Feline Vs Canin Heartworm: Same Parasite/ Different Disease Essay, Research Paper
Canine vs. Feline Heartworm:
Same Parasite / Different Disease
Feline Heartworm disease has caused headaches for veterinarians since it was first recognized in 1921. Although feline and canine heartworm disease are different they do have some attributes in common. Both are caused by the parasite, Dirofilaria immitis, and vectored by the mosquito. However, Veterinarians, accustomed to canine heartworm disease and the symptoms and diagnostic tools that go along with it, are not recognizing that heartworm infection in the cat is a completely different disease. Not only does feline heartworm disease have different clinical signs and a greater clinical importance of small worm burdens than canine heartworm disease, but it also does not respond to the same types of diagnostic tools. Another difficulty is that the clinical signs of feline heartworm disease are similar to other known cat diseases and therefore often misdiagnosed. This diagnostic error is most likely causing a great deficiency in the observed prevalence of feline heartworm disease2. Under reported disease rates cause a misguided sense of security, preventing owner awareness of the disease. With the proven efficacy of heartworm preventative for cats there is absolutely no reason for a cat become infected with heartworms. The next step in the battle against this seemingly simplistic, yet maddeningly evasive, disease is to research more efficient diagnostic tools specifically for feline heartworm disease and ensure cat owners are educated about the presence of and prevention for feline heartworm disease.
Feline and canine heartworm diseases each show their own unique characteristics. First, the susceptibility to infection of cats ranges from 61% to 90% compared to 100% susceptibility in dogs. Also, the prepatent period in cats is approximately 8 months, 2 months longer than the prepatent period of heartworms in dogs. Longevity of heartworms in the cat is 2 to 3 years, whereas in the dog longevity is 5 to 7 years. Furthermore, ectopic infections are found regularly within heartworm-infected cats but are only an occasional occurrence in the dog. The most important factor to remember about the heartworm infected cat is that the lungs are the organs showing the greatest pathology. The dog, however, shows pathology in the heart as well as the lungs3. All of these, small but significant, differences cause variation in clinical signs between the dog and the cat and impair the efficiency of canine catered diagnostic tools when they are used on the feline.
Since cats do not show the same clinical signs as dogs, deciding when to test for heartworms in cats is very difficult. In fact, many cats may not show any clinical signs up to the hour before death. Also, the nonspecific clinical signs of heartworm disease in cats are consistent with many feline diseases, making diagnosis through clinical signs alone nearly impossible. While dogs mostly suffer severe heart damage from heartworm infection, cats typically exhibit minimal changes in the heart 4. Roughly 40% of clinical signs in cats are respiratory in nature and include cough and dyspnea. All clinical signs may be acute in onset or chronic and intermittent. Acute signs include collapse, dyspnea, convulsion, vomiting and or diarrhea, blindness, tachycardia, syncope, and death. Most of these can occur as a result of a single-worm infection. Chronic signs include coughing, vomiting, dyspnea, lethargy, anorexia, and weight loss 1,2,4,6-9.
The clinical signs of feline heartworm disease are greatly related to two stages of the parasite’s life cycle. The first stage is the arrival of the L5 immature larvae in the distal pulmonary arteries. This can cause the lungs to react with inflammatory, asthma-like symptoms known as acute pneumonitis. Since acute pneumonitis is a classical sign of bronchial asthma, this is a good example of heartworm symptoms mimicking other known feline diseases, thus causing a misdiagnosis. The second stage of the parasites life cycle that causes clinical signs in the cat, is the death of the adult worms. This traditionally will cause a thromboembolism to occur1,6,7,10. Scientists have identified a specialized macrophage, not found in dogs, in the cat. This accounts for some of the major differences in signs. Research is suggesting that the presence of the parasite induces inhibition of these macrophages. When the parasite dies this inhibition is lifted, causing a major clinical reaction to occur 6,9. It is because of these major reactions that treatment with adulticide therapy should not be used for feline heartworm disease.
Despite these differences between feline and canine heartworm disease, canine diagnostic tests are still used attempt diagnosis of heartworms in the cat. The tests that are being used include microfilariae tests, serology tests for antibody and antigens of the heartworm, radiography, and echocardiography. The use of the microfilariae test is not helpful because an infected cat will very rarely have microfilariae in its blood stream. The antibody test may indicate feline exposure to heartworms but does not provide a definitive diagnosis of current infection. The antigen test detects *50% of natural infections and is only useful if there are adult female worms present in the feline heartworm population. Since cats have such small worm budens, single-sex populations are very common. As a result, there may not be any female worms present, making false negatives highly possible. A third test, radiography, is actually a quite helpful diagnostic tool for feline heartworm as long as the practitioner knows what to look for. In the dog, radiography is used to look for signs of right heart enlargement. In order to diagnose heartworms in the cat using radiography, enlargement and blunting of pulmonary arteries must be seen. Finally, echocardiography is 100% accurate in positive diagnosis of feline heartworms if a worm can be seen. Unfortunately, heartworms usually reside in areas of the heart where they will not be seen by an echocardiograph 1,5,7,6,11,12. Many of these tests, especially the blood test, are very useful tools for the diagnosis of heartworms in dogs. However, when diagnosing heartworms in cats, we are diagnosing a completely different disease and, therefore, research towards unique feline testing should be done.
Since diagnosis of feline heartworm disease is not reliable the true prevalence of this disease is greatly masked. Veterinarians need to ensure that their clients are being thouroughly educated of the risks, available prevention, and lack of options for cats that do contract the disease. Many cat owners are under the mistaken assumption that since they own cats, and not dogs, heartworms are not a risk. Furthermore, some cat owners may know about the risk of heartworms in cats but do not consider their strictly indoor cat a candidate. Practitioners need to inform these owners that these cats are still at risk for heartworms. In fact, they may be at an even greater risk due to a lack of resistance against heartworms that some outdoor cats obtain from failed infectious mosquito bites. Studies have even shown that nearly 50% of diagnosed feline heartworm disease are cats that live exclusively indoors1,9,10. Once cat owners have been informed about the risk of feline heartworm infection, they should be told that effective prevention is available. Feline heartguard (ivermectin) is proven to be 100% effective and is safe for all ages as well as breeding toms and queens2,7,9. The preventative should be given once a month for optimum protection. Lastly, owners should be informed of the poor prognosis for cats that are infected with heartworms. There is no approved treatment for feline heartworm infection, making prevention very important.
Veterinarians have limited options as to how to help a patient and owner once a diagnosis of feline heartworms is made. Practitioners should now begin to realize that they will never find canine heartworm disease in a cat. Therefore, they will not be able to diagnose feline heartworm disease with canine diagnostic tools. As a result, it is very important to continue research on potential diagnostic tools and treatment for feline heartworm disease. It is now the responsibility of the veterinary practitioner to inform his clients of the risk of heartworm and to make sure those clients understand the benefits of feline heartworm preventative in order to set everyone’s mind at ease.
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