Lyme disease is a zoonotic infection, meaning it spreads from animals to humans. It is, in fact, the most common tick-borne zoonotic disease in the United States, with an estimated 476,000 people diagnosed and treated for it each year. The bacteria that cause Lyme disease live in wild animal populations and reach humans through an intermediary: infected blacklegged ticks.
What Makes Lyme Disease Zoonotic
A zoonotic disease is any infection that originates in animals and can be transmitted to people. Lyme disease fits this definition because the bacteria responsible, primarily Borrelia burgdorferi in the U.S., maintain themselves in a natural cycle between ticks and wild animal hosts. Humans aren’t part of that cycle. We’re accidental hosts who get infected when a tick that previously fed on an infected animal bites us.
This is an important distinction. The bacteria don’t spread from person to person. You can’t catch Lyme disease from a handshake, a sneeze, or sharing food. The only route to human infection is through the bite of an infected tick, which picked up the bacteria from a wild animal reservoir.
The Animal Reservoirs Behind Lyme Disease
Several wild animal species serve as reservoirs, meaning they carry the Lyme bacteria in their bloodstream without becoming seriously ill and pass those bacteria to ticks that feed on them. In eastern North America, the white-footed mouse is the most important reservoir. These mice are highly efficient at infecting the ticks that bite them, keeping the bacteria circulating in the environment.
Mice aren’t the only reservoirs. Rats, voles, dormice, chipmunks, and various other small rodents are all competent hosts. Birds play a role too. American robins are considered a significant avian reservoir because they forage on the ground where ticks live and are frequently infested by them. European blackbirds carry the bacteria as well. Seabirds even maintain their own cycle of infection on Arctic islands. In Europe and Asia, additional Borrelia species (B. afzelii and B. garinii) circulate through similar rodent and bird populations.
Deer often get mentioned in connection with Lyme disease, but their role is different. Deer don’t efficiently transmit the bacteria to ticks. Instead, they’re important because they’re a major food source for adult ticks, supporting the tick population itself. More deer in an area generally means more ticks, which means more opportunities for transmission.
How the Bacteria Reach Humans
The transmission chain works like this: a larval tick feeds on an infected mouse or bird, picks up the bacteria, and carries them into its next life stage. As a nymph or adult, that tick attaches to a new host, which could be a person. Two tick species are responsible in the U.S. The blacklegged tick (also called the deer tick) transmits Lyme disease across the northeastern, mid-Atlantic, and north-central states. The western blacklegged tick handles transmission along the Pacific Coast.
These ticks are small. Nymphs, which cause most human infections, are roughly the size of a poppy seed and easy to miss on skin. They attach to a host and feed for several days. Transmission isn’t instant; the bacteria need time to migrate from the tick’s gut to its salivary glands before entering the host. This is why finding and removing ticks promptly reduces your risk significantly.
Can Pets Transmit Lyme to Humans?
Dogs and cats can get Lyme disease, but they don’t pass the infection directly to you. There’s no evidence that touching, grooming, or being licked by an infected pet can transmit the bacteria. The concern with pets is indirect: dogs and outdoor cats can carry infected ticks into your home and yard, where those ticks may then bite you. A tick that drops off your dog onto a couch cushion or bed can find its way to a human host.
This makes pets a practical risk factor even though they’re not a direct source of infection. Checking your animals for ticks after they’ve been outdoors, and using veterinarian-recommended tick prevention products, reduces the chance of ticks hitching a ride inside.
Recognizing Lyme Disease After a Tick Bite
The most recognizable sign of Lyme disease is the erythema migrans rash, which appears in over 70% of infected people. It typically shows up 3 to 30 days after a bite and often (though not always) expands into a bull’s-eye pattern. The rash is usually not painful or itchy, which means it can go unnoticed if it’s on your back or scalp.
Early symptoms also include fever, fatigue, headache, and muscle or joint aches. If untreated, the infection can spread to the joints, heart, and nervous system over weeks to months, causing more serious problems like joint swelling, facial paralysis, or heart rhythm irregularities. Early treatment with antibiotics is highly effective, which is why recognizing symptoms matters.
Where the Risk Is Growing
Since 1995, the geographic range of Lyme disease in the U.S. has expanded significantly. Areas that were once considered low-risk, particularly in the upper Midwest and parts of the mid-Atlantic, now report consistent cases. Over 89,000 cases were reported to the CDC in 2023, though the true number of people diagnosed and treated is likely several times higher. The expansion is linked to changes in tick habitat driven by warming temperatures, suburban development pushing into wooded areas, and growing deer populations.
Reducing Your Exposure
Because Lyme disease depends entirely on tick bites to reach humans, prevention comes down to avoiding those bites. The CDC recommends EPA-registered repellents containing DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol, or 2-undecanone. Treating clothing and gear with permethrin adds another layer of protection.
After spending time in grassy, brushy, or wooded areas, do a full-body tick check. Pay attention to hidden spots: behind the ears, along the hairline, under arms, around the waist, and behind the knees. Showering within two hours of coming indoors helps wash off unattached ticks. If you find one attached, remove it with fine-tipped tweezers by pulling straight up with steady pressure, getting as close to the skin as possible.