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Ask the Vet: Side Effects of Heartworm Treatment in Dogs

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Knowing the side effects of heartworm treatment in dogs is important considering the risks associated with this treatment and the importance of early detection of troublesome signs. In order to better understand these side effects, it helps to learn more about how heartworms operate and how heartworm treatment works in dogs. Veterinarian Dr. Ivana describes possible side effects and what dog owners should do if they notice them along with tips on keeping heartworm positive dogs calm during treatment. 

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How Heartworm Disease Impacts Dogs

There is only one parasite that causes heart problems in dogs – the heartworm parasite, also known by its scientific name, Dirofilaria immitis. The heartworms are transmitted to dogs through mosquito bites as microscopic larvae. These mature into male and female adults, which are the size of earthworms and live in the right side of the heart, particularly in the right ventricle and the pulmonary arteries (from which blood is carried to the lungs).

In a heavy infestation, the right side of the heart contains a mass of worms, and this obstruction leads to heart failure. In some cases, the worms may overflow into the right atrium (the top right chamber of the heart) and even into the large veins that return blood from the liver to the heart. When worms occupy the pulmonary arteries, the obstruction to the blood flow can cause blood clots to develop in the lungs.

The severity of disease depends on the number of worms and the size of the dog. Small dogs suffer more from small worm loads than do large dogs. Affected individuals show signs indicating varying degrees of heart failure, from reduced exercise tolerance through soft coughing to weight loss, visible pulsation of blood flow in the veins of the neck, fainting and even a dog suddenly collapsing and death.

If the pulmonary arteries are affected, a dog may cough up blood-tinged phlegm. If the veins from the liver are involved, a dog may show signs of liver failure, including jaundice, anemia, and ascites (a swollen, fluid-filled abdomen).

A blood test called a heartworm antigen test identifies the presence of adult female heartworms. Occasionally, a false negative test occurs (failure to detect worms when they are present). This result may occur in light infestations or when all the worms are male.

A microfilarial concentration test relies on identifying microfilariae (microscopic forms of the parasite, produced by adult females) under the microscope. Again, a false negative sometimes occurs. About 10 to 25 percent of infected dogs do not have microfilariae in the bloodstream.

A chest x-ray may show an enlarged right ventricle and enlarged pulmonary arteries. ECG sometimes reveals a disturbance of the heart rhythm. Echocardiography (examination of the heart structure by ultrasound scanning) reveals the adult worms themselves.

Side Effects of Heartworm Treatment in Dogs

The most effective initial treatment for heartworm disease in dogs is to stop all exercise. Medical treatment depends on the level of complications associated with the disease. Common heartworm treatments involves injections of arsenic containing drugs. There are several side effects of heartworm treatment to be aware of. Report to your vet if you notice any. It is much better to prevent heartworm than to try to treat it.

Compounds containing arsenic are given to kill adult worms, although arsenic causes a variety of side-effects. Drugs containing arsenic are strong and given by injection intramuscularly which usually traumatize the tissues surrounding the injection site. Pain, inflammation and swelling are likely to occur post-injection.

To minimize the pain, arsenic drugs are usually given in conjunction with pain medications. The associated swelling and tenderness usually resolve within a week. In cases of more severe injection reactions, firm nodules may appear at the injection site. More often than not, the firm nodules are permanent.

The usual arsenic-containing drug given is melarsomine. Melarsomine (Immiticide) has the widest margin of safety as it has a slow kill rate and kills over 90 percent of adult worms. Though safer than previous treatments, this therapy may lead to other problems.

Namely, dead worms may cause blood clots to form in the lungs (pulmonary thromboembolism). To break up any clots, the vet may dispense aspirin, which inhibits blood clotting. Plus, to minimize the risk of developing blood clots, physical exercise during treatment and at least one month after the treatment should be avoided. See your vet immediately should your dog show signs of increased respiratory rate and effort.

Arsenic treatments may be dangerous for elderly dogs. If your dog is old, your vet will discuss the pros and cons of their use. You and your vet may decide to treat only with daily low-dose aspirin and restricted exercise.

Arsenic treatments are often combined with doxycycline and macrocyclic lactones. The doxycycline destroys the symbiotic bacterium – Wolbachia (which lives with the heartworm) while the macrocyclic lactones kill the microfilariae and mature larvae.

Four to five weeks after drug therapy, a heartworm antigen test is performed. A negative test usually means that all adult worms have been destroyed. If the test is positive, your vet will discuss re-treatment. A microfilaria concentration test follows. If it is negative, no more treatment is needed and routine prevention can begin.

If it is positive, the next step is to kill circulating microfilariae. Either milbemycin or ivermectin are used off-label (outside their normally approved use). Ivermectin is most effective and has the fewest complications, but is not licensed for use in dogs and is dangerous for some breeds. Ivermectin is potentially lethal in certain sheepdog breeds, where the drug crosses the barrier that separates the bloodstream form the brain. Ivermectin must not be used in the Rough or Smooth Collie, Shetland Sheepdog, Old English Sheepdog, Australian Shepherd or other herding breeds or crosses including these breeds.

Ivermectin is associated with several side-effects such as vomiting, diarrhea, anorexia, lethargy, dizziness, weakness and tremors. The branded medicine Heartgard contains the same drug, ivermectin, but at a much lower concentration. Heartgard is safe and is fully licensed for prevention of heartworm infection.

Limiting Activity During Dog Heartworm Treatment

As stated, limited physical activity during and after the heartworm treatment is crucial. However, there is a difference between cage rest and exercise restriction. In some phases of the treatment, the vet will recommend cage rest, while in others restricted exercise will be advisable.

The cage rest is obvious and self-explanatory – the dog is safely secured in a crate or pen with no possibility of physical activity. Exercise restriction means avoiding activities that elevate the heart rate – no running, no playing and no chasing.

However, moderate on-leash walks are allowed. During the cage rest period it is important to help your dog survive boredom. This is best achieved by offering your dog companionship and providing tons of mental stimulation.

About the Author

Dr. Ivana Crnec is a graduate of the University Sv. Kliment Ohridski’s Faculty of Veterinary Medicine in Bitola, Republic of Macedonia.

ivana crnec

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