flies of the genus Calliphora (secondary blowflies) and eventually the tertiary blowfly Chrysomyia rufifacies. This latter blowfly has large, hairy maggots which consume the sheep's flesh at such a rate that they may penetrate the abdominal cavity, leading to peritonitis and death.

Blowfly strikes are almost always fatal unless the sheep is caught, the wool clipped from the infected area, the maggots scraped out, and disinfectant and insecticide applies to prevent further strikes. A sheep which has recovered after this treatment will have a serious wool break caused by the high fever present during maggot activity. This break in the wool greatly reduces its value.