Lyme disease is infection caused by several species of Borrelia bacteria, and is typically
transmitted by ticks, in particular deer ticks.
Small mammals, such as mice and squirrels, are the primary reservoir for Borrelia.
They host the larval and nymphal forms of deer ticks and infect them with the bacteria.
Deer host adult ticks, but do not carry Borrelia.
Lyme disease is most prevalent in areas where deer ticks are most abundant.
Not every tick bite leads to Lyme disease.
To cause the disease, the tick must be infected with Borrelia, and the infected tick must
attach to the human body long enough, typically for at least 36 hours, for the bacteria to
enter the body.
The best way to prevent Lyme disease is to avoid tick bites, by staying away from wooded
areas, or covering up the skin and using insect repellents; and if bitten, to remove the tick
as soon as possible.
In the early stage of the disease, the bacteria spread from the site of the tick bite, forming
a characteristic bull's eye-shaped rash, called erythema migrans, that expands over the course
of several days, then fades after 3 to 4 weeks.
The rash is warm to touch but typically not itchy or painful.
However, some patients may have an uncharacteristic rash, or no rash at all.
Patients may also have flu-like symptoms and swollen lymph nodes at this stage.
Without treatment, the bacteria spread through the lymphatic system and bloodstream to other
organs and other skin sites, and the disease progresses to the early-disseminated stage,
stage 2, which typically starts after a month, and lasts for a couple of months.
Symptoms are due to inflammation triggered by the bacteria in the affected organs.
In the joints, inflammation causes joint pain, most commonly in the knees.
In the nervous system, inflamed nerves can result in numbness and muscle weakness.
Inflammation in the brain may lead to meningitis or cognitive defects, such as impaired memory.
Rarely, infection can cause irregular heartbeats.
Some patients may also have multiple ring-shaped secondary skin lesions.
If left untreated, the late stage, stage 3, begins months to years after the initial infection.
The most common feature is joint pain, or Lyme arthritis.
Patients may also present with aseptic meningitis, Bell’s palsy, nerve pain, or cognitive deficits.
Some patients may be misdiagnosed as having fibromyalgia.
Diagnosis is based on symptoms, especially the characteristic rash, history of a tick
bite, and lab tests for antibodies against the bacteria, which are positive a few weeks
after the bite.
However, diagnosis can be difficult as many patients do not recall having a tick bite
or a rash.
In most cases, Lyme disease can be treated successfully with 2 weeks of oral antibiotics.
Patients with neurological symptoms may require intravenous antibiotics and a longer treatment
course.
The earlier the treatment begins, the higher the chance of full recovery.
A small number of patients may have symptoms that last for months after they finish treatment.
This condition is called post-treatment Lyme disease syndrome, PTLDS, and is likely caused
by an autoimmune response triggered by the bacteria rather than the infection itself.
Additional antibiotic treatments are not useful, but patients with PTLDS usually recover slowly
over time.