West Nile Virus and Cats

By Jean Hofve, DVM

Spring is just around the corner, and with it comes a new crop of mosquitoes and, potentially, a new round of West Nile Virus.

This virus, previously been found in Africa, Eastern Europe, West Asia, and the Middle East, arrived in the U.S. in 1999. Similar viruses are found in all the temperate and tropical parts of the world. It is transmitted by mosquitoes.

Most illness caused by West Nile is limited to mild, flu-like symptoms. However, people can develop a more severe form called West Nile encephalitis. Encephalitis is an inflammation of the brain, and can be fatal. Most victims were over 50 years old. In 2003, there were more than 9,000 cases reported in the U.S. (about twice as many as 2002), and 231 deaths (fewer than 2002). Colorado was by far the hardest hit state, followed by Nebraska and South Dakota. Evidently the mosquitoes enjoy our high, dry plains as much as we do!

An important question for pet guardians is: can cats and dogs get West Nile disease? Yes, they can. Dogs appear to be fairly resistant. When experimentally infected, their immune systems cleared the virus within a few days. In the real world, one study found 6 times as many dogs infected as people, but none of them became ill. Cats, however, are more susceptible.

A newly-published study by the Centers for Disease Control (CDC) and Colorado State University reported that cats can get West Nile not just from mosquito bites, but also by eating infected prey (birds and rodents). However, cats typically do not show clinical signs. A few cats developed a mild fever and were slightly lethargic, but more severe symptoms were not seen. Most cats do not develop large amounts of virus in their blood, so they cannot transmit the disease, but in a few cats, there was enough virus to pass on if they were bitten by another mosquito. However, CDC considers this risk very low.

How can you prevent West Nile from getting a foothold in your family? First, protect yourself. During mosquito season, use a mosquito repellent and/or wear protective clothing. DEET is the typically recommended repelling ingredient, but there is intriguing evidence that catnip works even better. Nepetalactone is the essential oil in catnip that gives the plant its odor. It was about 10 times more effective in repelling mosquitoes in lab tests. There are some “natural” sprays that include catnip along with other essential oils like pennyroyal and citrus. Do not use DEET or essential oil products on a dog or cat; they can be toxic.

Next, protect your animals. For dogs, there are products available that repel fleas, ticks, and mosquitoes, such as K9 Advantix. These are powerful chemicals and can have adverse effects; use with caution. Cats are more difficult. Because they are so chemically sensitive, there are few commercial products that are safe for cats. One possibility is Zodiac pet wipes, a topical weekly treatment. According to the company, the pre-measured ingredients on the cloth make it impossible to overdose, and they assure us that they are very safe for cats.

Control mosquitoes around your home. Eliminating all standing water, or if that’s not possible, treating standing water with a mosquito inhibitor to keep the mosquito population down. Don’t forget gutters, old tires, flowerpots, and other hidden sources of water. Clean up your yard. Mosquitoes can lay eggs in just a tiny amount of water; so get rid of small, discarded items that can hold water such as cans, bottles, and wrappers.

Of course, keeping cats indoors is a good idea. The more time cats and dogs spend outside, the higher their risk. If your cat has an outdoor enclosure, consider adding a layer of mosquito netting to allow your cat to get fresh air safely. Minimize outdoor time for all pets at dawn, dusk, and evening, when mosquitoes are most active.



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