The ox harbours two forms of Hæmatopinus and one of Trichodectes, the Hæmatopinus eurysternus and tenuirostrus, and the Trichodectes scalaris.
The sheep suffers from Trichodectes sphærocephalus and a Melophagus, the goat from the Hæmatopinus stenops and the Trichodectes climax, and the pig from the Hæmatopinus urius.
The symptoms are, with trifling variations, the same in all domestic animals, the principal being rubbing and itching. The animals scratch, bite and attempt to rub against hard objects, even abrading the skin when this is thin and the irritation is severe.
The parasites may, however, remain localised, and it rarely happens that they are present in any considerable number in all parts of the body.
Fig. 247.—Sheep louse (Trichocephalus sphærocephalus). a, Female; b, antenna; c, d, dorsal and side view of leg. Enlarged. (After Osborn, 1896; Bul. No. 5, Div. Entomology, Dept. Agr.)
In the ox they are principally found in the depression at the back of the base of the horns, and in the upper margin of the neck and the back. In the absence of treatment phthiriasis may become generalised over the entire surface of the body.
The trichodectes and the melophagus of the sheep choose similar points, but when the wool is long they may be found nearly all over the body.
In the pig the hæmatopinus is found on the neck, in the region of the poll, about the armpits, and round the eyes and ears.
These parasites, whose powers of increase are astonishing, keep the patients in a continual state of irritation, causing them to lose condition and, in the absence of treatment, to die of exhaustion.