The Biology of the Goat
Lice
Lice are members of the insect class. Like all the other insects lice have a head, a thorax which bears 3 pair of legs and an abdomen. They have antennae but usually do not have eyes or if they do, the eyes are much reduced. The ends of the legs have claw-like structures used to hold onto hair or feathers. Lice are all wingless and flattened in appearance. They are ectoparasites which mean that they live their entire life on the surface of mammals and birds. Lice are considered to be very old parasites and so are highly host specific. Few of them are able to survive on any but their correct host. Some lice can be as small as less than 1 mm, while others can be as large as 10 mm. Lice do not like warm weather or sunlight so they are most numerous in the fall and winter and can cause the most problems in goats that spend most of the time during bad weather in the barn close to one another.Goats can be host to two different kinds of lice:
Biting Lice
One type of louse is called a biting louse. They belong to an order called Mallophaga which contains over 3000 known species. Damalinia caprae is the species that parasitize goats.
The heads of biting lice are large. They have mouth parts that are adapted to chewing structures that are part of their diet such as hair, feathers, skin debris, scabs, wool wax, and even their own eggs. They do not actually bite but a few species are known to be able to suck tissue fluids from their hosts.
Species of biting lice that live on birds possess colonies of bacteria in the cells of their digestive system which help them digest the keratin of feathers. Biting lice, like the species that occur on goats and sheep are thought to damage hair by eating into the shaft almost like felling a tree.
These lice prefer to live at the base of the tail, between the legs, on the shoulders, head, neck and along the back, but can appear anywhere on the goat in a heavy infestation.
Sucking Lice
Sucking lice belong to an order of insects called Anoplura. There are about 300 species and they are known to only parasitize mammals. Linognathus stenopsis is the species that are found on goats. The sucking lice have narrow heads with piercing mouth parts that are retracted into the head when not being used to suck blood and tissue fluid from the host. Unlike biting lice, a few species of sucking lice are known to transmit diseases.
When the louse pierces the skin, saliva is injected into the wound which prevents the blood from clotting. The reaction to the saliva causes severe irritation to the goat such as biting, scratching, restlessness, loss of sleep and interruption of feeding. A large infestation can also cause anemia from loss of blood.
These lice feed almost constantly resulting in oozing blood which clots on the surface of the skin. This can cause a secondary bacterial skin infection as well as attract flies. These lice are usually found on the legs.
Life Cycle
Generations of lice live their entire life completing their full life cycle on the surface of the host. The only way that lice can infect another animal is by direct physical contact. Some species of lice can be transmitted by handlers and farm equipment.
The female louse attaches her eggs (called nits) to the hair shaft with a strong glue-like substance. The first nymph developes inside the egg and looks very much like an adult louse except that it is smaller and is not sexually developed. The first nymph hatches out of the egg in 1 to 2 weeks. A week or so later the first nymph will molt its skin becoming a second nymph which becomes a third nymph which will then metamorphose into the adult stage. The entire cycle from egg to adult is completed in about 4 to 6 weeks depending on the species.
Lice at any stage will die in a short time if they fall off of the host. Eggs cannot hatch at all if they fall off the host. Skin temperature is a critical factor in hatching of eggs. Higher temperatures prevent eggs from developing. This may account for the fact that populations of lice are higher in the winter than in the summer months or why some lice prefer the cooler temperatures of the legs in the summer and the warmer temperature of the host's body in the winter. One species of biting lice that infect horses are parthenogenic -- females can lay fertile eggs without mating.