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See your veterinarian if your pet becomes very itchy, or irritated, or if you notice other signs after applying spot-on flea and tick preventative products.

Spot-on flea and tick products for dogs and cats have greatly improved the safety and convenience of controlling external parasites on our pets. Using pesticides that are many degrees less toxic and frequently more effective than the organophosphate or carbamate dips and sprays of the past, these spot-on products help keep our pets (and our homes) free of annoying pests that can also carry serious diseases.

When used according to the label directions, spot-on products are well tolerated by most pets. However, as with any product that is applied directly on the skin, there is the possibility that certain individuals will have adverse local reactions to one or more ingredients in the product. Some individuals will have similar reactions to many different spot-on products with different active ingredients, suggesting that their sensitivities may be to some of the inactive ingredients. These reactions are restricted to the area of skin that comes in direct contact with the product, so they do not reflect a systemic toxicosis but rather a local hypersensitivity. Skin reactions to spot-on products can vary from mild tingling sensations to actual chemical burns of the skin in especially sensitive individuals.

The mildest form of skin reaction to spot-on products is epidermal paresthesia, which is defined as an abnormal sensation such as an itch or prickling of the skin. Paresthesia occurs when the applied product "tickles" the nerve endings in the skin, causing the characteristic sensation. Although paresthesia may occur with any spot-on product, it is most commonly associated with products containing concentrated pyrethroids such as permethrin, cyphenothrin, and etofenprox. Pyrethroid paresthesia is a syndrome that has been well documented in both humans and animals. Human descriptions of pyrethroid paresthesia vary from "tingling" to "pins and needles" to "burning" sensations in the skin at the site of pyrethroid contact. These sensations begin within 30 minutes of application and may last 8 to 24 hours if untreated. Some pets appear to experience similar discomfort following application of spot-on products; the fact that the products are generally applied between the shoulder blades can make the sensation particularly annoying to the pets. Affected pets, cats in particular, may become hyperactive and agitated as they try to walk away from the sensation; others may become quiet, subdued and reluctant to move. Skin at the product application site will appear normal (if redness is noted, you're dealing with contact dermatitis, see below).

Fortunately, treating epidermal paresthesia is fairly simple and entails bathing off the product with a mild dish soap (pet shampoos are too mild to remove all of the product). For pyrethroid paresthesia, applying vitamin E to the affected skin can provide quick relief— just use scissors to snip open a vitamin E capsule (the kind used as a vitamin supplement) and squeeze the oily contents onto the pet's skin and rub it in.

A rare but more significant skin reaction (in terms of sensitivity) to spot-on products is contact dermatitis, in which an inflammatory reaction develops in response to the topically applied product. These responses - sometimes referred to as hypersensitivity reactions - can occur upon the first use of a product, but more commonly occur after several uneventful exposures to the product. The body sets up an inflammatory response to the site of application, and the skin will appear red and irritated. In more severe cases, wheals or blisters may develop and the skin may actually ulcerate. Unlike paresthesia, which tends to occur within 30 minutes of application, oftentimes the onset of contact dermatitis is delayed for several hours, with the full extent of the injury taking 12 to 24 hours to develop. Depending on the degree of the inflammatory reaction, the level of discomfort can range from mild to quite severe. After bathing off the spot-on product, pets who develop more than mild redness at the application site should be seen by their veterinarians for further treatment to reduce the inflammation and discomfort.

The good news is that skin reactions to spot-on flea/tick control products are quite uncommon, and most pets will have no problems when these products are used. Unfortunately, it is not possible to predict which individual will react to a given product until the reaction has occurred. For this reason, it is a good idea to observe your pet closely for at least an hour following the application of any spot-on product for any signs of discomfort.

Although not life-threatening, skin reactions can range from mildly annoying to very painful; for this reason, a pet that has had a reaction to a spot-on product should never have the same product applied in the future. If a spot-on product is still desired for flea and/or tick control, try a product with different active ingredients than the one that caused the reaction. Following application, watch the pet carefully and be ready to bathe at the first sign of problems. Unfortunately, there are rare individuals that cannot tolerate any of the topical spot-on products, and other flea control options such as sprays or dips may be needed.

More serious problems can result when spot-on flea/tick products designed for dogs are inappropriately applied to cats. Certain concentrated pyrethroids (e.g. permethrin) that are not harmful to dogs can cause life-threatening tremors and/or seizures if applied to cats—these are not really adverse events but are instead true poisonings since the product was misused.  Always read the label before using any topical flea/tick product on cats, and never apply products intended for dogs onto cats.   

Frontline (fipronil) is toxic to rabbits. 

If your pet has a reaction to a flea or tick control product of any kind, you should report the reaction. All spot-on product labels will have a phone number that you can use to report the reaction to the manufacturer. By law, the manufacturer is required to send monthly reports of adverse reactions to the appropriate federal regulating authority. Alternatively, you can go to the agencies' websites to find out how to report reactions directly to them. Report reactions to flea/tick products containing heartworm preventatives, such as Revolution and Advantage Multi, to the Food and Drug Administration; report flea/tick products that do not contain heartworm preventatives to the Environmental Protection Agency.

Food and Drug Administration (FDA)