Please note: Certain pet products have linked to this blog post as us providing proof of flea resistance. Our comments were drawn from veterinary research but we at ULVH have conducted no study ourselves and shouldn’t be cited as a source. Thank you.

Flea and Heartworm Resistance to Medications: How to Protect Our Pets

For decades, fleas and heartworms have been relatively easy to prevent. A pet owner simply had to administer one of the many quality preventatives on the market. Even if a pet got fleas, one of the many flea medications available could kill these pests. 

Some fleas and heartworms have begun to evolve in recent years, though, developing resistance to the medications normally used to control them. The good news is that resistant super pests aren’t widespread—yet—and prevention is still the absolute best policy. Here is what to know about increasing resistance to preventatives and how to keep our pets protected.

Flea Resistance to Medications

Frontline products (Frontline Plus, Frontline Gold) and the K9 Advantix line are two of the better-known brands of topical flea preventatives that are reported to be losing effectiveness. These products have been in use long enough that fleas have adapted and become resistant to them. For the majority of pets, though, these products are still effective. 

If your pet has persistent flea issues, look to newer topical preventatives or recent 1 or  3-month oral treatments. These contain medications that fleas are not currently resistant to. 

Your Pet is Still Scratching—Is It Flea Resistance? 

Just because your pet is scratching after a flea treatment, or you’re still seeing fleas, it doesn’t mean that your pet is resistant to flea products. Fleas are normally more active in the 12 hours after treatment that it typically takes fleas to die. 

With topical flea treatments, a common problem is applying them correctly. Be sure to: 

  • Apply the flea product to your pet’s skin and not just the coat. 

  • Buy a preventative made for your pet’s weight and apply the right amount of product. 

  • Make sure your dog is completely dry if you apply after a bath. 

If your pet is still scratching despite doing everything right, it could be one of these reasons: 

  • Your dog may be one of the 40% who test positive for flea bite dermatitis. This skin allergy can cause itching until fleas are fully eliminated and the skin is healed.

  • Some dog flea medications are toxic to cats, and may cause their skin to itch. NEVER APPLY DOG FLEA PRODUCTS ON CATS. THIS CAN RESULT IN DEATH.

  • Your pet’s bed or home environment still contains fleas. Flea eggs continue to hatch and produce more fleas if your pet’s environment is not also cleaned and treated.

  • Generic versions of flea medications are often less effective. 

Heartworm Resistance to Medications

Mosquitoes carry heartworm disease, transmitting heartworm larvae from an infected pet to an unprotected pet. It takes 6 months from bite to adult infection and dogs can be infected by more than one mosquito. The adult heartworms create microfilaria that are ingested by the mosquito to be passed on to other dogs for the cycle to repeat.

Heartworm preventatives stop the transmission of heartworms in virtually 100% of cases—as long as they are administered without gaps in coverage. Inconsistency in heartworm medication is the number one reason pets get heartworms. Even a short break in coverage, or a delay in the start of dosing, can expose your pet. 

When heartworm disease is discovered early through testing, dogs are sometimes treated using a slow kill method. This means that a monthly dose of heartworm prevention is administered to gradually kill off the heartworms and microfilaria over a one-to-two year period. 

This method of treatment may be effective, but the downside is that the microfilaria that survive become resistant. Heartworm microfilaria that are resistant can continue to multiply in the bloodstream despite heartworm treatment. 

Starting up heartworm doses after your pet has been infected with heartworms could have a similar effect. Therefore, we recommend the following:  

  • Be consistent and regular with your heartworm preventatives. Add it to your calendar.

  • Always have your pet tested before starting heartworm preventatives or after a gap.

  • If heartworms are found, your veterinarian will review treatment options based on your pet’s individual case.

Can Pet Owners Prevent Heartworm Resistance?

The best way of avoiding heartworm resistance is to make sure that your pet is not infected with heartworms in the first place. Union Lake Veterinary Hospital recommends administering heartworm preventatives on your pet year-round without gaps. Consistency is key. Ask us if you have any questions or concerns about flea, heartworm or other preventatives for your pet. 

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