Female dogs go into heat, or experience an estrus cycle, when they reach reproductive maturity. This point can occur anywhere from 6 months of age, though 8 to 9 months is more common. A dog’s first heat may be as late as 18 months of age, which is more common in large breed dogs. Dogs come into heat about twice a year, and heat cycle lasts about 3 weeks. When a female dog is in heat, she is receptive to mating and can become pregnant; female dogs will not accept males for breeding when they are not in heat. During their heat cycle, reproductive organs become highly vascularized and blood flow to the vaginal area is increased. Discharge from the vagina can occur and may be tinged with blood. This can make an awful mess in the pet owner's home and people who professionally breed dogs have ways of dealing with this issue, either through use of dog “diapers” or use of a kennel. During the heat cycle, your dog’s behavior can change-- she may become excitable, agitated, lethargic, or even aggressive. For your average pet owner, dealing with a heat cycle is not something that fits in with their routine and can be stressful, so preventing it may be the best solution.
Understanding a Dog’s Heat Cycle
The heat cycle is a naturally occurring part of your dog's reproductive cycle when her body prepares for breeding, fertilization and pregnancy. Blood flow to the reproductive organs and reproductive tract is increased. There are 4 phases to a dog's reproductive cycle.
Beginning of heat, or proestrus: lasts 3 to 17 days and is characterized by production of estrogen and bloody vaginal discharge as the body prepares for mating and fertilization
Full heat, or estrus: lasts 4 to 7 days, luteinizing hormone is released, bleeding may continue, although it may decrease, female is receptive to mating, ovulation occurs
End of heat, or diestrus: lasts about 6 days, discharge ceases, if not pregnant the uterine lining is absorbed back into the female dog's body
Non-heat or anestrus: the final 2 to 3 month span of your dog's estrous cycle, when your dog is not hormonally active or sexually receptive
Preventing Heat Cycles
If your are not planning on breeding your dog during a heat cycle, not only can unwanted symptoms such as vaginal discharge and behavioral changes prove difficult to manage, but the chance of your dog becoming pregnant from an unplanned breeding is a very real possibility.
The best way to prevent heat cycles, if you are not planning on breeding your female dog, is to spay her. There are two spay procedures that are performed on dogs. The most common in North America is an ovariohysterectomy, where the ovaries and uterus are surgically removed. This procedure has the added advantage of preventing not only heat cycles and unwanted pregnancy, but also the incidence of reproductive cancers. It is routinely performed, relatively inexpensive, and recovery in dogs is generally uneventful. If your dog is experiencing a heat cycle, spay can be performed during the cycle, however, due to increased supply of blood to the reproductive organs, there is an increased risk of hemorrhaging, and many pet owners may find it advisable to wait until a few months after the heat cycle has ended.
An alternative procedure to ovariohysterectomy is ovariectomy. This procedure is more common in Europe and other parts of the world, and involves removal of the ovaries. This prevents heat and unwanted pregnancy, but the uterus remains in place. This procedure is less invasive, and often performed laparoscopically, however, it does not mitigate the risk of uterine cancers or infections occurring in the dog.
If you want to stop heat cycles, but are planning on breeding the dog at a later date, or if a medical condition or circumstances precludes performing a spay procedure, there are medications available to prevent or stop heat cycles.
Megestrol acetate can be given orally as soon as signs of proestrus begin. It is given for 8 consecutive days and is designed to stop the heat cycle. The disadvantage of this is that some unwanted symptoms of heat may have already occurred before the medication brings the estrus cycle to a halt. Your dog will go into heat again in about 6 months time.
Another oral medication available is Mibolerone. It prevents heat cycle from occurring, but must be given 30 days before your dog's heat cycle begins. This requires that the pet owner is aware of when their dog's estrus cycle is going to start, so it won't prevent a first heat, and a regular heat cycle pattern must be established in order for this to be a useful method for dog owners.
Both medications have side effects such as behavioral changes and incontinence and are not appropriate for dogs that have medical conditions such as kidney or liver problems. The drugs can be dangerous, overdose is possible, and they are not always effective at stopping heat cycles. They may also increase incidence of reproductive cancers and can not be used long term.
Your veterinarian can also administer Delvosteron injections to prevent estrus in your dog. These require a series of injections to be given: an initial injection, followed by one at 3 months, 4 months, and then at continuing 5 month intervals. The injections prevent your dog from coming into heat, but there is a potential decrease in fertility in the dog long-term. Also, a small increase in uterine infection, which can be life threatening, has been associated with these injections.
Advantages of Preventing Heat Cycles
The main advantage of preventing heat in your dog is preventing the incidence of unplanned and unwanted pregnancies in your dog. Unless you are a professional breeder, dealing with the consequences of heat cycle, pregnancy, and whelping is time-consuming and can be stressful for both your dog and you. Also there is a large population of unwanted and stray dogs in the U.S. and throughout North America, and most dog owners wish to avoid unintentionally contributing to the problem. Spaying your dog not only prevents heat cycles, but also prevents a host of other reproductive organ diseases which may cause problems for you and your dog later on and is the most recommended method of controlling heat cycles.
Many Benefits and Options
Heat cycles in dogs that are not being used for breeding are inconvenient and stressful for pets and pet owners. Besides the chance of an unwanted pregnancy, they are messy and can result in behavioral changes in your pet that most dog owners are not prepared to deal with. Spaying is recommended for dogs that are not part of a planned breeding program, as it not only prevents heat and pregnancy, but prevents your dog from getting several serious reproductive diseases, such as pyometra, uterine infection, and cancers. There are oral and injectable medications that can be used to prevent heat cycles in dogs, if the dog is going to be used for breeding at a later date, or in cases where spay is not a viable option. These medcications are associated with side effects however, are only recommended for short term use, and may compromise long term fertility or health. If your are not planning on breeding your dog, spaying her is highly recommended.