5 Rare fevers you’ve probably never heard of

Most fevers resolve by themselves and don’t warrant much intervention or worrying, for that matter. They are usually not a sign of anything untoward. However, there are a number of rare and sometimes dangerous infections, which can cause specific febrile illnesses. These rare fevers are often hard for a physician to diagnose. Because they are rare, they can be misdiagnosed and therefore mistreated. Additionally, some of these illnesses only occur in specific parts of the world, which makes it more likely that most of them are not widely known (or frequently encountered in clinical settings) outside their endemic regions. 

When reading about these rare fevers, you might notice that a lot of these fevers share the same cardinal signs and symptoms. These diseases may be caused by pathogens (infectious organisms) that are very different from one another but the clinical picture, especially in the early stages, might be very typical. Usually, the unique clinical features of an infection only manifest in the later stages, when the infection is quite severe. This makes it a huge challenge to diagnose these infections, especially considering their obscurity. 

In the section below, we’ve listed 5 rare fevers that you’ve probably never heard about. Read ahead to find out more about these infections, where they are found and how they are diagnosed and treated. 

1. Rocky Mountain Spotted Fever

Rocky mountain spotted fever is a rare and potentially life threatening infection that is spread by ticks. It was named after the Rocky Mountains of North America, where it was first discovered in the 19th century and the characteristic “spotted” rash which it causes.

The disease is caused by a type of bacterium called Rickettsia rickettsii. Humans mostly contract the infection through a tick bite - American dog ticks, rocky mountain wood ticks and brown dog ticks are the organisms that have been implicated, so far, with spreading the disease. The infection is endemic to parts of the United States, Mexico and Central America. The CDC classifies the disease under the category “Spotted fever rickettsiosis”, which includes other infections such as pacific coast tick fever and rickettsialpox. They estimate that around 5000 cases of the infection are reported in the US each year. 

Clinically, the infection usually starts with a fever and headache, subsequently, followed by a characteristic red, “spotted” (petechial) rash which usually first appears on the wrists and ankles. The classical “triad” of signs for rocky mountain spotted fever is elevated temperature, headache and a recent history of tick bite. The early signs are quite non-specific which makes it hard for physicians to diagnose the disease. The disease can potentially cause severe complications such as gangrene (necessitating amputation), cognitive deficits, ataxia (lack of coordination), deafness etc. The infection is treated using Tetracyclines (a class of antibiotics), predominantly, Doxycycline.

The disease used to be associated with a staggering 30% mortality rate prior to the antibiotic era. However, even today, it kills about 5% of the people that it infects. The disease is especially dangerous for those with a genetic condition called “Glucose-6-Phosphatase Deficiency”, often resulting in death just days after the initial infection. 

2. Rift Valley Fever

Rift valley fever (RVF) is a viral disease that affects both livestock and humans. It is caused by a virus called the RVF virus. The disease was first identified in 1931, in the Rift Valley of Kenya, during an investigation into a mysterious epidemic that infected sheep. 

Rift valley fever, in humans, primarily occurs through contact with the bodily fluids of infected animals (sheep, cattle, camels etc). Additionally, the disease can also spread to humans through mosquito bites. Cases of rift valley fever have only been reported within Africa and the Arabian peninsula. Multiple major outbreaks of the disease have occurred, mostly in sub-Saharan Africa.

In humans, rift valley fever can has two forms. The first one is the mild form of the infection, which causes typical flu-like symptoms such as fever, muscle pains and headaches and the second is severe, causing serious complications such as blindness, bleeding etc.Most people who get infected with the virus tend to have the mild version of the infection with only about 2% of those infected progressing to having a more severe form of the disease. In its severe form, rift valley fever can cause blindness (ocular form), meningoencephalitis or hemorrhagic syndrome (bleeding). Of these, the hemorrhagic-fever form of the infection has the highest mortality rate (~50%). Overall, the disease kills less than 1% of infected humans, although it is much more lethal in animals. 

There is no approved human vaccine against rift valley fever. The best way to prevent this disease, in humans, is to vaccinate livestock/animals against the virus. 

3. Sennetsu Fever

Sennetsu fever is a very rare infectious disease that belongs to a group of diseases known as the human ehrlichioses. It is caused by a bacterium called Ehrlichia sennetsu. This group of diseases (ehrlichioses) affects a wide range of animals, in addition to humans. They are caused by a genus of bacteria known as “Ehrlichia”. In addition to Sennetsu fever, we have identified several other types of ehrlichial infections in humans such as human monocytic ehrlichiosis and human granulocytic ehrlichiosis. These conditions tend to have similar clinical presentations and are caused by various species of bacteria belonging to the genus Ehrlichia. 

We have known about ehrlichioses in animals for quite some time. For example, canine ehrlichiosis, which is an ehrlichial infection that affects dogs, was identified all the way back in 1935. On the other hand, sennetsu fever which was the first identified human ehrlichiosis and was only discovered in 1954, in Japan. Many ehrlichioses are transmitted through ticks; However, we still do not know the vector organism for sennetsu fever. Although there is no consensus, many believe that the disease spreads through the consumption of uncooked fish. 

Sennetsu fever presents with high temperature (which is sudden in onset), myalgia severe headaches, chills and swollen lymph nodes.Some patients also experience nausea, vomiting and anorexia. Hepatosplenomegaly (enlargement of liver and spleen) is a common clinical finding in sennetsu fever. In many cases, sennetsu fever is associated with a drop in white blood cell (WBC) count and an increase in the concentration of liver enzymes (transaminases). 

Sennetsu fever is extremely rare and so far, no cases of the disease have been reported outside Japan and Malaysia.  

4. Cat Scratch Fever

Cat scratch fever (also known as cat scratch disease) is a disease that spreads to humans through cats. Cat scratch disease is caused by a bacterium called Bartonella henselae and spreads to humans through the bite or scratch of a cat.

In most people, the main symptoms of cat scratch disease only start to appear a few days (or even weeks) after the initial exposure. The first noticeable symptom is usually the appearance of a raised, fluid filled bump (papule) on the site of the bite/scratch. This papule is usually painless and is often missed by the patient. It remains for a period of 1-3 weeks, following which the other symptoms start to appear. After about 3 weeks, there is painful swelling of lymph nodes (lymphadenitis). The lymph nodes appear red and are quite warm to the touch. In some cases, the lymph nodes are filled with pus (suppuration). Usually, the lymphadenopathy persists for about 4 months but in some cases, the lymph nodes may take over a year to get back to normal. Other symptoms of cat scratch disease include generalised malaise, low-grade fever, headaches and body aches, much like a typical picture of a standard febrile illness. 

In most people, cat scratch fever is a self-limiting condition which, at best, only requires supportive therapy or in case of high-risk groups, antibiotic therapy. However, in immunocompromised individuals (eg. HIV positive patients, those on cancer therapy etc) and children, there are reports of Bartonella henselae infections causing serious complications. In children, granulomatous hepatitis (inflammation of the liver), splenitis (inflammation of the spleen) and bone infections (osteomyelitis), are commonly observed. Bartonella infections may also cause highly serious sequelae such as encephalopathy , endocarditis or an ocular (eye) manifestation called Parinaud’s oculoglandular syndrome. Endocarditis caused by Bartonella infections are associated with a high rate of mortality.

5. Colorado Tick Fever

Colorado tick fever is a rare viral infection that is caused by a type of Coltivirus called CTFV (Colorado Tick Fever Virus). The disease affects a very small number of people annually and is usually limited to the Rocky Mountain belt of the United States and Western Canada. The virus infects humans through the bite of the Rocky Mountain wood tick (Dermacentor andersoni). The disease doesn’t usually spread from person to person, except through blood transfusion (in rare cases). 

This infection is often confused with the first entrant on this list, Rocky Mountain Spotted Fever. These diseases both involve ticks, the Rocky Mountains and might even have similar clinical presentations. However, as mentioned previously, RMSF is a bacterial illness and CTF is viral.  

The symptoms of Colorado tick fever appear 3-6 days following the tick bite. In some cases, the incubation period can even extend upto 20 days. Common symptoms are fever, photophobia (increased light sensitivity), pain behind the eyes, chills, fatigue, nausea and muscle pains (especially in the legs and back). Hepatosplenomegaly (enlargement of the liver and spleen) is also a commonly noted finding. In some cases, a slight rash formation can be observed.

A peculiar feature of CTF is that patients show a “biphasic” fever pattern. This basically means that there is a sharp rise in body temperature over 2-3 days, following which the fever remits, only to return in a day or two. Most patients get better within days of getting infected. However, in a very small number of paediatric cases, the virus can cause a more serious illness involving the central nervous system, resulting in aseptic meningitis or encephalitis. However, this is extremely rare. 

There is no specific treatment for Colorado tick fever. As a first step, the tick is to be carefully removed from the skin using tweezers, following which an analgesic such as paracetamol is prescribed for the pain.