Lyme disease is an infectious disease that's found around the world. In the US it's most commonly caused by the bacterium Borrelia burgdorferi. And cousins of this bacteria, Borrelia garinii and Borrelia afzelii, are causes of Lyme disease in Europe and Asia. And new ones are still being found. In fact, in 2015, researchers from Mayo Clinic discovered a new bacterial strain causing Lyme disease and decided to name it Borrelia mayonii.

So these Borrelia bacteria are all spirochetes, which means that unlike other bacteria, they're long, thin, and spiral-shaped and they spin or twist to move around.

Lyme disease is classified as a zoonosis, meaning it's spread to humans from animals that exist as natural reservoirs, reservoir meaning that the bacteria typically doesn't cause serious disease to the animal. And Borrelia species can infect a wide range of animals, including small animals like mice, lizards, and birds.

The bacterium isn't known to spread directly from animals to humans, but it's said it needs a vector, meaning some sort of intermediate organism to spread from the animal to the human.

Depending on the region, there are different vectors. For example, in the northeastern US it's spread via Ixodes scapularis, commonly known as the black-legged deer tick. But other types of Ixodes ticks transmit Lyme Borrelia in western North America and Eurasia, like the sheep tick, the taiga tick, and the western black-legged tick.

Now, ticks need blood to survive. And I don't mean they have blood already. They need blood from a host or a blood meal, just like mosquitoes, leeches, vampires, and other not-so-pleasant creatures. Literally being blood suckers, it makes them obvious candidates for spreading bloodborne diseases, right? And ticks need to go through life stages: larva, nymph, and adult. Not too different from us: children, adolescents, and adults.

The larval tick will most likely get the bacteria while feeding on something like a mouse, since deer ticks in the larval stage prefer smaller animals. After larval deer ticks molt in the nymphal stage, they feed on a broader range of animals, meaning that they can spread the bacteria they picked up to new reservoirs. After molting again to the adult phase, they focus mostly on deer, which actually aren't the best host for Borrelia. So, since the larval and nymphal deer ticks spread Borrelia to more suitable hosts, these two stages are very important for Borrelia.

Now, both nymphal- and adult-stage deer ticks, although not their preferred meal, occasionally feed on the unknowing human. If you've ever had a tick on you, you'd probably agree that the first thing you'd want to do is get it off. The tough part about deer ticks, especially nymphal-stage deer ticks, is that they're really small, and therefore sometimes hard to notice. This means that they can be allowed to feed for long periods of time, thus allowing the tick to transmit the disease. The Borrelia bacteria are transmitted within the saliva of the tick as it’s feeding, and it typically takes about 36 to 48 hours of attachment time for the bacterium to move from the tick’s gut into the saliva and into the human.

If Borrelia infects a human, it causes diseases in three stages. The early localized stage of Lyme disease is usually days to weeks after the initial infection. As the bacteria spreads from this initial point, redness and inflammation expands as well. Sometimes the space between the initial bite and the outer radius of the rash is cleared of bacteria, leading to a bullseye-shaped rash, also known as erythema migrans, which is the classic sign of early Lyme disease. In addition, there might be some vague flu-like symptoms at this stage as well.

Next is the early dissemination stage, which is usually weeks to months later, when the bacteria starts to spread through the bloodstream, or disseminate, to places like the heart, the brain, and the joints. In general, relatively few bacteria actually invade these various tissues, but the immune reaction is usually pretty severe, and turns these tissues into a war zone, essentially killing the bacteria but damaging the tissue in the process.

Other instances of erythema migrans might start to pop up in places that don't relate to the original bite. And this is where Lyme disease can become really serious. When the bacteria infects the heart tissue, it's known as carditis. Although heart tissue inflammation can affect all sorts of heart functions, it often clinically presents as AV heart block, meaning the electrical signal moving from the upper chambers to the lower chambers is held up, or blocked, which changes the timing of the heartbeat.

Lyme disease is also known to cause swelling around the nerves that control the facial muscles, essentially pinching the nerve and causing facial nerve palsy, where facial muscles become weak or even paralyzed, which can make it hard to smile or close the eye. With Lyme disease, this condition is often bilateral, meaning it happens on both sides of the face.

If the disease spreads to the joints, it can cause arthritis in the knee, the wrist, and the ankles. Lyme disease can also spread to the meninges, which is the lining of the brain, which causes meningitis and sometimes terrible neck stiffness and headaches.

Not surprisingly, inflammation in various parts of the body can also cause fevers, fatigue, and other flu-like symptoms, just like we saw in the early localized stage.

If the disease is allowed to progress further to the late dissemination stage, which can generally be up to a year after infection, the major symptom is chronic arthritis of either one joint or a few joints, commonly involving the knee.

Now the diagnosis of Lyme disease is usually made by looking at antibodies against Borrelia proteins. The tricky thing is that sometimes antibodies cross-react. In other words, an antibody that's formed against a normal bacteria that lives in the gut might suddenly cross-react with a Borrelia protein because it looks similar.

To help tease apart who really has Borrelia and who doesn't, there are criteria about how many different types of antibodies a person needs to have in order to make a diagnosis more specific and weed out what can be considered false-positives.

To treat Lyme disease, antibiotics are the first-line choice of treatment, and if given early on in the disease, are very effective at wiping out the bacteria, although specific antibacterial medication given often depends on things like the stage of the disease and the age of the patient.

It's also important to monitor patients being treated with antibiotics because they can develop Jarisch-Herxheimer reaction, which is where they develop fever, sweating, and muscle pains in response to spirochetes breaking open and releasing a lot of antigens all at once, which is just super immunogenic.

Ultimately, once patients are treated for Lyme disease, they do really well. There can be a few weeks or months of ongoing symptoms of fatigue and muscle or joint pains, which is normal, but this resolves and is not really related to the ongoing persistence of the Borrelia bacteria in the body. In fact, there's actually no evidence that the bacteria lives on chronically in the body after treatment, and there's no reason to use prolonged courses of antibiotics or other medications to treat Lyme disease.

In terms of prevention, people who work or explore wooded areas or areas with brushes, leaf litter, and tall grass are most at risk of picking up a deer tick and being infected with Lyme disease. That said, taking preventative measures are important for everyone, like wearing hats and long-sleeve shirts, as well as using bug spray in areas where ticks are known to spread Lyme disease.