Sexually-transmitted diseases in animals.

The importance of sexually-transmitted diseases (STD) in human society needs no emphasis, and during the last two decades their increased prevalence has provoked much discussion. However, as Francis Bacon (1561-1626) remarked: 'Man is of kin to the beasts by his body', and the importance of coitus as a means of transmitting disease is as well known to veterinarians as to physicians. The presentation of STD differs in man and animals. In man, it is the symptoms and signs of the initial infection (often quite minor in themselves) which bring a patient, and his sexual contacts, to medical investigation: most human cases of STD do not present because of complications. This does not usually apply to animals. Although an animal may show local evidence of infection, more often the veterinarian's advice is sought because of the appearance of complications, and of these the most important are abortion and infertility. Because of this, STD have been studied most in species such as cattle, sheep, pigs, horses, and dogs, where regular breeding is practised and such complications become of major economic importance. Many animals lead a social life in groups, with a large number of females associating with each male. Mating is usually restricted to the period of oestrus, but in herds of domesticated animals most females come into oestrus at about the same time, so a sire may rapidly infect a large number of females; the communal use of good sires by neighbouring animal breeders may result in the introduction of STD into previously 'clean' herds. In herds with a number of males, an infection may spread from male to male via the female, a situation common enough among humans. Although there are many opportunities for the spread of STD between animals, this spread is limited by the resistance to infection of the female, the degree of which varies between individuals and at different stages of the oestrus cycle. During oestrus resistance is high and the marked bacterial contamination of the vagina and uterus which occurs