Sun, 08/21/2022 - 7:41pm

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How safe are flea and tick products?

Do some flea and tick preventive medications cause seizures in dogs?


There has been a lot of discussion on social media recently about dogs having seizures after taking certain flea and tick preventive products. These products belong to a class of chemicals called isoxazolines. Isoxazolines are a synthetic chemical class that effectively kills a wide range of parasites such as fleas and ticks. They can also be used to treat demodectic and sarcoptic mange, which are conditions of the skin caused by two different mites.

Most of the available products — such as afoxolaner sold as Nexgard (Boehringer Ingelheim), fluralaner sold as Bravecto (Merck), sarolaner sold as Simparica (Zoetis), and lotilaner sold as Credelio (Elanco) — are available in tablet or chewable form to be given orally to dogs. Nexgard, Bravecto and Simparica are also available as a topical solution, along with Revolution Plus (Zoetis), which is a combination of sarolaner and selamectin.

Isoxazolines are rapidly absorbed in the blood and distributed throughout the body of the dog. They act as an “ectoparasite.” This means that they kill parasites that live on or in the skin or in the fur or ears of dogs, such as fleas, ticks and mites. In order to be exposed to the active chemical, parasites must attach to the skin and start feeding on the dog.

Once the parasites have ingested the dog’s blood, the isoxazolines kill by acting on the parasites’ nervous system. They block the normal movement of charged chloride particles (ions) in and out of nerve cells, especially those associated with gamma-aminobutyric acid (GABA) and glutamate, two substances that convey messages between nerves (neurotransmitters). This blockage results in uncontrolled activity of the nervous system and the paralysis and death of the parasites.

Isoxazolines have a rapid onset of action. For example, fluralaner (Bravecto) kills fleas within eight hours and ticks within 12 to 24 hours in dogs. This means the fleas are dead before they can lay eggs, so the contamination of the environment is greatly reduced.


What are the risks associated with these products?


The most common side effects of the tablets when given to dogs are mild, temporary diarrhea, vomiting, lack of appetite and drooling. The reported rate for these cases is about one in 100 dogs.

When the topical solution is applied, the most common reaction is a mild, short-lived sensitivity of the skin including redness, itching or hair loss. The reported rate for this is also about one in 100 dogs.

The Food and Drug Administration (FDA) considers products in the isoxazoline class to be safe and effective for dogs. As a result of the reports from owners, the FDA is alerting dog owners and veterinarians of the potential for “neurologic adverse events” following treatment with these drugs. The symptoms include seizures, muscle tremors, unsteadiness and loss of coordination.

Few of these cases have been documented adequately, and none have been confirmed in the research literature. Since isoxazoline drugs target neuronal chloride channels with a clear preference for invertebrates (insects), they are considered to have a good safety profile. However, the potential for the action of the drug on the nervous system of vertebrates cannot be ruled out completely. Dogs with known neurologic disease should avoid taking this class of preventive.


What should I do if my dog has a reaction while using an isoxazoline product?


If your dog experiences any adverse reaction while using one of these products, first consult your veterinarian. The FDA continues to monitor adverse drug events for these products and encourages dog owners and veterinarians to report them. You can do this by reporting to the drug’s manufacturers, who are required to report this information to the FDA, or by submitting a report directly to the FDA.

To report adverse or suspected adverse drug reactions for these products, contact the appropriate manufacturers at the following phone numbers:

   Merck Animal Health (Bravecto): 800-224-5318

   Elanco Animal Health (Credelio): 888-545-5973

   Boehringer Ingelheim (Nexgard): 888-637-4251

   Zoetis (Simparica, Revolution Plus): 888-963-8471

If you prefer to report directly to the FDA, see How to Report Animal Drug and Device Side Effects and Product Problems (

Nexgard is recommended for dogs under the age of six months. Simparica and Bravecto are recommended for dogs over the age of six months. Isoxazoline products are approved for use in breeding, pregnant and lactating dogs.

The risks of not administering flea and tick preventives to your dog may be significant for both your dog and your family. Cases of Lyme disease, flea infestations and anaplasmosis by far exceed the number of reported adverse side effects of these preventives. Killing ticks through preventives also prevents human family members from contracting tick-borne diseases such as Lyme and anaplasmosis.


How do Seresto collars work?


The flea and tick collars of old are plastic bands soaked in insecticide and coated in a stinky white powder. The Seresto collar uses modern technologies that store the active ingredients inside of the collar so there is no powdery residue or smell.

The Seresto collar slowly and continuously releases two active ingredients, flumethrin and imidacloprid, on to your dog’s skin over the course of eight months. These chemical compounds repel and kill fleas, ticks and chewing lice, as well as the mites that cause sarcoptic mange (scabies). They work through simple contact, which means the bugs do not have to bite your dog in order to be affected. This is especially important for dogs with flea-bite hypersensitivity, who will scratch relentlessly after being bitten.

After applying the collar, Seresto kills any fleas on your dog’s body within 24 hours. If the collar is worn continually for the next eight months, any new fleas that land on your dog will be killed within two hours.

Seresto collars kill fleas before they can lay eggs, which helps reduce flea larvae in the dog’s environment. However, immature fleas that are already in the environment are not affected, so fleas may emerge for six weeks or more.

A wide range of tick types are susceptible to Seresto. Ticks that are already on your dog before the collar is applied may not be killed. Remove any existing ticks before putting the collar on your dog. Once in place, prevention of new ticks starts within 48 hours, and any new ticks will be repelled or killed in as fast as six hours.

Seresto collars are water resistant and remain effective after bathing, shampooing, swimming, and rain or sun exposure. With repeated exposure to water, the length of time the collar is effective is reduced from eight months to five months. The collars are considered safe for dogs that are seven weeks of age or older. Consult your veterinarian before applying a Seresto collar to debilitated, geriatric, pregnant, breeding or nursing dogs.

The collars are not labeled with an expiration date. Keep the collar in its original sealed packaging in a cool, dry location. Once the packaging is open, the collar will be good for eight months.


Are Seresto flea and tick collars safe?


Recent reports from pet owners of severe reactions attributed to the Seresto collars have gained national attention. The collars were developed by Bayer and are now manufactured and sold by Elanco Animal Health. Most flea and tick products, including Seresto collars, are regulated by the Environmental Protection Agency (EPA).

It was stated that the EPA has received more than 75,000 incident reports involving Seresto Collars since they were first introduced in 2012. Those reports included 1,700 deaths of pets and about 1,000 incidents of harm to humans. According to veterinarians at several poison-control centers, these reports encompass a wide range of symptoms, ranging from ruptured eardrums to heart, liver and kidney failure. It is their opinion that the randomness of the complaints makes it unlikely that the collar is the source of the problem.

Since anyone can report anything to the EPA, the raw data can be misleading. Without a veterinary examination or necropsy (animal autopsy) to rule out other causes of illness or death, an accurate cause-effect connection cannot be established. Veterinary toxicologists rate Seresto collars as having very low toxicity potential, even when the collars are accidentally ingested.

The prevalence of counterfeit products may be the source of the problem. Fake collars are packaged to look like the real thing and are usually considerably cheaper in price. These knock-offs may not only fail to protect against fleas and ticks, but often contain ingredients that are harmful to both dogs and humans.

At this time, it is the opinion of the veterinary community that until there is actual scientific proof resulting in warnings from the profession, the Seresto collar is a safe product that will continue to be recommended as part of flea and tick control programs.



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