Heartworm disease is caused by a worm (Dirofilaria immitis) that is about 14 inches long. Dogs become infected through mosquito bites. The mosquitos carry the larva that they acquired from biting other dogs or coyotes.
Heartworm live on the right side of the heart which is responsible for pumping blood to the lungs. These worms prefer to live in the pulmonary arteries which take blood from the right side of the heart to the lungs. The worms are nourished by the blood that is coursing past them. These worms cause a large inflammatory response and a tendency for the blood to inappropriately clot.
A mild infection has fewer than 25 worms. When the worm burden increases to more than 25 worms, the worms back up into the right ventricle, the chamber that pumps blood to the lungs. The worms take up space where the blood should be, so the heart has more difficulty pumping blood. With more than 50 worms, the worms back up into the right atrium, the chamber that collects blood from the rest of the body. A severe infection of over 100 worms causes caval syndrome. There is not enough room in the heart chambers for blood to enter therefore less blood is able to be pumped through the heart. Many dogs die rapidly from caval syndrome.
The adult worms mate and give birth to baby worms (larvae) called microfilariae. The microfilariae are released into the bloodstream. These worms can live for up to two years and then die of old age. If a mosquito bites a dog with microfilariae, the microfilariae can be slurped up by the mosquito when it takes a blood meal. The microfilariae will mature in the mosquito and can then infect another dog when the dog is bitten by the mosquito.
When the mosquito bites a dog, the larvae leave the mosquito and enter the skin through the bite wound. The larvae live in the skin for 3 months and mature. The young heartworm then enters the bloodstream and migrates to the heart and into the pulmonary arteries.
There are a couple of ways to test for heartworm disease. The traditional snap tests that your veterinarian runs in the office look for proteins from adult female worms. A Knott’s test can also be performed to look for microfilariae in the blood. It can take 5-7 months from the time that the dog is infected with heartworm for one of the tests to be positive.
The current heartworm preventatives kill the microfilariae that are living in the skin. Once the worms become adults, the heartworm preventative is ineffective at killing the worms. At this point, other medications will need to be used to eliminate the adult worms. It is important to give Heartworm medication year-round to prevent these microfilariae from becoming adult worms.
What does heartworm disease look like in a dog? For the first 5-7 months, the microfilariae are growing, maturing, preparing to mate in the skin. The dog will have no symptoms and there will be no indication that the dog has been infected with heartworm.
The young worm will migrate to a pulmonary artery. The worm is large and takes up a lot space in the artery. The dog’s immune system recognized foreign proteins present and launches an attack, but the immune system is designed to work against smaller invaders such as bacteria and viruses. The heartworm is too big to be killed effectively this way and the result is inflammation and thickening in the arteries. Blood flow becomes turbulent and the tendency to form abnormal clots is increased. Adding to this unpleasant mix is a bacterium called Wolbachia pipientis, which normally lives inside the heartworm but is released in large numbers every time the heartworm molts to a new developmental stage, gives birth to its young or dies. These bacteria contribute heavily to the resultant inflammation occurring in the pulmonary arteries and lung tissue itself. Blood vessels get stiff and cannot operate normally, creating a high-resistance area that the heart must pump through. The inflammation calls in numerous immune cells, which generate even more inflammation as they attempt to destroy the large heartworm. The lung itself becomes inflamed and eventually becomes scarred, creating an even larger high-resistance area for the to pump blood through.
The real damage occurs from worms that have died in place. The dead body of the heartworm breaks apart and is carried through the vasculature of the lung until it lodges somewhere and obstructs blood flow. The arteries that are supposed to form delicate branches, branching tinier and tinier, are now blunted and closed off, like a tree branch broken off close to the tree trunk. This leaves more areas of the lung blocked off and unable to receive blood and participate in oxygen exchange. The lungs become scarred, which makes it even harder for the heart to pump blood. Due to the significant damage, right sided heart failure can occur.
Symptoms of heartworm disease
- There may be no symptoms depending on the activity level of the dog.
- Shortness of breath/panting
- Easy tiring/intolerance of exercise
- Fluid accumulation in the abdomen or chest
- Nose bleeds
- Sudden death
- Glomerular disease – inflammation in the kidneys
The good news about heartworm disease: it can be prevented. Each year, a dog needs a heartworm test to be sure the dog is negative prior to the next dose of preventative. Heartworm preventatives are given on the same day each month. That dose will kill any of young heartworms. The goal is to eliminate the young worms before they become adults. By giving the preventative each month, there is better chance that no worms will escape the preventative and develop into adult worms.
When heartworm prevention is not given regularly, a dog may become infected with heartworm. At this point, additional treatment will be needed to prevent permanent lung damage, heart failure or death. A heartworm positive dog will be started on an antibiotic to kill the Wolbachia along with a preventative to kill any microfilariae. The dog will receive three very painful injections in the muscle along the spine. The dog will need to be kept on strict rest (crated) for at least 3 months. As the worms die, they can be released into the bloodstream and cause an embolism which could result in death.
Your best plan of action is to give Heartworm preventative year-round and control mosquitos in the environment. Take to your veterinarian about the options available for heartworm prevention.