Heartworms pose a very serious and life threatening condition in dogs if left untreated. A severe infestation can be fatal for any dog, however dogs that are very young, senior dogs or dogs with other health or respiratory problems are at high risk for death due to these internal parasites. Although heartworms historically were only found in the southern United States, mostly along the Mississippi River and the southern Atlantic and Gulf coastal areas, they are now found in almost every country including southern areas of Canada, all of South America, Australia, Europe and even into Japan.
Heartworms are a very specific species of internal parasites similar to roundworms. Their scientific classification name is Dirofilaria immitis, and although they prefer living in dogs they are also found in as many as 30 other species of mammals. They are most commonly found in other animals similar to dogs such as wolves, foxes and coyotes but they can also be found in cats, sea lions, ferrets, wild cat species and even in humans, although this is relatively uncommon.
Heartworms are spread by mosquitoes so virtually anywhere mosquitoes are found there is always a chance for heartworm. Very cold climates typically aren't as severely affected since the lifecycle of the mosquito is too short for the heartworm to use the insect as a host, however there are mosquito species that survive longer than others, so they can definitely harbor the parasites.
Heartworm Life Cycle
Heartworms, as the name implies, live in the heart and lungs of the dog as well as in the major blood vessels, particularly around the heart region of the chest. There are both male and female heartworms and mature males can reach lengths of up to seven inches in length and females up to twelve inches. Once mature the male and female heartworms breed, with the females producing huge numbers of eggs throughout their life. It is not uncommon for a female heartworm to produce thousands of very tiny eggs during her lifetime and some may even produce more than just a few thousand.
These tiny, microscopic eggs do not immediately hatch in the host dog. This is actually a safety precaution for the heartworm population since if any or all of the eggs hatched in the host dog they would choke off the blood supply, resulting in the death of the host animal and death of the heartworms. Rather these eggs simply stay their microscopic size, floating through the blood system of the host dog. As the eggs move about the body they travel into the tiny blood capillaries and vessels that are close to the surface of the skin. During these times they can be sucked up by a feeding mosquito, which then triggers the egg to hatch into a larvae within the unsuspecting insect.
While in the mosquito the larvae feed on the blood the mosquito ingests, however they do not feed so much as to result in the death of the insect. Again, the mosquito's survival is very closely linked to the survival of the juvenile heartworms. During this time the heartworm larvae move to the mouth and sucking areas of the mosquito and simply move from the mosquito to the dog when the mosquito bite is occurring.
Once in the dog the larvae begin to move through the body, feeding and growing as they migrate to the heart, lungs and blood vessels. This does not happen quickly and can take several months to occur. When the heartworms reach the destination organ they are sexually mature and begin to reproduce immediately, continuing the cycle.
Signs of Heartworms
The really frightening thing is that there are no early signs of heartworms, once you see the signs the dog is already seriously infected. Typically most owners will first notice a lack of energy in the dog, poor coat condition and mild to moderate coughing either during exercise or play. In really sedate dogs or dogs that are never exercised there may be few if any symptoms at all.
Once there are a significant number of worms impeding the functioning of the heart and lungs the signs and symptoms become more pronounced. These can include fainting, seizure type activities, abnormal lung sounds, chronic cough and choking sounds, fluid accumulation in the abdominal cavity, kidney, lung and heart failure and eventually death.
In most cases diagnosis is made by either a routine blood test that detects the presence of the larvae, known correctly as microfilariae, within the blood of the dog. Sometimes the larvae can be present without an actual adult in the body, especially if caught just after the mosquito infected the dog. This is relatively rare but is the best case scenario for treatment.
In addition the vet will also consider the dog's history, complete a chest x-ray and often request an ultra-sound to determine how significant and advanced the heartworm problem is within the heart, lungs and blood vessels.
When there is a mild to moderate infection of heartworms there is a good prognosis for full treatment and recovery for the dog. The more pronounced the infection the greater the likelihood is for complications during treatment as the dead worms collect in the blood vessels, kidneys, liver and lungs. In some cases dogs with extreme heartworm infestations and other health related issues are not treated as the treatment is likely to be fatal to the dogs. The primary health issue may need to be corrected before starting on the heartworm treatment to try to protect the dog as much as possible.
Treatment includes killing both the adult heartworms in the heart, lungs and blood vessels as well as the microfilariae in the body. This includes both oral medications as well as deep intramuscular shots to kill the adult worms. During this time the dog has to be kept indoors and exercised at a minimal amount. In addition he or she needs to be closely monitored for any signs of distress that may indicate blocked blood vessels or pulmonary thromboembolism, a blockage in the lungs due to circulation of the dead worms from the heart and lungs.
The best way to treat heartworm is to use a monthly heartworm preventative. This may be part of a topical flea treatment or may be a tablet or liquid given to the dog once a month. In some areas heartworm treatment is needed only in spring and summer seasons when mosquitoes are present, but in warmer climates it should be continued all year round.
It is important to have your dog tested for heartworms before starting a monthly treatment, your vet can do a simple blood test to help you get started.