Fruit Valley Veterinary Clinic

Heartworm disease in dogs: disease and prevention

SUMMARY: Heartworm infection is regularly found in Oswego. Adult heartworms mainly infect the large blood vessels in the lungs. Heartworms are transmitted by mosquito bites. Heartworm infection is preventable by a variety of safe products. Heartworm infection can be detected by a blood test and can be eliminated through medical treatment. All dogs in the Oswego area should receive prevention from at least June 1st through Nov. 1st every year. Testing should be done depending on previous prevention, age, and time of year. Cats can also get heartworm and may benefit from prevention.

What is heartworm disease? Heartworm disease is an infestation of parasitic worms in the heart and adjacent blood vessels. These worms look similar to roundworms, with a length up to 14 inches. A dog can be infected by 1 to 100s of heartworms which impair circulation and can cause heart failure and many other issues.

How is heartworm disease spread? Heartworms are transmitted via mosquitos. When a mosquito ingests the blood of a dog with heartworm larvae , it can pass them on to infect another dog weeks later during warm weather. All it takes is one mosquito bite to acquire heartworms! Any dog that goes outside or is exposed to mosquitos can become infected with heartworm.

How likely is my dog to get heartworm disease? Currently our clinic diagnoses usually less than a dozen dogs with heartworm infection per year. Most cases occur in dogs not receiving prevention who spend substantial time outdoors. We estimate that the prevalence of heartworm infection in unprotected outdoor dogs in Oswego is about 20%. In the southern US, the prevalence is about 45 to 55%. If you live in an area infested with mosquitoes and aren’t giving monthly prevention, your dog runs a significant risk of developing heartworm infection.

What would heartworm disease do to my dog? A dog with a small number of heartworms or an early infection may show no clinical signs of the disease, although they quickly suffer damage to the blood vessels in their lungs and lose some athletic potential. Dogs with more severe infections may show weight loss, a decreased appetite, a chronic cough, and may tire easily. Due to heart failure, they may accumulate body fluids and develop a swollen belly. Over the long term, heartworms eventually die and then cause blood clots and embolisms that can cause heart failure or enough sudden death.

In the most severe cases, blood is blocked from returning to the heart; and the dog will become listless, weak, inappetent, and jaundiced due to inefficient liver and kidney function. This situation requires immediate surgery to remove the worms and avoid death.

How can we prevent heartworm disease? Prevention is the best way to deal with heartworm disease, All it take is a once-a-month pill or topical product that kills heartworm larvae (microfilariae). This medication needs to be given within a month after mosquito season begins and continued until after the first hard frost (at least June 1st - November 1st). An additional benefit of some once-a-month heartworm preventives is that they may control roundworms, hookworms, and whipworms (intestinal parasite) as well. Some preventives also control or kill fleas. Some sources are recommending heartworm prevention for all 12 months of the year; currently we have seen no published evidence to support this recommendation for our region.

How can we tell if my dog has heartworms? By testing a blood sample we can detect most adult heartworm infections. All dogs should be tested during the first spring after they are born. In general, it is ideal to do this in the springtime before starting prevention, since any larvae deposited the previous year will be mature enough to detect by then. Provided that heartworm prevention is given every year and the previous tests have been negative, we feel comfortable testing every 2 or 3 years thereafter. However, if you start prevention late or end early, or miss several doses, we recommend testing again the following spring. The American Heartworm Society recommends annual testing, but we have found that dogs receiving adequate preventive rarely test positive.

What if I forget to give my dog a dose of prevention? Each package of prevention comes with stickers to put on your calendar to remind you when to administer the preventive, but if you miss a dose, all is not lost. You have the option of continuing with the prevention as soon as you remember. However, this is not always 100% effective, so it is recommended to test your dog for adult heartworms the following year.

Why should I have my dog tested? There are two main reasons for testing. First, some dogs may have heartworm infections that could result in serious adverse effects if given certain heartworm preventives. Second, if heartworms are detected, then the infection can be treated. Having heartworms can decrease the lifespan of a dog and cause poor health. A positive test should not be a death sentence for an otherwise happy, healthy dog; it should lead to treatment in order to prolong your dog’s life. If you would not consider treating for heartworm, then there is less reason to test.

How can we treat heartworm infection? Treatment of heartworm infection has a high rate of success, but does present some risk when the worms die off and begin to disintegrate. To minimize problems, we may pretreat with an antibiotic to hurt and shrink the heartworms before we kill them. To kill adult worms, we administer a series injections, depending on how extensive we suspect the infection is.. Following treatment, your dog may receive steroids and should remain quiet and avoid running around for a month, to decrease the formation of clots and emboli. Treatment generally costs several hundred dollars for a relatively healthy dog. Another step is to eliminate any larvae with a microfilariacide. After treatment, you should continue giving this monthly (May-Nov) as prevention against acquiring a new infection.

What if I can't afford treatment? If you cannot afford treatment, we can still devise strategies to minimize further problems. It is still important to give your dog the monthly prevention to limit the number heartworms your dog will have and prevent transmission.

Interpreting the test results

Two kinds of testing are done to detect the presence of heartworms in dogs. The test we usually run first is an "antigen" test. This detects molecules in the blood that came from the uterus of the adult female worm. A positive antigen test means that there are adult female heartworms (and probably also male ones) in the dog's heart and lung blood vessels. The test does NOT tell us how many worms are in the heart. If the dog is young or only missed heartworm prevention for a short time, then there are probably a small number of worms (perhaps less than a dozen). If the dog is older and has never had heartworm prevention, then the infection may be dozens or hundreds of worms.

The second test we run on the blood of positive dogs is a microfilaria test. Microfilaria are the larvae or "babies" of the adult heartworms. Many dogs with heartworm may not have any of the microfilaria because the females are sterile or there are no males. Also, the monthly heartworm preventives damage the female heartworm's uterus and induce sterility. A negative test for microfilaria makes it more likely that the dog has only a few worms. Seeing many microfilaria makes it more likely that there are many adult heartworms in the dog.

Prevention for the heartworm-infected dog

If the dog has no microfilaria detected in its blood, then it can begin monthly heartworm prevention to prevent it from getting any more heartworms and to sterilize the female heartworms.

If the dog has microfilaria in its blood, then caution must be used when giving a preventive. The monthly preventives will kill off the baby worms. If there were many baby worms, the dog could have a reaction to the dead baby worm bodies. This usually only occurs in dogs with large numbers of baby worms. The signs of the reaction may include: lethargy, labored breathing, vomiting and high heart rate. This usually happens within 2 to 6 hours of giving the preventive and the dog usually recovers within 24 hours. Prednisone can help prevent the reaction.

It is rare that these dogs need any kind of treatment.

We believe that the reaction to dead microfilaria is less likely if Heartgard (ivermectin) is used initially for the first 2 months. However, we strongly feel that Interceptor (milbemycin oxime) provides more wide spectrum control of intestinal parasites (especially whipworms) and we recommend Interceptor for the rest of the season.

Treatment for heartworms

The only known drugs that can kill adult heartworms are arsenic-containing ones. The newest drug is Immiticide (melarsomine) and, although expensive, it is dramatically improved in safety and efficacy. Before treatment with melarsomine, dogs should be on monthly heartworm prevention for at least 3 months. This ensures that all heartworms in the dog are mature enough to be killed by the melarsomine. Also, the dogs should receive doxycycline (10 mg/kg BID) for 4 weeks. The doxycycline antibiotic helps eliminate a bacterium that is necessary for health of the heartworms. In effect, the doxycycline causes the heartworms to become sickly and shrink. It also reduces the infective threat to other dogs. For young healthy dogs that probably have low numbers of heartworms, we recommend a 2 day course of treatment with Immiticide followed by 4 to 6 weeks of aspirin therapy and exercise restriction to minimize the risk of thromboembolism (throwing blood clots internally from the dead worms). With dogs that are older and are suspected of having large numbers of heartworms, radiographs and blood and urine analysis should be considered to make sure that there are not other complicating problems (heart failure, diabetes, cancer, kidney disease). Also we strongly recommend "staged" heartworm treatment for these dogs. In staged treatment, the dog is given a one day treatment that will probably kill half of the heartworms. Then aspirin or prednisone and restriction are done for 4 weeks and then the full 2 day treatment is done to kill off the rest of the heartworms. Then follows another 4 to 6 weeks of aspirin or prednisone therapy and exercise restriction.

After Treatment

After treatment for heartworms with Immiticide, heartworms will be dying within your dog's major blood vessels within the lungs over several days. These worms are 6 to 15 inches in length, but quite slender. We cannot know how many worms are there, but it is safest to assume that there may be many. Once the worms are dead, the worms may start to break down and attract inflammatory blood cells and develop clots of blood (thrombi). If these thrombi break loose and travel with the blood flow until they get stuck in a smaller blood vessel, then thromboembolism has occurred. Thromboembolism can cause dangerous high blood pressures that can lead to hemorrhage and sudden death. In order to avoid these dire events, we recommend that your dog be restricted to as little aerobic exercise as possible for 4 to 6 weeks. Also, since it is known that aspirin has the ability to reduce blood-clotting, we recommend giving aspirin for 1 month unless there is some contraindication or problem with using aspirin.

To confirm that the heartworm infection has been eliminated, a heartworm antigen test sometime after 4 months from complete Immiticide treatment can be done. It is of course important to use an appropriate heartworm preventive to avoid further infections.

Therapy for infected dogs that are not going to receive heartworm treatment

If heartworm treatment is not an option, the dog should be put on heartworm prevention to prevent further infection by more heartworms and to reduce the risk for other dogs. Also, Interceptor would help protect the dog from intestinal parasites. Since the heartworms can live for 7 years and it is impossible to predict when they will die, an appropriate dosage of aspirin can allow some dogs to outlive heartworm infections and become heartworm free. A course of doxycycline may help reduce the heartworm burden.

Please call if you have any questions.