Rabbit fever is an infectious disease caused by the bacterium Francisella tularensis, one of the most infectious pathogens known to science. As few as ten inhaled bacteria can trigger life-threatening illness in a person.1PubMed Central. Francisella tularensis enters macrophages via a novel process involving pseudopod loops The disease goes by its medical name, tularemia, and earned the nickname “rabbit fever” because hunters and outdoors workers historically caught it while handling infected rabbits. But rabbits are far from the only source, and the ways this disease reaches people are more varied than the folksy name suggests.
The Bacterium Behind It
Francisella tularensis is a small, hardy, gram-negative bacterium that can survive for weeks in water, soil, and decaying animal carcasses. What makes it so dangerous is its behavior once inside the body. The bacterium targets immune cells called macrophages, the very cells that are supposed to destroy invaders. It slips into them using an unusual engulfment process, then stalls the cell’s internal defenses for a few hours before escaping into the cell’s interior fluid, where it multiplies freely.2Trends in Microbiology. Francisella tularensis and the Host-Pathogen Interaction In effect, the bacterium hijacks the immune system’s front-line soldiers and turns them into safe houses for reproduction.
Not all strains are equally dangerous. The species is divided into subspecies that differ in where they’re found and how sick they make people. Type A (subspecies tularensis) is the most virulent and occurs mainly in North America. Type B (subspecies holarctica) is milder and widespread across both hemispheres. A third subspecies, mediasiatica, is confined mostly to Central Asia.3Taylor & Francis Online. Tularemia – a re-emerging disease with growing concern Within Type A, subgroup AIb causes the most severe disease and the highest fatality rates compared to other Type A subgroups or Type B infections.3Taylor & Francis Online. Tularemia – a re-emerging disease with growing concern This means the geographic region where someone is exposed can influence how serious the illness becomes, with central and eastern North America harboring the more aggressive strains.
How People Get Infected
Tularemia reaches humans through a surprisingly wide range of routes, and understanding them is key to avoiding the disease. The most common pathway is a tick or deer fly bite. When an infected tick feeds, it deposits bacteria directly into the skin, and even a brief attachment can be enough. Handling sick or dead animals, especially rabbits, hares, and rodents, is the second classic route. Bacteria enter through tiny cuts or abrasions in the skin, which is why hunters, trappers, and veterinary workers are at elevated risk.
Inhalation is a less common but far more dangerous route. Breathing in contaminated dust, for instance while mowing over a rabbit nest or handling contaminated hay, can deliver bacteria straight to the lungs.4PubMed Central. Tularemia, lawn mowers, and rabbits’ nests Drinking or swallowing contaminated water can lead to infection in the throat and gut. And touching your eyes with contaminated hands can cause an eye-specific form of the disease. There is no documented person-to-person spread, which is a small reassurance but also means every case traces back to an environmental or animal source.
This diversity of transmission routes is part of what makes tularemia tricky. A landscaper mowing a field, a child playing near a creek, a hunter skinning a rabbit, and a hiker bitten by a tick are all plausible patients. The disease doesn’t have one obvious risk profile the way, say, a foodborne illness does.
Symptoms and the Six Clinical Forms
How rabbit fever presents depends heavily on how the bacteria got in. Doctors recognize six clinical forms: ulceroglandular, glandular, oculoglandular, oropharyngeal, respiratory (pneumonic), and typhoidal.5PubMed Central. Tularemia: a re-emerging tick-borne infectious disease Across all forms, high fever is usually the first signal, often appearing abruptly three to five days after exposure. But the other symptoms diverge from there.
- Ulceroglandular: The most common form. A painful skin ulcer develops at the site where bacteria entered, typically where a tick bit or where skin contacted an infected animal. The nearest lymph nodes swell and become tender. There is usually a central dark scab (eschar) at the wound.6PubMed Central. A case of ulceroglandular tularemia presenting with lymphadenopathy and an ulcer on a linear morphoea lesion surrounded by erysipelas
- Glandular: Swollen, painful lymph nodes appear without any visible skin ulcer. The transmission route is the same as ulceroglandular, but the skin lesion at the entry point either heals quickly or never forms. Because there’s no obvious wound, doctors sometimes miss the connection to tularemia, and absent or mild fever can further delay diagnosis.6PubMed Central. A case of ulceroglandular tularemia presenting with lymphadenopathy and an ulcer on a linear morphoea lesion surrounded by erysipelas
- Oculoglandular: Infection enters through the eye, causing painful redness, swelling, and discharge, along with swollen lymph nodes near the ear or jaw. This often happens when someone rubs their eye after touching a contaminated surface.
- Oropharyngeal: Results from eating or drinking contaminated food or water. Sore throat, mouth ulcers, and swollen neck lymph nodes are typical.
- Pneumonic: The most dangerous form. Bacteria reach the lungs either by direct inhalation or by spreading through the bloodstream from another site. Symptoms include cough, chest pain, and difficulty breathing. Without treatment, pneumonic tularemia can progress rapidly and has the highest fatality rate of any form.
- Typhoidal: A systemic illness with high fever, exhaustion, and sometimes abdominal pain, but without obvious localized signs like a skin ulcer or swollen lymph nodes. This form is the hardest to diagnose because it looks like many other febrile illnesses.
The ulceroglandular and glandular forms together account for the majority of reported cases. The pneumonic and typhoidal forms are less frequent but carry the most risk.
Why Pneumonic Tularemia Is Especially Feared
When F. tularensis reaches the lungs through inhalation, the resulting pneumonic tularemia can be devastating. Animal studies give a stark picture of how quickly the disease progresses. In one experiment using the highly virulent SCHU S4 strain, rabbits exposed to aerosolized bacteria developed fever by the third day, followed by a steep drop in food and water intake. Blood work on day four showed a collapse in lymphocyte and platelet counts alongside rising markers of liver stress and inflammation. On average, the animals became moribund just over five days after exposure, and none survived at any inhaled dose.7PLOS ONE. Pneumonic Tularemia in Rabbits Resembles the Human Disease as Illustrated by Radiographic and Hematological Changes after Infection Researchers note that the disease course in rabbits closely mirrors what happens in humans, which is partly why the pneumonic form has drawn so much attention from both clinicians and biodefense planners.
In humans, early symptoms of pneumonic tularemia can look like a bad flu or community-acquired pneumonia: fever, dry cough, chest tightness, and fatigue. The initial resemblance to common respiratory infections is one reason cases get misdiagnosed. By the time a physician thinks to test for tularemia, the patient may already be seriously ill. The window between onset and effective treatment is narrow, and any delay worsens the prognosis.
Treatment With Antibiotics
The good news is that tularemia is treatable with antibiotics, and outcomes are generally good when treatment starts early. The traditional first-line drugs are aminoglycoside antibiotics like streptomycin and gentamicin, delivered by injection. For milder cases or situations where injection isn’t practical, oral antibiotics are also effective.
Research on primates exposed to aerosolized F. tularensis showed that ciprofloxacin, a fluoroquinolone antibiotic, achieved complete survival regardless of when treatment began after fever appeared. All ten animals treated with ciprofloxacin survived, and none had detectable bacteria in their tissues afterward. Doxycycline, a tetracycline-class antibiotic, also performed well: all ten animals survived when treated 48 hours after fever onset, and nine of ten survived when treated at the 24-hour mark. However, some doxycycline-treated animals still had residual live bacteria in their tissues after recovery, suggesting ciprofloxacin may do a more thorough job of clearing the infection.8PubMed Central. Efficacy of Doxycycline and Ciprofloxacin for Treatment of Pneumonic Tularemia in Cynomolgus Macaques By contrast, about 80% of untreated animals in the same study died, underlining how critical antibiotic therapy is.
Treatment courses typically last 10 to 21 days depending on the severity and form. For the ulceroglandular form, a shorter course often suffices. For pneumonic or typhoidal disease, longer treatment is standard. Relapses can occur if the antibiotic course is cut short, particularly with bacteriostatic drugs (those that stop bacteria from growing rather than killing them outright).
Recovery Can Take Longer Than Expected
Even when antibiotics resolve the acute infection, recovery from tularemia isn’t always quick. In one clinical report, two patients who responded well to antibiotic treatment and had no relapse still experienced lingering fatigue, muscle pain, and shortness of breath for two to three months after their illness.9PubMed Central. The clinical spectrum of tularemia—Two cases This kind of post-infectious fatigue isn’t unique to tularemia, but it catches many patients off guard. If you’ve had rabbit fever and still feel wiped out weeks later, that’s a recognized pattern, not a sign that treatment failed.
Swollen lymph nodes can also take weeks or even months to return to normal size, and in some cases they suppurate (fill with pus) and require drainage. This is especially common in the ulceroglandular and glandular forms. Patients sometimes worry that persistent swelling means the infection is still active, but it often reflects the body’s ongoing inflammatory response rather than live bacteria.
No Licensed Vaccine Exists
Despite decades of work, no tularemia vaccine has been licensed for use in the United States or anywhere else for routine human use.10PubMed Central. Adaptive Immunity to Francisella tularensis and Considerations for Vaccine Development A live-attenuated vaccine based on the milder Type B subspecies was developed over 50 years ago and has been used on a limited, investigational basis for lab workers at high risk. But it has never gained formal approval for general use in humans or animals.11PubMed. Development, Strategies, and Challenges for Tularemia Vaccine
The difficulty lies partly in the bacterium itself. F. tularensis is so adept at evading the immune system that generating long-lasting, reliable immunity has been a persistent challenge. Early-stage candidates including killed-bacteria vaccines and subunit vaccines have shown promise in animal models, and researchers have explored oral and inhaled delivery routes that may offer better protection against the pneumonic form than the traditional skin-scratch method.12PubMed Central. Respiratory and oral vaccination improves protection conferred by the live vaccine strain against pneumonic tularemia in the rabbit model But none of these are close to reaching the clinic.
Without a vaccine, prevention comes down to practical measures: wearing insect repellent in tick-prone areas, using gloves when handling wild animals, avoiding drinking untreated water in rural areas, and being cautious when mowing or clearing brush in areas where rabbits and rodents are common. If you find a dead rabbit or hare, don’t touch it with bare hands.
The Bioweapon Shadow
The extreme infectiousness of F. tularensis, the fact that just a handful of inhaled organisms can cause fatal pneumonia, has made it a focus of bioweapons research for nearly a century. Japan studied and probably used tularemia against civilian and military prisoners during World War II, conducting involuntary human experiments. In the United States during the 1950s and 1960s, voluntary experiments were carried out on prison inmates and non-combatant soldiers. Soviet scientists reportedly went further, allegedly developing a vaccine-resistant strain and testing it as a biological weapon in the early 1980s.13PubMed. From Squirrels to Biological Weapons: The Early History of Tularemia
Today, F. tularensis is classified as a Tier 1 Select Agent in the United States, the highest category of concern for potential bioterrorism agents. This classification is shared with anthrax, smallpox, and botulinum toxin. The classification drives ongoing investment in diagnostic tools, stockpiled antibiotics, and vaccine research, even though natural tularemia cases in humans are relatively rare. In the U.S., only about 100 to 200 cases are reported in a typical year. The concern isn’t the natural disease burden but the catastrophic potential of a deliberate aerosol release in a populated area.
Climate, Geography, and a Shifting Map
Tularemia has historically been associated with temperate climates in the Northern Hemisphere, with hotspots in Scandinavia, the central United States, and parts of Russia and Turkey. But the geographic range appears to be expanding. A modeling study found that climate alone could explain a large share of where tularemia occurs, and that between 2017 and 2020, climate-driven suitability for the disease increased across arid and semi-arid zones in East Africa compared to a 2012–2015 baseline.14PubMed. Climate-driven potential for tularemia in East Africa: skill testing and ecological consistency of a transferred risk model The implication is that warming temperatures and changing rainfall patterns could bring rabbit fever to regions that have rarely or never dealt with it.
The ecology of outbreaks is also more complex than a simple story of “infected rabbits and ticks.” In the Netherlands, a 2015 tularemia outbreak among hares prompted investigation into whether common voles, whose populations had surged that winter, might be amplifying the cycle. The hypothesis was that a population boom of susceptible rodents could fuel bacterial spread and spill over into hares and then humans. Investigators, however, did not find F. tularensis in the voles, leaving the question of what drives outbreak timing partly unanswered.15PubMed Central. Environmental surveillance during an outbreak of tularaemia in hares, the Netherlands, 2015 This kind of ecological uncertainty matters because effective public-health response depends on understanding not just how the bacterium infects people, but what environmental conditions cause case numbers to spike.
Diagnosis Is Often Delayed
One of the most frustrating aspects of tularemia is how often it goes unrecognized at first. Most physicians outside endemic areas will never see a case in their careers, and the early symptoms, fever, fatigue, swollen lymph nodes, overlap with dozens of more common conditions. The ulceroglandular form at least offers a visible clue: the skin ulcer with a dark eschar. But the glandular, typhoidal, and pneumonic forms can be mistaken for strep throat, mononucleosis, cat-scratch disease, lymphoma, or ordinary pneumonia.
Laboratory confirmation adds another delay. Standard blood cultures often fail to grow F. tularensis because the bacterium is slow-growing and requires special media that most hospital labs don’t stock. Serological testing (looking for antibodies in the blood) is the most common diagnostic method, but antibodies may not reach detectable levels until the second week of illness. PCR-based tests that detect bacterial DNA are faster but not universally available. In practice, the diagnosis often hinges on a physician thinking to ask about animal contact, tick bites, or outdoor activities, and connecting those dots to an unusual clinical picture.
If you develop a fever with swollen lymph nodes or an unexplained skin ulcer after spending time outdoors, mention your activities to your doctor. That one piece of context can be the difference between a prompt tularemia workup and weeks of diagnostic wandering.
Cats as an Underappreciated Source
When people hear “rabbit fever,” they picture hunters and rural settings. But domestic cats are an underrecognized link in the transmission chain. Cats that hunt rabbits and rodents can become infected, and they can pass F. tularensis to their owners through bites, scratches, or simply close contact while the animal is shedding bacteria. Cats can also carry infected ticks into the home. Unlike dogs, cats are quite susceptible to tularemia and can become seriously ill themselves, so a cat that suddenly develops a high fever, lethargy, and swollen lymph nodes after catching prey outdoors warrants veterinary attention and caution from the owner.
The risk is low in purely indoor cats, but any cat with outdoor access in an area where tularemia occurs is a potential bridge between wildlife reservoirs and human households. Wearing gloves when handling a sick cat, keeping cats indoors during tick season, and applying veterinary-approved tick prevention are straightforward steps that reduce the risk substantially.