It has long been known that dogs can contract the heartworm parasite and suffer from congestive heart failure and can ultimately end in death, but very few know that their feline friends can also suffer from this same parasite. There are quite a few differences amongst the two species. In both species, the infestation starts with the bite of an infected mosquito. The mosquito injects the heartworm larvae under the skin where it begins to travel through the tissues of the animal headed towards the heart. In dogs, these larvae migrate into the heart and attach directly to the heart muscle and grow into adult worms. The heart has plenty of room for the worms to accumulate and grow. The worms then produce offspring which are released into the blood stream for the next mosquito to collect and carry onto the next unsuspecting victim. The heartworm parasite behaves differently in cats. The migrating larva prefers to attach to the pulmonary artery which leads to the lungs and they tend to stay in an immature form and not complete their life cycle to become adult worms but persist in an immature state. There is much less room in the artery compared to the heart. Therefore, it only takes a few worms to occlude the vessel and lead to serious problems.
Testing for heartworms is more difficult due to this difference in how the worm reacts in the different species. In dogs, the preferred test is the antigen test. This is an excellent test because it detects a protein that is carried on the adult female heartworm. If the test is positive, then the dog has had or does have at least one adult female heartworm. Because even one worm can be detected with this test, it is an excellent choice for screening our dogs. Unfortunately, since the worms rarely grow to be adults in the cat, this test will not work as the protein does not form until the worm matures. Instead, we rely upon an antibody test. This simply means that the cat was exposed to heartworm larvae which the animal’s immune system has reacted against but it does not determine if the larvae ever made it in their migration to the pulmonary vessel. Therefore, it is common to get a positive antibody test but have no current heartworm larvae in the cat. It can, however, rule out the heartworm disease.
The symptoms of heartworm vary amongst the two species as well. Heartworm disease in dogs is more slowly progressing as the worms accumulate and grow. Congestive heart failure is a common end result. In cats, clogging of the pulmonary artery can lead to a sudden crisis. The blood cannot flow to the lung properly or the worm breaks off in this high velocity area and causes a sudden obstruction within the lung itself. This can result in the symptoms of coughing or vomiting but unfortunately a common result is simply sudden death.
Treatment of heartworm disease is very effective in the dog if caught before the congestive heart failure becomes too advanced. It is expensive and scarring of the heart and long-term inflammation caused by the worms may need to be addressed, but there is a very effective treatment protocol to aid in ridding the worms from our dog friends. Cats, however, are a different matter. They are too sensitive to the drugs used to kill the worms and cannot be given this medication. The worms can be manually removed via ultrasound guidance and a retrieval device by an expert surgeon, but this is obviously quite risky. Most of the time, the best we can offer is to treat the secondary inflammation these worms can cause to the lungs and pulmonary vessels. This is palliative at best!
Heartworm prevention is key for both species! Many pet owners are aware of this problem in dogs and routinely administer a heartworm preventative to their dogs. There are many convenient options from a monthly oral medication, a monthly topical medication or an injection of a long acting medication that can last up to twelve full months now. Unfortunately, most cat owners are not even aware that this is a silent but deadly killer. Heartworm prevention is also available to our cats in a monthly chewable tablet or a monthly topical application but rarely purchased. When a cat suddenly dies, it is rarely sent to a pathologist to determine the cause of death. Education of cat owners is instrumental in prevention of heartworm disease in cats. If they only knew…….