Short answer: autumn brings a specific set of pet poisonings: rodent bait (fall is rodent-control season), antifreeze (the Merck Veterinary Manual says most cases occur in fall, winter and early spring), wild mushrooms, acorns, moldy compost, and autumn crocus. Several of them are deceptive: pets can look fine — or seem to recover — for a day or more before the serious damage appears. If you suspect your pet ate any of these, call your vet, the ASPCA Animal Poison Control Center at (888) 426-4435, or Pet Poison Helpline at (855) 764-7661 right away, and take the packaging or a photo with you.
1. Rodent Poison
As mice and rats move indoors, more bait goes down in homes, garages and sheds — and pets find it. Since the EPA restricted consumer sales of the most potent anticoagulant poisons, the Merck Veterinary Manual reports that bromethalin has become the most common rodenticide exposure in pets, with vitamin D3 (cholecalciferol) products also increasing. “Pet-resistant” bait stations help, but Merck notes pets can still break into them.
| Poison type | What it does | When signs appear (Merck) |
|---|---|---|
| Bromethalin | Brain swelling: tremors, seizures, weakness, paralysis. Cats are “exquisitely sensitive.” | 4–36 hours at high doses; 1–5 days at lower doses |
| Cholecalciferol (vitamin D3) | Dangerously high calcium, kidney damage. One soft bait holds 10–21 mg — far above the 0.1 mg/kg level where signs can start, even in a large dog. | 12–48 hours |
| Anticoagulants | Stops blood clotting: bruising, nosebleeds, coughing blood, weakness | Bleeding typically 3–7 days later |
The delays are the danger. A dog that ate an anticoagulant bait can look completely normal for days. Treatment works best — and is often simple — when it starts right after the ingestion, so call even if your pet seems fine, and have the product’s name or EPA registration number ready.
2. Antifreeze (Ethylene Glycol)
Antifreeze gets added or flushed from car cooling systems in the fall, and drips and open containers are easy to lap up. US manufacturers have added a bittering agent since 2012, but Merck notes the products may still appeal to some animals. The amounts are tiny:
| Pet | Minimum lethal amount of undiluted ethylene glycol (Merck) |
|---|---|
| 10-lb cat | ~6 mL — about 1.3 teaspoons |
| 10-lb dog | ~20–30 mL — about 5 teaspoons |
| 40-lb dog | ~80–120 mL — about 3–4 fl oz |
| 70-lb dog | ~140–210 mL — about 5–7 fl oz |
Ready-to-use antifreeze is often diluted, so a larger volume of it would be needed — but any suspected ingestion is an emergency.
Early signs look like drunkenness — vomiting, wobbliness, drinking and urinating a lot — within 30 minutes to 12 hours. Then comes a dangerous false recovery, before kidney failure sets in around 12–24 hours in cats and 36–72 hours in dogs. There is an antidote (fomepizole), but it has to be given early. Clean up drips, store containers sealed and up high, and consider pet-safer propylene glycol–based products.
3. Wild Mushrooms
Fall is peak fruiting season for many toxic mushrooms, and dogs are especially vulnerable “because of their indiscriminate eating habits and a tendency to roam,” according to Merck. The most dangerous species — death cap and related Amanita, Galerina and Lepiota mushrooms — cause liver failure, and their warning signs don’t start for 6 to 24 hours. Merck notes that a long delay before symptoms is often associated with the worst outcomes. Some Cortinarius species damage the kidneys days later.
You can’t tell a safe mushroom from a deadly one by looking at it. If your dog ate a wild mushroom, photograph it (and any others growing nearby), put a sample in a paper bag, and call right away — don’t wait to see if symptoms appear.
4. Acorns and Oak
Acorns contain tannins, which can irritate the stomach and — in larger amounts — injure the kidneys and liver. But Merck’s June 2026 review points out that in dogs, the main problem is often a physical intestinal blockage from swallowed acorns. Signs include vomiting, diarrhea (sometimes bloody), not eating, belly pain, and straining or not passing stool. Rake up acorns in areas where your dog spends time, and call your vet if your dog has eaten more than a nibble or shows any of these signs.
5. Compost Piles and Moldy Food
Fall garden cleanup fills compost bins — and moldy food can grow tremorgenic mycotoxins. Merck lists moldy walnuts, peanuts, dairy and compost or food waste as common sources. Signs — vomiting, tremors, wobbliness, seizures — can start within an hour or two, and there’s no known safe amount. Keep compost fully enclosed and moldy leftovers out of open trash. Windfall grapes deserve special mention: there’s no known safe amount of grapes or raisins for dogs.
6. Fleas and Ticks Don’t Stop in September
The Companion Animal Parasite Council says flea and tick “seasonality” is changing and increasingly unpredictable, and recommends year-round prevention rather than stopping when the weather cools. Several tick-borne diseases, including Lyme disease, remain a risk well into fall.
7. Fall Plants
- Autumn crocus (Colchicum): dangerous. Pet Poison Helpline says every part contains colchicine and can cause severe poisoning, organ failure and death. It isn’t the same as the spring crocus.
- Mums (chrysanthemums): usually only stomach upset, per Pet Poison Helpline.
- Sago palm: not seasonal, but increasingly common in decor and landscaping — its seeds can cause liver failure.
8. Darker Walks and Stiffer Joints
When the clocks change, evening walks happen in the dark. Reflective collars, leashes and lights help drivers see your pet. Cooler weather is also when owners often notice senior pets moving more stiffly. That’s worth a vet visit — and never a reason to reach for human pain medicine, which can seriously poison a dog or cat. See our guide to the early signs of arthritis in dogs.
Your Fall Checklist
- Place any rodent bait only where pets absolutely can’t reach it — or use traps instead.
- Wipe up antifreeze drips and store containers sealed and high.
- Walk your yard after rain and remove mushrooms and fallen acorns.
- Close the compost bin and keep moldy food out of reach.
- Keep flea, tick and heartworm prevention going year-round.
- Save the poison-control numbers in your phone.
Halloween is right behind all of this — see our guide to Halloween candy, chocolate and xylitol risks.
The Bottom Line
The fall poisonings to take most seriously — rodent bait, antifreeze and toxic mushrooms — share one trait: the pet often looks fine at first. Call as soon as you know or suspect your pet ate something. That early phone call is when treatment is simplest and most effective.
This article is for informational purposes only and does not replace guidance from your veterinarian. If you think your pet has been poisoned, call your vet or an animal poison control line immediately.
Sources
- Merck Veterinary Manual — Overview of Rodenticide Poisoning; Bromethalin; Anticoagulant rodenticides; Cholecalciferol (reviewed November 2025)
- Merck Veterinary Manual — Ethylene Glycol Toxicosis; Mushrooms Toxic to Animals; Oak Toxicosis (June 2026); Tremorgenic Neuromycotoxicosis in Dogs
- Pet Poison Helpline — Autumn crocus; Chrysanthemum; Sago palm; Grapes
- Companion Animal Parasite Council — The Case for Year-Round Flea and Tick Control
- ASPCA — Top 10 Toxins of 2025; Animal Poison Control
