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Lyme Disease



Content Reviewers:

Yifan Xiao, MD

Lyme disease, or Lyme borreliosis, is an infectious, blood-borne bacterial disease that is transmitted by ticks.

It’s caused by Borrelia burgdorferi species.

Now, the genus Borrelia contains several species.

Lyme disease in people is caused primarily by Borrelia burgdorferi in North America and by B. afzelii, B. garinii, and B. burgdorferi in Europe and Asia.

In domestic animals, only B burgdorferi is confirmed to cause Lyme disease..

Borrelia are spirochetes, which means spiral-shaped bacteria.

They have outer surface proteins, abbreviated as Osp, which play a role in virulence; and sets of flagella that run between the cell wall and outer membrane, which they use to spin or twist to move in a wave-like motion.

Hard-shelled, Ixodes ticks, or deer ticks, are the vector for B. Burgdorferi, meaning they are the intermediate organism that spreads the bacteria.

In the northeast and Midwest USA, I. scapularis, the black-legged deer tick is the main vector; while on the Pacific coast, it’s I. pacificus, the western black-legged tick.

In Europe and Asia I ricinus and I. persulcatus are the primary vectors.

Ticks like environments with moderate humidity and temperature so they’re often found in wooded areas, thick brush, marshes, and tall grass.

The ticks are small, and even adults are only about 3 mm long, so they can be hard to notice.

Now Ixodes ticks feed on the blood from hosts throughout their life stages of larva, nymph, and adult.

When they hatch as larvae, they are uninfected.

When they feed on infected hosts as larvae or nymphs, they can pick up the B. burgdorferi bacteria.

In the younger stages of their life, they often feed on smaller animals like rodents, birds and even lizards.

When they grow into adults, they move on to larger mammals like dogs, cats, or horses.

A tick infected with B. Burgdorferi can transmit the bacteria to humans and animals through their saliva during feeding.

In the first few hours after attachment, the bacteria in the tick’s midgut switch their outer surface protein from OspA to OspC, which helps with transmission and provides protection against the host’s immune system.

After 24 hours of attachment, the bacteria passes from the ticks digestive system to the host’s skin, and then into the blood vessels that the tick is feeding on.

So, the longer a tick is attached, the more likely transmission will occur.

After the bacteria get into the skin, they cause a local infection that activates local immune cells, leading to an inflammatory response.

After several days, the bacteria can disseminate through the bloodstream to distant tissues, like the heart, kidneys, and joints.