Wednesday, August 4, 2010

Heartworm Risks for Cats

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 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.

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