How Lyme Disease Spreads — and How It Doesn’t

In the United States, the bacteria that cause Lyme disease — Borrelia burgdorferi and, rarely, Borrelia mayonii — reach people through the bites of infected blacklegged ticks.

Infected ticks are most common in forested areas of those regions. The share of ticks carrying the bacteria ranges from none to more than 50%, depending on the area and the tick's life stage; blacklegged ticks in most of the Southeast are almost never infected.

The life stages of the blacklegged tick, with a dime for scale. Image from CDC's page on how Lyme disease spreads.

The 24-hour window

In most cases, a tick must be attached for more than 24 hours to pass on the bacteria. Removing a tick within 24 hours greatly reduces the chance of infection — so avoiding tick bites and removing ticks promptly prevent Lyme disease.

A tick's life

Blacklegged ticks live 2 to 3 years and pass through four stages: egg, larva, nymph and adult. Larvae and nymphs each need a blood meal to develop, and adult females need blood to make eggs.

• Larvae and nymphs pick up the bacteria by feeding on an infected animal, usually a rodent, and keep it as they develop. Nymphs and adult females can then spread it at their next meal.
• Infected females don't pass the bacteria to their eggs.
• Deer feed ticks and help them spread to new areas, but deer aren't infected and don't infect ticks.

The blacklegged tick's life cycle. Image from CDC's page on how Lyme disease spreads.

When the risk is highest

Where Lyme disease is common in the East, people can be bitten by infected ticks from spring through fall. Nymphs seek hosts from April through July; adults are most active in early spring and fall.

Nymphs are the biggest danger: they are plentiful and about the size of a poppy seed, so they're easy to miss — many patients never notice a bite. Adult females, about the size of a sesame seed, can also spread the bacteria but are more likely to be seen and removed in time.

How Lyme disease does not spread

• Touching, kissing or sex: no credible scientific evidence; animal studies don't support sexual transmission, and the bacterium's biology doesn't fit it.
• Pregnancy: untreated Lyme disease can infect the placenta, and passing it to the fetus is possible but extremely rare. With appropriate antibiotics, there's no added risk of adverse birth outcomes. No studies have assessed the development of children whose mothers had Lyme disease in pregnancy.
• Breast milk: no reported cases. If you're breastfeeding and diagnosed, tell your doctor so they can choose a safe antibiotic.
• Blood transfusion: no cases linked, but the bacteria can survive in stored blood. Don't donate while being treated; after finishing treatment you may be eligible — see the Red Cross criteria.
• Pets: dogs and cats can get Lyme disease but don't spread it to their owners directly. They can, however, carry infected ticks into your home or yard; tick prevention products for pets help.
• Game meat: eating it won't give you Lyme disease — though always cook meat thoroughly, and hunting and dressing animals can bring you close to infected ticks.
• Air, food, water, mosquitoes, flies, fleas or lice: no credible evidence.
• Other ticks: lone star, American dog, Rocky Mountain wood and brown dog ticks can't transmit Lyme disease bacteria, though they spread other tick-borne diseases.

Life stages of blacklegged, lone star and American dog ticks. Image from CDC's page on how Lyme disease spreads.

Sources

Based on "How Lyme Disease Spreads," Centers for Disease Control and Prevention; a work of the United States government in the public domain.