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"Doctor, I don't know what happened," said Mrs. Tears. She was obviously in distress as she spoke on the phone. "Pixie just died." "Had Pixie done anything unusual in the past few days?" I asked. "He threw up last week and gagged a bit," Mrs. Tears said, "but he got over it. He has been fine since then, even earlier today. He woke me up this morning at his usual time. I fed him. He did not eat all of his food this morning, but he ate. I went to work, and when I came back, I found him dead." "Was he inside?" I asked. "Had he been outside at all?" "He does go outside," replied Mrs. Tears, "but he was inside all day today. I do not see anything that he could have gotten into in the house. Nothing is disturbed except the spot where he died. I do not see any marks on him, just a little blood coming out of his mouth. What do you think happened?" "There are so many things that could have happened," I replied. "He could have had a heart attack. Cats do not always give warnings when they have heart problems. An autopsy might help to identify the cause of death if it is something obvious. If nothing is obvious, biopsies of some of the tissues could reveal something." "I would really like to know what happened, Doctor," said Mrs. Tears. "I would like to have an autopsy done." Mrs. Tears brought Pixie's body in that day. Pixie was a lean domestic short hair. He was five years old. As Mrs. Tears had mentioned earlier, there were no obvious signs of trauma on Pixie's body. There were scars from previous fights. Pixie had liked to let other cats know that he was the king of the neighborhood. Aside from that, there was just a trickle of a bloodstain coming out of his mouth. We proceeded with the autopsy. As we examined the inside of the heart, we found three long, thin worms sitting in the heart chamber and blood vessels, heartworms. The rest of the autopsy was unremarkable. The probable cause of death was heartworms. Heartworms are not commonly found in cats. Cats appear to have a natural resistance to heartworms. Cats become infected only 5 - 20% of the times that dogs become infected. However, when they do acquire heartworms, cats react more severely to them. Because their hearts are so much smaller than those of the dog, just a few worms can cause a lot of damage in cats. Many cats with heartworms do not show any signs beforehand, unlike dogs. When they do show signs, they are so variable that they do not point specifically to heartworms. Signs can range from simple lethargy, loss of appetite, and coughing to vomiting blood, having difficulty breathing, and sudden death. Physical examination is often unrevealing. Sometimes a heart murmur or heart arrhythmia can be heard, but lots of other things can cause these. Researchers have improved the tests for feline heartworms. Since cats usually have less that ten heartworms, (typically two to four), some cats with heartworms will turn up negative when tested. The small number of heartworms may not be enough to elicit a reaction with the test. However, the testing is better and worth doing if a cat is suspected of having heartworms. Radiographs and echocardiograms may also help in diagnosing feline heartworms.		Cats at risk for heartworms are usually cats that go outside. Heartworms are transmitted by mosquitoes. Studies have shown male cats to acquire heartworms more than female cats, probably because male cats roam more. Despite the propensity for heartworms to be found more in outside cats, a study by North Carolina State University showed that one-third of cats diagnosed with heartworm disease were inside cats according to the owners. Since heartworms are transmitted by mosquitoes, they are considered a seasonal problem in the northern areas of the United States. Spring and summer are their warm seasons, the mosquito seasons. In the southern states, heartworm season starts earlier and ends later. Definitely, dogs in the southern states should be on heartworm preventatives by early spring, if not year-round. The cutoff times are not so clear. In the past, cat owners have not worried about defining the heartworm season. There was no reason to do so. There were no heartworm preventatives for cats. Now there is a feline heartworm preventative available. It is given once a month. Owners of cats that get outside often should consider it. When cats get heartworms, they usually only get adult heartworms. This is in contrast to dogs, which generally get both adult and baby heartworms. You would think that would make treatment simpler for cats. The truth is that, for cats, treatment of heartworms can be as deadly as the disease itself. Unfortunately, because of their smaller sized hearts, cats react more adversely to the death of the heartworms when treated. Cats have fewer arteries branching out from the heart than the dog. If an artery gets clogged in a dog, an alternative artery may take over. The cat has fewer alternative arteries. If a couple of the cat's arteries get clogged, it can be severe enough to result in death. Admittedly, the prevalence of heartworm disease in cats is low. Seven percent of cats in the southeast acquire heartworms. Seven percent is not a lot, but that percentage parallels and sometimes exceeds the percentage of feline leukemia and feline aids cases. At present, there is no safe, reliable treatment for any of these diseases, but two of them, feline leukemia and feline heartworms, can be prevented. Prevention is much better. Dr. Reggie R. Cox practices at Hufford Animal Hospital, a small animal practice in Pasadena, Maryland. She is a graduate of the University of Missouri, Columbia