I have reservations about topical flea and tick products. Although side effects are rare, they do occur. No product or medication is free from potential side effects, however, there are ways to mitigate some potential issues.
Skin irritations are described below. My other concern with topical products is their potential exposure to children who might touch an animal that was recently treated.
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 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, 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.
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 totally 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.
To alleviate these concerns, I recommend a chewable tablet that is good for three years. Consult with your veterinarian about the choices you have in choosing these products.
Dr. Lamb is the Veterinarian at the Manchester Animal Hospital.
