Heartworms in Dogs
Heartworms in dogs can be deadly. Heartworm disease in dogs is now found in all 48 of the lower United States. Fortunately, heartworm disease is easily prevented. Dogs should be tested annually before giving them any kind of preventive medication.
Heartworms in dogs is caused by the Dirofilaria immitis worm. This worm is transmitted in the microfilaria stage by the mosquito. Basically, a mosquito draws blood from an infected dog. Later, when it bites another dog, the microfilaria is transmitted to the new host. The microfilaria is actually going through an entire life cycle during this process, or molting. It molts twice while being carried inside the mosquito over the course of two weeks or longer. They are developing as larva at this stage and moving into the mosquito’s salivary glands, ready to move into the new host. Once the microfilaria have moved into the new dog it takes them around six or seven months to go through two more molts before they become sexually mature. It’s not until this stage that they can be picked up by a heartworm test at your veterinarian’s office.
The young worms begin to make their way to your dog’s heart, through his tissues, between 70 and 90 days after entering his body. Once the worms reach the dog’s heart they grow fast. They stay in the heart until the worm, or the dog dies. The worm can live 5 to 7 years.
The worms are sexually mature after they have been in the heart for about three months. At this point they can begin making more microfilaria. These worms will continue to grow in your dog’s heart, wrapping around it and blocking off its functions. In general, the smaller the dog, the fewer worms it takes to cause a serious problem.
Symptoms of heartworms in dogs include the following:
- dull coat
- weight loss
- difficulty breathing
- swollen abdomen
- congestive heart failure
Your dog may have heartworms for months, or even longer, before you notice any symptoms. By the time you begin noticing symptoms, your dog’s heart may already be infested with the adult worms. The heartworm blood test will not pick up the microfilaria for the first few months after your dog is bitten by an infected mosquito. The test will reveal that your dog has heartworms if the worms are already present in your dog’s heart. Once the worms begin living in your dog’s heart, there is arterial damage to your dog within days.
A heartworm test is a normal part of most annual check-ups for dogs and you should have your dog checked for heartworms each year whether he has been taking heartworm preventive or not. It is always possible that you could have accidentally skipped a dose or two, or you had a package of medicine that had something wrong with it. It’s best to make sure that your dog does not have heartworms, or to find out if he does. Then you and your veterinarian can pursue a good course of treatment.
If you suspect for some reason that your dog may have heartworms then your veterinarian will make sure he has your dog’s complete medical history. He or she will perform a physical exam on your dog and take x-rays of your dog’s chest. Your vet will also probably need to have a blood panel taken on your dog, as well as perform the heartworm antigen test. This test checks for microfilaria from worms in your dog’s heart. If the presence of microfilaria suggest that your dog has heartworms, then the other tests will be used to determine how much your dog’s health has been compromised.
If your dog does have heartworms then you and your veterinarian will need to discuss treatment options. The first consideration will need to be your dog’s overall health. His heart, liver, and kidneys will need to be evaluated to be sure they can stand up to any proposed treatment. In the U.S. today the treatment of choice is usually arsenic-based, which can be difficult on your dog’s organs. Treatment today is safer than in the past, but there are still possible side effects. The drug used for treating heartworms today is usually Immiticide.
After receiving treatment your dog will need to have his exercise restricted for several weeks. During this time the large adult worms in your dog’s heart are dying. They are breaking off from the heart muscle and being passed through your dog’s system. It is important to keep your dog resting during this time in order to prevent these large segments from traveling to your dog’s lungs where they could cause respiratory failure or death. Several weeks later your dog will receive a dose of medication to eliminate the microfilaria in his bloodstream. Once this is done, treatment is considered to be completed.
If your dog has a very advanced case of heartworms, called Vena Cava Syndrome, or Caval Syndrome, surgery may be considered to remove the worms from the heart, but this is only in very advanced cases.
There is another treatment for heartworms that is sometimes used. Ivermectin may be administered year-round at three times the normal dose used for heartworm prevention. Used at this dose it will eventually kill adult heartworms in the heart. However, this is not usually the preferred treatment in the U.S. This treatment is said to be less effective that using Immiticide, the arsenic-based treatment. And, it is said that it will take 18 months for the adult heartworms to begin to die, making it risky if your dog already has a heavy infestation of heartworms. However, this treatment option is free of the side effects often found in the arsenic-based treatment.
The best prevention for heartworms in dogs is to use a regular monthly heartworm preventive. There are many good preventive medications on the market today. Some of them use ivermectin, some use milbermycin, and some use moxidectin. All of these drugs are effective at killing heartworm microfilaria in the bloodstream when used as a preventive. The most important thing to remember about these preventive is that they must be given regularly. When they fail to work it is almost always because the owner forgot to give the dog their preventive dose. They are 99 percent effective when used correctly. Even when up to four doses are skipped, ivermectin products have been shown to be 95 percent effective in protecting dogs from heartworms.