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How Weather and Extended El Niño Impact Flea and Tick Season
Published on July 28th, 2026
Updated on July 28th, 2026
Written by Dr. Angela Silva
With shifting weather patterns and extended El Niño cycles bringing milder winters, fleas and ticks have officially become a year-round threat to our pets.
For years, pet parents thought of flea and tick prevention as a summer chore—something to start in the spring and pack away when the first winter frost hit. However, shifting climate patterns, including extended El Niño events, are rewriting the rules on parasite seasons.
When regions experience a rise in warmer winters and increased rainfall as is with El Niño fleas and ticks are staying active much longer than they used to. Here is a helpful overview of how weather impacts parasite activity and why year-round prevention is the safest bet for your pet, reviewed by our Petco Chief Veterinarians.
What is El Niño and how does it affect parasites?
El Niño is a climate pattern characterized by unusually warm ocean temperatures in the Equatorial Pacific, which leads to significant weather shifts across North America. For many areas, this means:
- Milder, warmer winters: Ticks become active anytime the temperature rises above freezing (32°F). Warmer winters mean ticks never truly go dormant.
- Increased moisture and humidity: Fleas thrive in warm, humid environments (ideally between 70°F and 85°F with 70% humidity). The extra rainfall brought on by El Niño creates the perfect breeding ground for flea populations to explode.
Because of these extended periods of favorable weather, what used to be a five-to-seven-month flea and tick season has effectively become a year-round threat in many parts of the country.
The Myth of the "Winter Freeze"
It's a common myth that winter freezes kill off all the fleas. While that's true for the freezing yard outside, our cozy, heated homes act like a winter resort for them. When a pet brings in fleas, their eggs slip off into the carpet fibers like seeds. The cold weather actually slows down their life cycle—making them go dormant in protective cocoons deep in the carpet. The moment we turn our central heating on, we simulate summer, and those dormant fleas can continue to hatch.
Risks of extended flea & tick exposure
When parasite seasons stretch throughout the year, the risk of your pet contracting a vector-borne disease or suffering from discomfort increases.
- Flea Allergy Dermatitis (FAD): The most common skin disease in dogs and cats, caused by an allergic reaction to flea saliva. It leads to intense itching, hair loss, and secondary skin infections.
- Tick-Borne Diseases: Ticks are notorious carriers of serious illnesses for both humans and pets, including Lyme disease, Ehrlichiosis, and Anaplasmosis.
- Tapeworms: Pets can easily ingest tapeworm infected fleas while grooming themselves, leading to tapeworm infections.
Why Petco veterinarians recommend year-round prevention
Because weather patterns are increasingly unpredictable, the Companion Animal Parasite Council (CAPC) and Petco veterinarians strongly recommend year-round, broad-spectrum parasite control for both dogs and cats. Trying to time your pet's prevention around the weather is risky, and a single missed month during an unseasonably warm winter can lead to a severe infestation.
What are the best flea and tick prevention options?
| Prevention Type | Brands | Frequency | Key Benefits and Coverage | Pet Type |
|---|---|---|---|---|
| Oral Chewable (Broad-Spectrum All-in-One) | Simparica Trio, NexGard PLUS, Credelio Quattro | Monthly | Prescription Required. Protects against fleas, ticks, heartworm disease, and roundworms/hookworms. Chewables are highly effective, taste like treats, and won't wash off during swimming or bathing. | Dogs |
| Oral Chewable (Flea & Tick Focus) | NexGard, Bravecto | Monthly (NexGard) or every 12 Weeks (Bravecto) | Prescription Required. Focused flea and tick protection. Great for pets with specific sensitivities or those on separate, dedicated heartworm-only regimens. | Dogs |
| Topical Spot-On (Cats - Broad Spectrum) | Revolution Plus, Bravecto Plus, NexGard Combo | Monthly (Revolution/NexGard) or Every 2 Months (Bravecto) | Prescription Required. All-in-one monthly spot-on defense tailored for cats. Prevents fleas, ticks, heartworms, roundworms, hookworms, and ear mites. Applied directly to the skin at the base of the skull. | Cats |
| Topical Spot-On (Flea, Tick & Mosquito) | K9 Advantix II, Frontline Plus | Monthly | Over-the-Counter. Convenient topical defense applied directly to the skin. K9 Advantix II also repels and kills mosquitoes. Note: K9 Advantix is strictly for dogs and is highly toxic to cats until dry. | Dogs & Cats |
| Prevention Collars | Seresto | Up to 8 Months | Over-the-Counter. Continuous low-dose release of active ingredients. Odorless, non-greasy, and water-resistant. Ideal for low-maintenance, long-term flea and tick protection. | Dogs & Cats |
Always consult your veterinarian before starting a new medication. Your Petco veterinarian will determine the right dose for your pet based on their species, age, and weight.
How to stay protected with Petco:
If your pet needs a new prescription or an annual checkup, booking an appointment at your local Vetco Total Care hospital or Vetco Vaccination Clinic is a great first step. Our veterinarians can help you select the most effective preventative for your specific region's climate.
Managing refills with ease
Once your Petco veterinarian prescribes your pet's preventative, you can fill it in-store or online through the Petco Pharmacy. We highly recommend setting up Autoship for your monthly preventatives. Autoship automatically delivers your pet's medication right to your door on your schedule, ensuring you never miss a dose during an unexpected warm front.
