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What is Heartworm in Dogs?
by Dr. N.J.Hayes, DVM
Heartworm is a mosquito borne parasite, - Dirofilaria immitis, that is transmitted through mosquito bites to dogs and, occasionally, cats (see handout for cats). Even if your dog is the only dog living in your area, it is important to remember that heartword is also transmitted to wolves, coyotes, foxes and some other animals.
Because the disease is borne by mosquitoes, it is very common in areas where mosquitoes are found. This is especially true in the southern states, (such as Florida and the gulf area, as well as the Midwest and Mississippi valley areas.) This disease will affect virtually every dog in the areas described - unless they’re on a prophylactic or preventive medication.
It was originally thought that mosquitoes wouldn’t be able to penetrate through the hair on longhaired dogs to reach the skin and feed; however, most of the feeding takes place on the belly, since most dogs have very little hair in this region. There is some feeding on the ears and muzzle where the hair is thinner and shorter.
One infected dog can serve as a dispersal point for the disease from which many other dogs are infected. The mosquitoes may repeatedly feed upon the same dog, (thus re-infecting it) or fly over to infect the neighboring dog. Mosquitoes have not been known to respect property boundaries or fences.
The heartworm lifecycle has many stages.
- There is the microfilarial stage:
This is, essentially, when the baby heartworms circulate in the dog’s bloodstream. When a mosquito bites a dog that has heartworm disease, it sucks up some of the microfilaria. Then, when the mosquito flies over and bites another dog, it injects the baby microfilaria into that dog. Once in the pet's body, the microfilaria mature through several life stages and migrate towards the heart.
- After reaching the heart,
 they mature into adult worms living in the right ventricle, right atria and the pulmonary arteries. As adults, this is where they cause all of the damage, because:
- The space they occupy cannot be filled by blood
- that causes high blood pressure
- and changes in the artery linings.
- Eventually heartworm disease leads to cardiac failure and death.
From the time that the larvae enter the dog until the microfilarial stage (the baby worms) can be detected in the blood is about six to seven months. At about one year after infection the heartworms are fully grown. The male heartworms can reach 4-6 inches long, and the females can reach 10 to 12 inches.Their life span within a dog appears to be 3 to 5 years. Click here For a more detailed explanation of the Heartworm Life Cycle.
How is Heartworm Disease Diagnosed?
Heartworm disease is usually detected by using blood tests. Commonly used is the antigen test, which tests for antigen of the reproductive tract of adult worms. The higher the concentration of worms, the stronger the positive result will be. This test can find as few as 2 worms.
There is also a test to look for microfilaria. It’s called a filter test. This test is less sensitive in detection of heartworm, because not all adult worms produce microfilaria. It is possible for a dog to have all female or all male heartworms and therefore no microfilaria. This test is rarely used.
If no blood test is used to determine the presence of heartworm while it is still in the microfilaria stage, the onset and severity of symptoms of heartworm disease will reflect the number of adult heartworms present, - the dog's age, - and the activity level of the dog. Large numbers of worms will usually result in severe heart and lung disease.
Until the number of mature heartworms exceed 50 in a 50 lb dog, most of the worms live in the lower lung lobes. As the numbers of heartworm increase, they move into the right chambers of the heart. At this stage, the presence of the worms has usually caused inflammatory changes in and around the arteries in the lower portion of the lungs. If the infestation continues, the heart may enlarge, become weakened because of the increased workload, and congestive heart failure may occur.
Because of the increased demands on the heart and lungs, an active dog is more likely to develop severe heartworm disease with a lower infestation of worms per pound than an inactive dog. If left untreated, heartworm disease may progress to the point where the dog may suddenly collapse and die within two or three days without surgical removal of the worms. Surgical removal of worms is rarely done, as it is highly risky.
What is the treatment?
If your dog has been diagnosed with heartworm, it will require treatment. Treatment involves the use of a drug called Immiticide ™.The treatment is usually - but not always as follows:
- This drug is given into the muscle two days in a row.
- Hospitalization is recommended in case of severe reactions.
- This drug has side effects which can affect the liver and kidneys as well as cause muscle pain.
- Once treated, they must remain quiet and without exercise for four to six weeks.
If the dog becomes too active, the dead adult heartworms can be dislodged from the heart and cause thrombosis in the lungs and stroke. Both of these effects are life threatening.
- After four to six weeks, the microfilaria must also be killed. This involves a day stay at the hospital and another drug is administered orally. In most cases this drug is ivermectin™. Some breeds are very sensitive to this medicine. It cannot be used, for example, in collies and shelties.
Is there a way to keep my dog from getting Heartworm Disease?
Fortunately, there is a way to prevent heartworm disease. A simple, once a month pill can prevent your dog from getting heartworm - as well as some intestinal parasites. Our hospital recommends Interceptor™. We recommend this medication be given year-round.
The monthly pill prevents the microfilaria from becoming adults. Because they work only on that stage of the lifecycle, it is important that they be given in a timely fashion. If you miss pills, your dog could become infected with heartworm. For this reason, even dogs maintained on the pills year-round should be tested as recommended by your veterinarian.
If you have any questions regarding heartworm disease or other common ailments of pets, please feel free to give us a call at 685-4707.
We would be happy to answer all of your questions.
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