Easily prevented with medication, heartworms are difficult and costly to cure. Dogs contract heartworms in only one way- through the bite of an infected mosquito. All it takes is one mosquito bite. There is no way to tell if a mosquito is infected. This is why preventing heartworms in the first place is so critical. After a dog has been bitten by the infected mosquito, it takes about 7 months for the heartworms to develop. At this time, they begin lodging in the heart, lungs and surrounding blood vessels and they begin to reproduce. One worm can grow up to 12 inches long and can live 5 to 7 years. A dog can have as many as 250 worms in its body. If your dog already has heartworms, you may be concerned that it could be spread to your other dogs or members of your family. The answer is no. This specific parasite only affects humans in very rare cases, but cannot be spread from a dog to a human. Heartworms are not contagious to your other dogs because the only way they can be contracted is from a mosquito bite. It is relatively easy to prevent heartworm disease in your dog. At a cost of $35 to $80 a year, depending on the weight of your dog, it is worth it. There are monthly pills available, as well as topicals that you put on the skin monthly. There is also an injectable medicine available, which is given once every 6 months. You may be wondering how you can tell if your dog has heartworms or not. This is difficult in the beginning, as initially there are no symptoms. However, the more worms that crowd the heart, the more the dog will become winded. When heartworm disease moves into its more critical stages, you may notice symptoms of abnormal sounds coming from the lungs, for instance. The dog may pass out due to a loss of blood to the brain. If the worms go untreated, the dog will eventually die. The treatment for heartworms involves an injectable, arsenic-based product known as Immiticide. The dog receives two or three injections that kill adult heartworms in the dog’s blood vessels that surround the heart. With everything that is involved, treatment for heartworms can cost up to $1,000. Many veterinarians begin their heartworm testing in April, for heartworm exposure that may have happened during the previous mosquito season. This is because evidence of the disease may be detectable at this point in time after the bite occurred. Logically, the earlier heartworms are detected, the better chance there is of your dog surviving them.