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This week's case is about a one and a half year old blue point Siamese. Romeo moved from New Jersey to Arizona at the beginning of 2011. His owner brought him into the clinic because he had been coughing for a few months. His previous veterinarian had done x-rays and saw that he had bronchitis - inflammation of the airways - but was not able to determine the cause. His owner told me that Romeo was mainly an indoor cat, but he did sit outside in a large cage for a while some days. Hearing that the cat had some outdoor contact, my first thought was to rule out heartworm disease.
Heartworms are injected into a cat by a mosquito. The heartworm larva will then travel throughout the blood stream and pass through two more larval stages before becoming a worm. We obtained a blood sample from Romeo and ran an antibody and antigen heartworm test. The antibody test would show if he had any exposure to heartworms sometime in the last months or years. The antigen test detects if there is an adult female worm in the cat's body. Cats are actually good at fighting off the heartworm infection and will only have immature worms or one or two adult worms if any survive. Dogs, on the other hand, tend to have many worms in the heart and lungs.
This ability to resist the parasite makes it difficult to definitively diagnose heartworm disease in cats since many infected cats have no worms that will cause the antigen test to turn positive. The cat may also completely fight off the parasite and have his antibody test turn negative after months or years. But even if a cat has beaten the parasite, his lungs may still have lasting damage from the heartworm. While fighting off the parasite, the immune system will mount an inflammatory response that leads to significant lung disease. This is called Heartworm Associated Respiratory Disease (H.A.R.D.). The disease can be mild to severe and can cause various symptoms including coughing, difficulty breathing, vomiting, weight loss and lethargy.
Romeo's antigen test was negative but his antibody test was positive. This did not tell us if he had any heartworms currently in his system but it did tell us that he was exposed at some point. I prescribed steroids for Romeo to help reduce the inflammation in his lungs. This decreased the irritation and mucus secretions so that he would not cough. I hoped it would also prevent further damage to his airways. He immediately improved and is no longer coughing. He is back to himself, and his owner is protecting him from being infected with heartworm again with the regular use of Revolution, a monthly heartworm preventative.
We still do not know how much damage has been done to Romeo's lungs and if he will need to be on steroids for the rest of his life or not. He likely was exposed to the heartworms back in New Jersey before he moved here. However, heartworm disease is becoming more prevalent in Arizona and can be spread by mosquitoes here just as easily as anywhere in the country. We strongly encourage heartworm preventative for all cats living in the valley.
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Dr. Judy Karnia
The recent bursting of the downstream dam at Tempe Town Lake has brought several environmental and health related concerns to the forefront. One of the most concerning is that the standing water could result in an increase in mosquitoes (see this article from azcentral.com for more information on other concerns about the lake). We are all familiar with the risks of disease to people from mosquitoes, but our pets are also at risk.
The number one risk to pets from mosquitoes is heartworms. Previously it was believed that only dogs were at risk from these parasites, but new research is showing that cats are also victims. Unlike dogs, cats are considered incomplete hosts to the heartworm, which means the worms rarely develop into adults, and even if they do, they rarely make it to the heart. However, we have no learned that cats are actually at risk and that heartworms can cause fairly significant health problems.
Mosquitoes become the carriers for heartworm larvae when they feed off a heartworm positive dog. They will pick up heartworms in the larval stage known as L3. The mosquito will then feed on a cat, transferring the larvae to the cat. Within the cat, the larvae can continue to develop through the L4 and L5 stages (L5 is the last larval stage before the adult worm).
Earlier this year, Dr. Karnia attended a seminar on heartworm disease in cats. The information presented during the seminar said that 75% of cats are susceptible to heartworm infection and 10% of those cats can actually develop the adult worms. The worms can actually live within the cats for 2-3 years, and during that time even one worm can cause significant damage. Because heartworms tend to settle in the lungs of cats, they can cause thickening of the arterial and bronchial walls and can block the arteriole, compromising the cat's ability to breathe. In rare cases, the worms can actually have aberrant migration to the cat's brain, eye or even abdominal fluid. Even worse, when the adult worm dies, the toxins of the decomposing worm can kill the cat too.
Even the larval forms can cause damage to the lungs before the cat is able to fight off the infection. Damage to the lungs can occur just from the presence of an L5 larval stage worm.
Unfortunately, diagnosing heartworm disease in cats can be very difficult. The symptoms are often similar to other health conditions, and frequently the damage to the lungs can be diagnosed as asthma related rather than from heartworms. Antigen tests are only positive if there is an adult female worm present in the cat's body at the time of testing. The antibody test is equally frustrating as a negative test only rules out a current heartworm exposure and does not say whether the cat has been exposed or infected at another time in his life.
So how do we deal with the problem? They say the best defense is a good offense, so the best way to keep your cat safe from heartworms is to give him a monthly parasite control that is effective against heartworms. Even indoor cats should receive it. Here at Scottsdale Cat Clinic, we recommend Heartgard, a chewable tablet that many cats will actually eat on their own; or Revolution, a topical parasite control medication.
Contact us for more information on heartworm disease and preventatives.
