Relapsing fever is a bacterial infection that causes recurring episodes of high fever separated by periods where you feel partially or completely well. It’s caused by Borrelia bacteria transmitted through tick or louse bites, and its hallmark pattern of fever that disappears and returns is driven by the bacteria’s ability to change its surface disguise faster than your immune system can keep up.
How the Fever Cycle Works
The first episode of fever typically lasts about 3 days for the tick-borne form and around 5.5 days for the louse-borne form. It begins abruptly with chills, muscle and joint pain, headache, and malaise. Then the fever breaks, sometimes dramatically, and you enter an afebrile period where you may feel fine or still somewhat run down. About 7 days later (9 days for the louse-borne version), the fever returns.
This cycle can repeat multiple times. Outbreaks studied in the western United States have shown patients averaging between 1.5 and 3 relapses, though more are possible without treatment. Each relapse tends to be shorter and less severe than the one before, but the unpredictability of the pattern is what makes the illness so distinctive and disorienting for the people experiencing it.
Why the Fever Keeps Coming Back
The relapses aren’t random. They’re caused by a sophisticated survival trick the Borrelia bacteria use to stay one step ahead of your immune system. The bacteria coat themselves in a surface protein that your immune cells learn to recognize and attack. Once your antibodies clear the bacteria expressing that protein, the fever breaks and you feel better.
But a small subset of bacteria have already switched to producing a completely different surface protein, one your existing antibodies can’t recognize. These survivors multiply rapidly in your bloodstream, causing the next wave of fever. Your immune system then builds new antibodies against this second disguise, clears those bacteria, and the fever breaks again. The cycle continues because the bacteria carry dozens of archived “disguise” genes and can swap them into their active expression site through a process called gene conversion, essentially copying a new gene into the slot where the old one sat. Each new variant buys the bacteria enough time to multiply before the immune system catches up.
Tick-Borne vs. Louse-Borne Forms
There are two main forms of relapsing fever, distinguished by how the bacteria reach you.
Tick-borne relapsing fever (TBRF) is spread by soft-bodied ticks that live in rodent nests inside cabin walls, attics, and crawl spaces. In the United States, most cases involve ticks that emerge at night, feed for less than 30 minutes while a person sleeps, and drop off before anyone notices. The bite is painless and brief enough that most people never realize they were bitten. Exposure is most common during summer stays in rustic, rodent-infested mountain cabins in western states, though it can also happen in winter when fires warm the cabin and activate dormant ticks hiding in the woodwork. A smaller number of cases occur in the south-central U.S. and are linked to cave exploration, where a different tick species transmits a different Borrelia strain.
Louse-borne relapsing fever (LBRF) is transmitted by body lice and occurs in settings of overcrowding and poor sanitation. It has historically caused large outbreaks in refugee camps and during wartime. Today it’s largely confined to parts of East Africa, particularly Ethiopia. LBRF tends to cause longer initial febrile episodes and carries a higher risk of serious complications. Tick-borne relapsing fever, by contrast, is endemic in parts of Africa, Asia, Europe, and the Americas.
Symptoms Beyond the Fever
The fever itself is hard to miss, often spiking high and arriving with shaking chills and rigors. But relapsing fever affects more than your temperature. Common symptoms during febrile episodes include severe headache, muscle pain, joint pain, nausea, vomiting, rapid heart rate, and flushing. Some people develop abdominal pain or a cough. Between episodes, you might feel completely normal or have lingering fatigue and general unwellness.
The transition from a febrile episode to recovery can itself be dramatic. The fever “crisis” involves a rapid drop in temperature sometimes accompanied by heavy sweating and a temporary drop in blood pressure. This is the body’s inflammatory response resolving as antibodies clear the current wave of bacteria from the blood.
How It’s Diagnosed
Relapsing fever is diagnosed by finding the corkscrew-shaped Borrelia bacteria on a blood smear examined under a microscope. The key is timing: the bacteria are abundant in the blood during febrile episodes but largely absent between them. A blood sample drawn while you have a fever is far more likely to show the organisms than one drawn during an afebrile interval. This means the diagnosis can be missed if blood is only drawn when you’re feeling better between episodes.
Because the tick bites are painless and happen at night, many patients don’t report a tick bite, which can delay diagnosis. If you develop recurring high fevers after staying in a mountain cabin, camping in a rural area, or exploring caves, mentioning that exposure history to your doctor significantly improves the chances of a correct diagnosis.
Treatment and the Herxheimer Reaction
Relapsing fever responds well to antibiotics. Treatment typically clears the infection and prevents further relapses. However, there’s an important complication that occurs during treatment itself: the Jarisch-Herxheimer reaction. This is a sudden, intense inflammatory response triggered by the rapid die-off of bacteria in the bloodstream after the first dose of antibiotics.
Symptoms of this reaction include high fever, shaking chills, rapid breathing, flushing, muscle pain, and a drop in blood pressure. It typically begins within hours of starting antibiotics. In louse-borne relapsing fever, more than half of treated patients experience this reaction, and in rare cases it can be life-threatening. For this reason, patients are monitored closely for at least four hours after their first antibiotic dose. The reaction is temporary, usually resolving within a day, and does not mean the treatment isn’t working. In fact, it’s a sign the antibiotics are doing their job, just too effectively for the body’s inflammatory system to handle gracefully.
Preventing Exposure
Because most U.S. cases are linked to rodent-infested cabins, prevention centers on keeping rodents and their ticks out of sleeping areas. Practical steps include sealing gaps around plumbing, windows, doors, foundations, and floor vents. Small openings can be plugged with steel wool, while larger ones may need hardware cloth or metal sheeting. Eliminating food sources matters too: store food in sealed containers, keep bird feeders and compost bins away from the structure, collect uneaten pet food, and avoid stacking firewood directly against the building.
If rodent nests are found and removed, the ticks don’t necessarily leave with them. Soft ticks can survive for long periods without a meal and will seek new hosts, including humans. Professional pest control operators can treat infested structures with targeted “crack and crevice” applications similar to those used for cockroaches and other wall-dwelling pests. When sleeping in a cabin that may harbor ticks, inspect walls and ceilings around the bed for cracks, move the bed away from walls, and keep bedding from touching the floor. These simple adjustments reduce the chance of a nighttime tick encounter considerably.