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San Francisco’s mild weather lets fleas breed almost all year, and numbers often climb in the warm weeks of fall. Indoor cats are not protected: fleas arrive on people, on other pets, and through shared hallways and old carpets. Over-grooming, small scabs and thinning fur are more common clues than the fleas themselves.
Why fall is flea season in San Francisco
Fleas like warmth and moderate humidity. In much of the country, a cold winter knocks them back. San Francisco rarely gets cold enough for that, and September and October are often the warmest weeks of the year in the Mission. That combination means flea populations that built up over summer are often at their largest just as people assume the season is over.
Once heating goes on later in the year, apartments stay comfortable for fleas indoors. So a problem that starts in fall can carry right through to spring if nobody spots it.
Five ways fleas reach a cat who never goes outside
1. On you
Adult fleas can jump onto shoes and pant legs from a friend’s dog or a neighbor’s pet. They do not live on people, but a ride home is all they need.
2. The dog in the household
A dog who walks around the neighborhood every day brings fleas home, and they can easily move on to the cat. If there is a dog in the apartment, the cat is exposed.
3. Shared buildings
Stairwells, hallways and shared laundry rooms connect every unit in a building. A neighbor’s pet with fleas can seed eggs in shared carpet that end up in your apartment.
4. The last tenant
This one surprises people. Flea pupae sit in carpets and floor cracks, protected in a cocoon, and can wait for months. When new tenants move in, the warmth, movement and vibration trigger them to hatch. Itchy bites on your ankles in the first weeks after moving into an apartment where a pet used to live are a classic sign.
5. Wildlife around the building
Rats, opossums and raccoons living under buildings, in crawl spaces and in backyards carry the same fleas that bite cats. Rodents that find their way indoors can bring those fleas to a cat who never sets foot outside.
What fleas look like on a cat
Usually, you do not see fleas at all. Cats groom constantly and remove most adult fleas before you ever notice one. What you see instead is what the fleas leave behind.
- Over-grooming, especially the belly, inner thighs and base of the tail, sometimes to the point of thin or bald patches
- Tiny crusty bumps you can feel under the fur on the back and neck
- Flea dirt, which looks like black pepper in the coat. Comb it onto a damp paper towel: flea dirt dissolves into reddish-brown streaks, because it is digested blood.
- Small, flat, cream-colored segments near the tail or in bedding, which look like sesame seeds when dry. These are tapeworm segments, and cats pick up tapeworm by swallowing fleas while grooming.
- Bites on people, often around the ankles
Some cats are allergic to flea saliva. For them, a few bites can cause intense itching, scabs and hair loss, even when you cannot find a single flea. In kittens and frail older cats, a heavy flea burden can cause anemia, with pale gums, weakness and low energy. That needs to be seen right away, and our pet diagnostics team can measure it with a simple blood sample.
Clearing fleas from an apartment
The fleas you see on a cat are only a small part of the problem. Most of the population lives in the home as eggs, larvae and pupae. Clearing them means working on the pets and the space together.
- Protect every pet in the home at the same time, with a product we recommend for each one, and keep going for at least three months. Stopping early lets the next wave of pupae restart the cycle.
- Never use a dog flea product on a cat. Some products made for dogs can cause tremors and seizures in cats, even when a cat simply grooms or cuddles up against a dog who was recently protected. If you have a dog and a cat, ask us for a dog product that is safe to use in a home with cats.
- Vacuum every day for the first few weeks, including under the bed and sofa, along baseboards, and inside cat carriers. Seal the contents in a bag and take it straight out of the apartment.
- Wash bedding, blankets and washable cat beds in hot water every week until the problem has cleared.
- Talk to your building. If you share hallways, let your landlord or neighbors know. Fleas do not stop at your front door.
Expect to see an occasional flea for several weeks, because pupae keep hatching. That is usually the life cycle rather than a sign the plan is failing, but phone us if you are still seeing fleas after the first month.
If your cat trembles, twitches or has a seizure after contact with a flea product, or a kitten is weak with pale gums, call us right away. When we are closed, the nearest open emergency hospital is the right place to go.
Our pet care services team can recommend the right year-round flea protection for your cat and any other pets at home. If your cat has sore skin, open scabs or hair loss, our medical services team will check the skin and look for other causes of itching too.
Indoor life keeps cats safe from a lot of city hazards. For keeping it interesting in a small apartment, see our guide to indoor cat enrichment in the Mission.
Frequently asked questions
She is strictly indoors. Does she still need flea protection?
In San Francisco, usually yes. Fleas reach indoor cats through people, other pets, shared buildings and wildlife, and the mild climate means there is no real off-season. We can help you decide what makes sense for your household.
I am getting bitten, but I cannot find fleas on my cat. What is going on?
That is common. Cats groom most fleas off, and newly hatched fleas in carpets will bite whoever walks past. Check your cat for flea dirt and over-grooming, and look at where your cat sleeps.
Can I use my dog’s flea product on my cat if I use a smaller amount?
No. Some dog products are dangerous to cats in any amount. Always use a product made for cats, and ask us which one fits your cat.
How long does it take to clear fleas completely?
Plan on at least three months of consistent protection for every pet, plus regular vacuuming, and longer if you keep seeing fleas. Pupae hatch in waves, so protection needs to be in place as each wave emerges.
Are fleas dangerous, or just annoying?
They are more than a nuisance. They can trigger allergy, carry tapeworm, spread the bacteria behind cat scratch fever, and cause anemia in young, small or frail cats. A kitten with fleas and pale gums should be seen right away.
We just moved in and the previous tenant had a dog. What should I do?
Ask us to start flea protection for every pet right away, vacuum thoroughly for the first few weeks, and watch for ankle bites. Catching it early is much easier than clearing an established problem.
Speak to the team at SF Vet Hospital
If your cat is over-grooming, scratching or losing fur, phone us and we will help you sort out the cause and a plan. For trembling, seizures or a weak kitten, phone right away, or go to the nearest open emergency hospital if we are closed.
Published by the team at SF Vet Hospital.
This article is for general educational purposes only and is not a substitute for an in-person veterinary examination or advice about your individual pet. If you have an urgent concern about your pet, contact SF Vet Hospital or your nearest emergency veterinary hospital right away.
SF Vet Hospital
2001 Harrison Street, San Francisco, CA 94110
Phone: (415) 907-7576
Email: staff.sfvh@gmail.com
Hours: Monday to Saturday 8:30 AM to 7:00 PM; Sunday 9:00 AM to 7:00 PM