Canine heartworm disease begins when a dog is bitten by a mosquito that is carrying juvenile heartworms. The larvae (juvenile heartworms) are deposited on the skin where they eventually burrow through and migrate to the bloodstream. The worms eventually make their home in the pulmonary arteries and heart. The worms incite an inflammatory response that causes damages to the dog's blood vessels and heart. They also make it difficult for the heart to pump blood because of the obstruction within the vessels. Adult heartworms can grow 10-12 inches in length!!!
Signs of Heartworm Disease
Some dogs can go for many years without showing obvious signs of heartworm disease. This is one important reason why we recommend yearly screening. When a pet does start experiencing damage to the pulmonary vessels and heart some of the signs we see include coughing, reluctance to exercise or play, fatigue, weight loss, and loss of appetite. Eventually, if the pet is not treated the heartworms can lead to heart failure and even sudden death.
Testing for Heartworms
There is a simple blood test performed that will detect a heartworm infection in your dog. The test is performed in house and you will have results in about 10 minutes. All dogs older than 6 months of age need to have a heartworm test performed prior to starting a preventative. Annual testing is recommended for all dogs.
If for some reason, a dog is not properly tested and has missed a dose of medication, it is possible for an infection to establish yet no microfilariae (immature heartworms) will be detected. People commonly ask why they must continue annual testing in animals that are on preventive medication; this is one reason.
Also note: because the heartworm tests on the market either look for microfilariae or for adult worm proteins, they will not detect infection with immature worms. This is why it takes 5 to 7 months from the time of exposure to get a valid heartworm test and this is also why there is no point in testing puppies less than 5 to 7 months of age.
Preventing Heartworms
Once a negative status has been established, the dog is placed on a monthly preventative. Our hospital recommends Interceptor or Trifexis. Interceptor is a monthly heart worm medication that also helps protect against roundworms, hookworms, whipworms, and tapeworms. It is important to prevent hookworms and roundworms because these parasites can by transmitted to you or your family. Trifexis is our other monthly heart worm preventative, it also prevents fleas and treats for hookworms, roundworms, and whipworms. Your dog should be on a monthly heartworm preventative all year- round for protection.
It is important to give the preventative on a timely schedule. The medications are eliminated from the dogs body shortly after they are given and do not protect from future infections. They actually kill any immature heartworms that your pet may have acquired since his/her last monthly dose.
Prevention is not only much more cost effective but also much safer for your pet than treating Heartworm disease.
Treating Heartworms
Once a dog is diagnosed with heartworm disease there are several things to consider. If it is not treated the disease will worsen and cause more damage to your pet. Unless there is a medical reason that makes your pet a poor candidate for treatment heartworm positive dogs should be treated. However, the treatment also has the potential for some serious side effects. As the medication is killing the adult heartworms there is a possibility that one of the worms may break off and become lodged somewhere else in your pet's body acting like a clot. This is one reason why your veterinarian will stress how important it is that you keep your pet calm during the time of treatment. The medication that is used (melarsamine hydrochloride) is extremely effective in eliminating the adult worms; however some dogs are not cleared with a single treatment.
Cats and Heartworms
While it is true that the feline infection is not as common as the canine infection, the feline infection has recently been found to be a much more widespread problem than previously believed. In the past, a common statistic was that within a given geographic area, the feline heartworm infection rate was approximately 10% of the canine infection rate. Recent research indicates this is not so; in heartworm endemic areas, the incidence of feline heartworm infection rivals or surpass FeLv or FIV infections. An incidence of 2% to 14% of all cats has been reported for endemic areas, making heartworm a concern for any cat living where there are mosquitoes.
- The cat is not a natural host for the heartworm, which means the migrating larval heartworm is not likely to complete its life cycle. The migrating worm uses molecular sign posts to tell it how to get to its host's pulmonary arteries. The worm is prepared to read CANINE directions and may get lost in the feline body, ending up who knows where. Most of the larvae that actually make it to the pulmonary artery die soon afterwards due to the massive immune attack from the feline body. Very few larval heartworms survive to adulthood in cats.
- Whereas a moderate heartworm infection in a dog would involve 25 to 50 adult heartworms, infected cats typically have less than six adult worms. Because the feline heart and blood vessels are so small, these few worms can wreak havoc. In a dog, six worms or fewer might not be considered worth treating. In a cat, a single worm could easily represent a lethal infection.
- Whereas worms found in the canine heart can reach lengths up to 14 inches, the average length of worms found in feline hearts is only 5 to 8 inches long.
- While an adult heartworm can expect to live 5 years in a dog, it will only live 2 to 3 years in a cat, probably due to the cat's strong immune reaction.
The cat's immune system is extremely reactive against heartworms. For this reason, it is virtually impossible to detect microfilariae (immature heartworms) in an infected cat. (The cat's immune system removes them too quickly.) Also, symptoms of infection tend to be more immune-related than heart-failure related. Cats develop more of a lung disease, complete with respiratory distress, and chronic coughing or vomiting. Feline heartworm disease is often misdiagnosed as feline asthma. Sudden death may occur just as it may occur in infected dogs.
In dogs, diagnosis is usually not complicated. A blood sample is tested for proteins that can only be found in the body of the adult female heartworm. In cats, heartworm disease is not dependent on adult worms so this kind of testing has limited applications. Furthermore, there may be too few adult worms to generate a positive antigen test.
Our hospital recommends placing your cat on Revolution for a parasite preventative. It is a topical medication that is applied once a month. It is not only a heartworm preventative but also prevents fleas, ear mites, hookworm and roundworm infections.