Flea, tick and heartworm calendar by region

Tick season is a life-stage story — and on the West Coast it runs backwards

Almost every piece of national tick advice collapses three things into one word. Different tick species have different seasons, each species has three life stages with different seasons, and only one of those stages does most of the damage. Three facts do most of the work:

• Nymphs cause most Lyme disease, not adults. They quest from April through July and they are the size of a poppy seed, so they are missed. In most cases a tick has to be attached more than 24 hours to transmit, which is why a daily hands-on check earns its place next to any product.
• Winter is not an off-season. CDC has blacklegged tick adults questing any time winter temperatures rise above freezing, and the brown dog tick lives indoors and never stops at all.
• The West Coast is inverted. Adult western blacklegged ticks quest from late November through April or May and are most active in winter on the north coast and in the Sierra foothills. A Californian who stops treating in November stops at the beginning of the season.

What a ZIP code can tell you, and what it can't

A ZIP code box invites you to believe there is a per-ZIP parasite dataset behind it. There isn't, for anyone. A ZIP code is a mail delivery route; it was never drawn around a habitat. The evidence underneath this tool is regional in the first place: CAPC's prevalence maps are county-level counts of positive test results reported by clinics and laboratories — a record of what was tested and found, rather than of what is out there — and CDC's tick surveillance is county-level presence or absence, with the explicit warning that a county not marked as established does not mean ticks are absent.

So this tool maps ZIP codes to twelve regions, names the places in each one so you can catch a bad match yourself, and labels the rows that rest on inference rather than US case data — the flea months, for instance, which come from the species' published biology because no month-by-month national flea dataset exists. Military and territorial mail routes return nothing at all rather than a confident wrong answer. Your vet sees what actually walks through the door in your area. If their advice differs from this page, follow theirs.

Heartworm is the argument that's already over

In some countries seasonal heartworm dosing is a live disagreement between reputable sources. In the United States it is not. The American Heartworm Society's recommendation is the same in every state and every month: “Think 12” — test every 12 months, give a preventive 12 months a year. Heartworm has been diagnosed in all 50 states.

The interesting part is why, because the transmission season genuinely is shorter in the north. A heartworm larva needs about 130 heartworm development units above a 14 °C threshold to become infective, and in Montana or Maine that takes a summer. But sheltered yards and urban heat islands run 8 to 17 °F above the modeled weather-station temperature those calculations use, mosquitoes overwinter, development resumes the moment it warms, and only about three-quarters of US dogs are on a preventive at all — which keeps an untreated reservoir going in every region. The AHS position is that the risk may never reach zero and, where it does, it cannot be predicted accurately enough to dose against.

That is why the calendar above shows the mosquito row going quiet in winter across most of the country while the heartworm row stays lit every month. They are answering different questions, and the gap between them is the point.

Common questions

When is tick season in the United States?

It depends which tick, which life stage and which coast. In the East and Midwest, blacklegged tick nymphs quest from April through July and cause most Lyme disease cases because they are the size of a poppy seed; adults are out in the fall and again in early spring, and the CDC notes they will quest any time winter temperatures rise above freezing. Lone star ticks run from about April to August. On the West Coast the pattern is inverted: adult western blacklegged ticks quest from late November through April or May and are most active in winter on the north coast and in the Sierra foothills. There is no single national tick season, which is why this tool asks where you are.

Do I need tick and flea prevention in winter?

In most of the country, yes. Blacklegged tick adults are active on any winter day above freezing, the brown dog tick breeds indoors and has no season at all, and on the West Coast winter is the peak of adult tick activity rather than the gap in it. Fleas are worse: the eggs, larvae and pupae live in the house rather than on the animal, and a heated house keeps them going. Seasonal treatment fails at the shoulders — the weeks either side of the peak, when owners have stopped or not yet started.

Does my dog need heartworm prevention all year, even up north?

The American Heartworm Society recommends it everywhere: "Think 12" — test every 12 months, give a preventive 12 months a year. Heartworm has been diagnosed in all 50 states. The reasoning is not that mosquitoes bite in January in Minnesota; it is that transmission risk may never reach zero and, where it does, it cannot be predicted reliably. Weather-station temperatures under-read sheltered yards and urban heat islands by 8 to 17 °F, larval development in the mosquito resumes as soon as it warms, and an uninterrupted monthly habit is what keeps a dog covered. Alaska is the one place where the local risk is genuinely close to zero, and even there the question is one for a vet who knows the state.

Why does the calendar show mosquitoes stopping but heartworm dosing carrying on?

Because they are answering different questions, and this is the most common mistake in seasonal parasite advice. The mosquito row shows when transmission is possible where you live — a heartworm larva needs roughly 130 heartworm development units above a 14 °C threshold to become infective. The dose row shows when to treat, and the recommendation there is year-round for the reasons above. Reading a quiet January on the risk row as permission to skip a January dose is exactly the wrong conclusion.

Which ticks are in my area?

The calendar names them by species, because in the US the species is the answer: the blacklegged tick carries Lyme disease, anaplasmosis and babesiosis; the lone star tick carries ehrlichiosis and is behind most US alpha-gal syndrome; the American dog tick and the Rocky Mountain wood tick carry Rocky Mountain spotted fever; the brown dog tick carries it in the Southwest and is the one tick that breeds indoors. What the tool cannot do is tell you about your street. CDC surveillance is county-level, and it says plainly that a county not marked as established does not mean ticks are absent.

How often should I deworm my dog?

CAPC recommends year-round broad-spectrum parasite control covering heartworm, intestinal parasites, fleas and ticks together, which is why most US dogs get their intestinal cover inside the same monthly product as the heartworm dose. Where that is not what a household is doing, the floor is four broad-spectrum treatments a year, plus a fecal examination at least twice a year for an adult animal. This tool moves you to monthly where the risk is real — high hookworm and roundworm regions, a young child in the house, a hunting or scavenging animal, or regular boarding and dog parks — and otherwise shows the quarterly minimum.

Do indoor cats need flea, tick and heartworm prevention?

Cats need heartworm prevention in particular, because there is no approved treatment for heartworm infection in cats — the drug used in dogs is not safe for them. Even immature worms cause heartworm associated respiratory disease, and one or two worms can make a cat very ill. Infected mosquitoes come indoors, so an indoor cat is not exempt. One hard rule applies to every product: FDA’s guidance is that if a product is for use on dogs, do not use it on cats.

What is alpha-gal syndrome, and should I worry about it?

It is an allergy to a sugar found in most mammals, which can develop after a tick bite and can make red meat dangerous to eat, sometimes for years. In the United States it is most often associated with the lone star tick, and most reported cases are in Southern, Eastern and Central states. It is a reason tick prevention on a dog matters to the whole household: a dog that brings ticks inside brings them to people.

Do fleas die off over winter?

No. Development slows from about three weeks in summer heat to five to seven weeks in cool conditions, and the eggs, larvae and pupae are in the carpet, the bedding and the yard rather than on the animal. Heated houses do the rest, which is why an infestation reappears in February in a house nobody treated. The exception is the desert Southwest and the high plains, where the air is dry enough to be genuinely hard on the larvae — and irrigated yards and crawl spaces are enough to get around even that.

Do I need an account to use this?

No. There is no sign-up and no email capture, and nothing you type is sent anywhere — the calendar is worked out in your own browser. You can print it, or download it as repeating calendar reminders that work in Apple Calendar, Google Calendar and Outlook.