Heartworm Disease: Everything you need to know

What is heartworm disease?

Heartworm disease is a serious and potentially fatal disease in pets in the United States and many other parts of the world. It is caused by foot-long worms (heartworms) that live in the heart, lungs, and associated blood vessels of affected pets, causing severe lung disease, heart failure, and damage to other organs in the body. ​

Dogs. The dog is a natural host for heartworms, which means that heartworms that live inside the dog mature into adults, mate, and produce offspring. If untreated, their numbers can increase, and dogs have been known to harbor several hundred worms in their bodies. Heartworm disease causes lasting damage to the heart, lungs, and arteries, and can affect the dog’s health and quality of life long after the parasites are gone. For this reason, heartworm prevention for dogs is by far the best option, and treatment—when needed—should be administered as early in the course of the disease as possible. Learn more about heartworm medicine for dogs.

Heartworm Prevention for Dogs

Heartworm prevention for dogs is an important concern for every pet owner. Prevention is an important part of providing essential care, and heartworm disease prevention for dogs is something every owner can do. Consider this:

• Dogs have been diagnosed with heartworm disease in every state in the U.S.
• Heartworms are spread by mosquitoes, so any area of the country that has mosquitoes—even just a few of them—can also have heartworm disease.

• Dogs don’t just need prevention during warm-weather months. Heartworm preventives work by treating heartworms that already infected the pet within the past month or longer; meanwhile, preventives need to be given on time, every time to be effective. 
• The American Heartworm Society estimates that more than a million dogs in the U.S. have heartworm disease—and heartworm disease can be fatal.
• Heartworm preventives are safe, relatively inexpensive, and easy to give, but if a dog becomes infected, heartworm treatment can be costly and difficult, requiring multiple veterinary visits and months of exercise restriction.
• While there are drug-free strategies owners can put in place to reduce a pet’s exposure to mosquitoes, there’s no such thing as a “natural” heartworm preventive.

Heartworm preventives come in different forms, including monthly chewable pills and topical “spot on” medications, as well as an injectable medication that is given every 6 or 12 months. Heartworm preventives are available only by prescription from veterinarians.
Some preventives only prevent heartworms, some protect pets from heartworms and intestinal parasites, and some protect pets from many different parasites, including heartworms, intestinal worms, fleas, ticks, and mites. Because veterinarians know which parasites are common in the area in which they practice, owners should talk to their pet’s doctor about what product or products will be best for their pets.

What do I need to know about heartworm testing?

Heartworm disease is a serious, progressive disease. The earlier it is detected, the better the chances the pet will recover. There are few, if any, early signs of disease when a dog is infected, so detecting their presence with a heartworm test administered by a veterinarian is important. The test requires just a small blood sample from your pet, and it works by detecting the presence of heartworm proteins. 

Some veterinarians process heartworm tests right in their hospitals while others send the samples to a diagnostic laboratory. In either case, results are obtained quickly. If your pet tests positive, further tests may be ordered.

When should my pet be tested?

All dogs should be tested annually for heartworm infection, and this can usually be done during a routine visit for preventive care. Following are guidelines on testing and timing:

• Puppies under 7 months of age can be started on heartworm prevention without a heartworm test (it takes at least 6 months for a dog to test positive after it has been infected), but should be tested 6 months after your initial visit, tested again 6 months later and yearly after that to ensure they are heartworm-free.
• Adult dogs over 7 months of age and previously not on a preventive need to be tested prior to starting heartworm prevention.  They, too, need to be tested 6 months and 12 months later and annually after that.
• You need to consult your veterinarian, and immediately re-start your dog on monthly preventive—then retest your dog 6 months later. The reason for re-testing is that heartworms must be approximately 7 months old before the infection can be diagnosed.

Annual testing is recommended, even when dogs are on heartworm prevention year-round, to ensure that the prevention program is working. Heartworm medications are highly effective, but dogs can still become infected. If you miss just one dose of a monthly medication—or give it late—it can leave your dog unprotected. Even if you give the medication as recommended, your dog may spit out or vomit a heartworm pill—or rub off a topical medication. Heartworm preventives are highly effective, but not 100 percent effective. If you don’t get your dog test, you won’t know your dog needs treatment.

What happens if my dog tests positive for heartworms?

No one wants to hear that their dog has heartworm, but the good news is that most infected dogs can be successfully treated. The goal is to first stabilize your dog if he is showing signs of disease, then kill all adult and immature worms while keeping the side effects of treatment to a minimum.

Here's what you should expect if your dog tests positive:

• Confirm the diagnosis. Once a dog tests positive on an antigen test, the diagnosis should be confirmed with an additional—and different—test. Because the treatment regimen for heartworm is both expensive and complex, your veterinarian will want to be absolutely sure that treatment is necessary.
• Restrict exercise. This requirement might be difficult to adhere to, especially if your dog is accustomed to being active. But your dog’s normal physical activities must be restricted as soon as the diagnosis is confirmed, because physical exertion increases the rate at which the heartworms cause damage in the heart and lungs. The more severe the symptoms, the less activity your dog should have.
• Stabilize your dog's disease. Before actual heartworm treatment can begin, your dog’s condition may need to be stabilized with appropriate therapy. In severe cases of heartworm disease, or when a dog has another serious condition, the process can take several months.
• Administer treatment. Once your veterinarian has determined your dog is stable and ready for heartworm treatment, he or she will recommend a treatment protocol involving several steps. The American Heartworm Society has guidelines for developing this plan of attack. Dogs with no signs or mild signs of heartworm disease, such as cough or exercise intolerance, have a high success rate with treatment. More severe disease can also be successfully treated, but the possibility of complications is greater. The severity of heartworm disease does not always correlate with the severity of symptoms, and dogs with many worms may have few or no symptoms early in the course of the disease.

Test (and prevent) for success. Approximately 9 months after treatment is completed, your veterinarian will perform a heartworm test to confirm that all heartworms have been eliminated. To avoid the possibility of your dog contracting heartworm disease again, you will want to administer heartworm prevention year-round for the rest of his life.

GO TO www.heartwormsociety.org/

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