Like Y. pestis, Francisella tularensis is a
Gram (-) bipolar rod whose major reservoir is small mammals (especially
rabbits and hares ["rabbit fever"], as well as squirrels,
voles, muskrats, beavers, and a variety of other species).
Francisella tularensis infection has long been recognized
as a direct zoonotic infection of rabbit hunters. They contract
the organism while skinning the rabbits.
Tularemia is also an occupational risk for veterinarians
handling infected animals, especially cats.
In one study in Alaska, 14% of veterinarians had positive
antibody titers to F. tularensis, compared to 1% of the
general U.S. population.
7.7% of cases of tularemia in the U.S. between 1977 and 1998
were due to contact (bites/scratches) with infected cats, and
during an outbreak in Sweden in 2000, ownership of cats was a
specific risk factor for infection.
People may also be exposed by consuming undercooked game
meats (infection requires 108 organisms) or inhalation of the
organism (infection requires only 101-2 organisms).
Tularemia has also been reported to occur as a waterborne
infection in Russia and Italy (The bacteria can survive for weeks
to months in water and mud.), and outbreaks may be initiated
when environmental conditions favor sudden increases in rodent
populations, as happened in post-war Kosovo in 1999-2000.
Like Y. pestis, F. tularensis is transmitted
between infected animals in the natural setting via bloodsucking
arthropods, especially ticks (Dermacentor and Amblyomma
in the U.S.) and biting flies, e.g. deer flies.
The organism passes transovarially in ticks, thereby providing
an efficient way of maintaining the organism in nature.
Tick transmission is also a mode of transmission to humans.
Globally, tularemia is found most commonly in the Northern
Hemisphere. In the U.S., tularemia in human beings is reported
most commonly in Arkansas (23%), Missouri (19%), South Dakota
(7%), Oklahoma (7%), Kansas, Montana, and Martha's Vineyard [Massachusetts].
However, during the period from 1990-2000, there were 1,368 cases
reported from 44 states.
An outbreak on Martha's Vineyard during the summer of 2000
involved 15 cases, 11 of which were the dangerous pneumonic form.
These cases were associated with employment as lawn mower operators/landscapers.
A previous pneumonic tularemia outbreak had occurred on the island
in 1978.
Tularemia topics
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