In this context heat refers to the fertile period of the female dog. It is not a disease but is often misinterpreted due to the symptoms that occur as a result.
Some of the first signs of heat are that male dogs are more attracted to the female dog and are seeking contact with her. Female dogs in heat also lick their genital area more frequently. Soon after beginning heat, a vaginal discharge occurs. This discharge can be bloody and can be a watery or slimy. The dog’s vulva is usually swollen. At a later stage the vaginal discharge changes its color and becomes yellowish and clear. The labia, the vaginal lips, are now soft and flaccid and are usually only partially swollen. Around this point in time the female actively encourages male dogs to mate with her. The symptoms of heat usually last for about three weeks. Sometimes they can last longer, in which case it’s called prolonged heat. If the vaginal discharge persists and turns brownish, consult a veterinarian since these could be signs of a disease of the uterus.
Heat is part of the estrous cycle of the female dog. The average dog has her first heat between the 7th and 10th month of life. It can be the case that females have their first heat much later (at about 1.5 years of age), which should be no cause for concern. Bigger breeds or obese animals often have their first heat later. Depending on the breed of the female dog, she may undergo heat up to four times a year. However, most females go into heat two times a year.
Heat is a natural process and usually doesn’t require treatment. There are several methods for preventing a female dog from going into heat. For more detailed information on this topic read the corresponding advice section. Also, your vet can prescribe drugs that can delay the onset heat and its symptoms, something that is often done when owner and dog are going on vacation.
For further information concerning heat and heat suppression read the article on heat supression in the advice section.