Microbe of the Week is written by undergraduate researcher, Maria Bergquist.
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Image of Francisella tularensis. Photo credit: CDC Public
Health Image Library
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Microbe of the Week: Francisella tularensis
Welcome to microbe of the week, where we break down the
different disease-causing microbes that lurk inside our tick vectors! This
week’s microbe is Francisella tularensis, the causative agent of Tularemia in
the United States.
What is Francisella tularensis?
Francisella tularensis is a bacterial species which is known
for its durability in the environment, lasting for weeks in low temperatures on
hay, infected carcasses, or in water. Further, the CDC reports that as few as
10 to 15 organisms can lead to an infection. Due to these traits, and its
potentially life-threatening symptoms, Francisella tularensis has caused concern
that it could be used as an agent for bioterrorism.
The ticks that can transmit Francisella tularensis include the American
dog tick (Dermacentor variabilis), the wood tick (Dermacentor andersoni), and
the lone star tick (Amblyomma americanum). It can also be spread by deer flies
or infected animals. Typically, the infected animals are rodents,
such as mice or rabbits, but can also include domestic cats and even pet
hamsters. Humans can also be infected by environmental sources where Francisella
tularensis is present, including hay, straw, and wet soil. Cases are predominantly found in Kansas, Arkansas, Missouri,
and Oklahoma, though there was a spike of cases in Colorado in 2015.
Symptoms
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Images of lesions caused by Francisella tularensis.
Photo
credit: CDC Public Health Image Library
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Symptoms of Tularemia depend greatly on the mode of
infection. A skin lesion or ulcer may appear at the site of a tick bite along
with swelling of the lymph nodes near the area where the bite occurred. The
bacteria may also enter the eye or mouth due to handling infected environmental
sources (i.e. infected animal carcasses). Symptoms of these modes of entry
include irritation and inflammation of the eye or sore throat, mouth ulcers,
and tonsillitis when infection occurs due to ingestion. Tularemia caused by
inhalation of dust or aerosols containing the organism is the most serious form
of Tularemia. Symptoms of this includes cough, chest pain, and difficulty
breathing.
See the CDC’s website for more details on the symptoms of Tularemia:
Treatment
Quoting directly from the CDC, “Tularemia can be difficult
to diagnose. It is a rare disease, and the symptoms can be mistaken for other,
more common, illnesses. For this reason, it is important to share with your
health care provider any likely exposures, such as tick and deer fly bites, or
contact with sick or dead animals.”
Treatment requires the use of antibiotics
which include streptomycin, gentamicin, doxycycline, and ciprofloxacin.
Link to the CDC webpage on treatment for Tularemia:
A Condensed History
- 1909 to 1912 — George W. McCoy leads investigation on bubonic plague in ground squirrels during which time he discovers an unknown, highly infectious pathogen. McCoy and Charles W. Chapin name the unknown pathogen Bacterium tularense (now Francisella tularensis), after Tulare County in California where it was found.
- 1913 — First ever microbiologically proven case of Tularemia confirmed in Cincinnati, Ohio.
- 1921 — Edward Francis determines two diseases in Utah are caused by Bacterium tularense: deer fly fever and rabbit fever.
- 1923 — Ralph Parker and Roscoe Spencer investigate RMSF in wood ticks (Dermacentor andersoni) and discover ticks capable of spreading Tularemia.
- 1928 to 1935 — Tularemia officially reported in Russia, Norway, Canada, Sweden, and Austria, though historical evidence suggests the disease was present in many of these countries centuries prior.
- 1932 — Lee Foshay introduces antiserum to treat Tularemia, though half of those treated experienced serum sickness.
- 1944 — Streptomycin (discovered just one year prior) is determined successful in treating Tularemia.
- 1956 — Testing on prisoners of the Ohio State Penitentiary confirms a partial immunity after first infection.
- 1940 to 1970 — Testing on humans by the United States Army took place to determine the bioterrorism threat of Francisella tularensis. During this time both US and British investigators released F. tularensis into open water near the Caribbean islands and Honolulu to determine how far the bacteria could spread before becoming non-infective.
- 1974 — Bacterium tularense renamed Francisella tularensis in honor of Edward Francis.
- 2001 — Anthrax attacks in United States spark increased research in Francisella tularensis as a biowarfare agent.
- 2018 — CDC reports 229 cases of Tularemia in the US.
An incredibly in-depth paper on the history of Francisella
tularensis is available here:
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