What causes heartworm disease?
Heartworm disease is a serious and potentially fatal disease in dogs. It is caused by a blood-borne parasite called Dirofilaria immitis .
Heartworms are found in the heart and adjacent large blood vessels of infected dogs. The female worm is 6 to 14 inches long (15 to 36 cm) and 1/8 inch wide (5 mm). The male is about half the size of the female. Adult worms produce offspring called microfilaria. One dog may have as many as 300 worms. Infection is acquired through bites by mosquitoes that have previously bitten an infected dog. It is not possible for the parasite to be transmitted directly between dogs without a mosquitoe as an intermediate.
How do heartworms get into the heart?
As many as 30 species of mosquitoes can transmit heartworms. The female mosquito bites an infected dog and ingests the microfilariae during a blood meal. The microfilariae develop further for 10 to 30 days in the mosquito and then enter the mouthparts of the mosquito. The microfilariae are now called infective larvae because at this stage of development, they will grow to adulthood when they enter a dog. The mosquito usually bites the uninfected dog and deposits the larvae under the skin. These will continue to develop and get larger as they undergo their migration towards the heart.
When fully developed, the infective larvae enter the bloodstream and move to the heart and adjacent vessels where they grow to maturity in two to three months and start reproducing more microfilaria, thereby completing the full life cycle. |
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Where are heartworms found?
Canine heartworm disease occurs all over the world. In the United States , it was once limited to the south and southeast regions. However, the disease is spreading north and is now found in most regions of the United States and Canada . Prevalence of the disease in this region is not high however, infection is possible wherever mosquitoes are present.
What do heartworms do to the dog?
Adult heartworms : Adult heartworms cause disease by clogging the heart and major blood vessels leading from the heart. They interfere with the valve action in the heart. By clogging the main blood vessels, the blood supply to other organs of the body is reduced, particularly the lungs, liver and kidneys, leading to malfunction of these organs.
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Most dogs infected with heartworms do not show any signs of disease for as long as two years. Unfortunately, by the time clinical signs are seen, the disease is well advanced. The signs of heartworm disease depend on the number of adult worms present, the location of the worms, the length of time the worms have been present, and the degree of damage to the heart, lungs, liver, and kidneys from the adult worms and the microfilariae. |
The most obvious signs are a soft, dry cough, shortness of breath, weakness, nervousness, listlessness, and loss of stamina. All of these signs are most noticeable following exercise, when some dogs may even faint.
Microfilariae (Young heartworms): Microfilariae circulate throughout the body but remain primarily in the small blood vessels. Because they are as wide as the small vessels, they may block blood flow in these vessels. The body cells being supplied by these vessels are deprived of the nutrients and oxygen normally supplied by the blood. The lungs and liver are primarily affected.
How is heartworm infection diagnosed?
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In the spring, a blood sample will be pulled from your dog and sent to a veterinary laboratory where it will be tested for the presence of adult worms. The test looks for specific proteins produced by the worms that would be found circulating in your dog's blood. Dogs with less than five adult heartworms will not have enough protein to give a positive result, so there may be an occasional false negative result in dogs with early infections. Because the detected protein is only produced by the female heartworm, a population of only male heartworms will also give a false negative. Therefore, there must be at least five female worms present for the most common heartworm test to diagnose heartworm disease. |
Radiographs (X-rays)
A radiograph of a dog with heartworm will usually show heart enlargement and swelling of the large artery leading to the lungs from the heart. These signs are considered presumptive evidence of heartworm disease. Radiographs may also reveal the condition of the heart, lungs, and vessels. This information allows us to predict an increased possibility of complications related to treatment.
How are dogs treated for heartworms?
There is some risk involved in treating dogs with heartworms, although fatalities are rare. In the past, the drug used to treat heartworms contained arsenic so toxic effects and reactions occurred more frequently. A newer drug is now available that does not have the toxic side-effects. We can now successfully treat more than 95% of dogs with heartworms.
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Treatment to kill adult heartworms : An injectable drug to kill adult heartworms is given for two days. It kills the adult heartworms in the heart and adjacent vessels. In more serious infections, these injections may be divided and given thirty days apart. |
Complete rest is essential after treatment
The adult worms die in a few days and start to decompose. As they break up, they are carried to the lungs, where they lodge in the small blood vessels and are eventually reabsorbed by the body. This can be a dangerous period so it is absolutely essential that the dog be kept quiet and not be allowed to exercise for one month following treatment. The first week after the injections is critical because the worms are dying. A cough is noticeable for seven to eight weeks after treatment in many heavily infected dogs.
Prompt treatment is essential if the dog has a significant reaction in the weeks following the initial treatment, although such reactions are rare. If a dog shows loss of appetite, shortness of breath, severe coughing, coughing up blood, fever, and/or depression, you should notify us. Response to antibiotics, cage rest, and supportive care and intravenous fluids is usually good in these cases.
Treatment to kill microfilaria
Approximately one month following treatment to kill the adults, the dog is returned to the hospital for administration of a drug to kill the microfilariae. Your dog needs to stay in the hospital for the day.
Response to treatment : Dog owners are usually pleasantly surprised at the change in their dog following treatment for heartworms, especially if the dog had been showing signs of heartworm disease. The dog has a renewed vigor and vitality, improved appetite, and weight gain.
How can I prevent this from happening again?
As can be seen it is much easier to prevent the disease than treat the disease. Prevention involves a once monthly tablet given to kill any larvae injected by a mosquitoe into your dog during the past month. In this area, preventative treatment begins June 1 st around which time, the mosquitoe population begins to be present and continues until November 1 st until all mosquitoes are gone. It is important to remember for travellers that go to Florida or southern states for the winter that their dogs should be on prevention while down there as the southern USA has mosquitoes present all year long. Therefore, heartworm transmission season is also all year long. For further advice regarding heartworm transmission, treatment and prevention, please discuss with your veterinarian during your yearly health exam.
