It’s mid-February when an otherwise healthy 42-year-old
stockbroker presents to your office with fever, chills, and malaise.
He reports no sick contacts, no exotic travel but mentions having
spent the previous weekend on a fruitful hunting trip that included
shooting and trapping rabbits. CXR reveals hilar adenopathy. Your
differential includes influenza, tuberculosis, sarcoidosis, histoplasmosis,
and tularemia. What should you do?
A gram-negative coccobacillus, tularemia was first identified
in 1911 by a scientist investigating an outbreak of what was initially
thought to be bubonic plague in Tulare County, California. The disease
has multiple means of transmission, and waterborne epidemics were
seen in Europe and the Soviet Union in and around World War II (WWII).
It has been suggested that the Soviet outbreak, which occurred at
the Eastern front during WWII, was a result of a deliberate biological attack
by the Russians. In fact, tularemia has been a favorite agent for
bioweapons research since the 1930s. In 1969, the World Health Organization
(WHO) estimated that 10 kg (20 lb) of aerosolized F. tularensis could infect 50,000 people,
killing approximately 4,000. These numbers may be outdated because
antibiotic and vaccine-resistant strains of weaponized tularemia—reported
to have been developed at Biopreparat—would be far more
Francisella tularensis, the causative
agent of tularemia, is a small, nonmotile, facultative aerobic,
intracellular gram-negative coccobacilli. Tularemia grows in aerobic
environments. Despite its inability to form spores, it is nonetheless
quite hardy and able to persist for several weeks in water, soil,
vegetation, or in animal products. F. tularensis has
three different species of which biovar tularensis is the most virulent
form, and it is the one most commonly seen in the United States.
The organism is considered to be one of the most communicable bacterial
pathogens known. Disease hosts include rabbits, rats, and other
small mammals that attain the infection through direct contact or
by insect vectors, such as ticks and mosquitoes.
Rabbits and other small mammals act as vectors for tularemia.
Photo provided by PDPhoto.org.
F. tularensis is found worldwide,
though global disease incidence has not been determined. Tularemia
occurs sporadically throughout the continental United States, but
predominantly in the rural areas of the South, Southwest, or Midwest.
Since the 1990s, fewer than 200 cases are reported annually and occur
in a bimodal seasonal pattern. The majority of cases are diagnosed
in the summer/fall and are thought to be secondary to arthropod
(tick) transmission. A smaller fall/winter peak coincides with
hunting seasons. Most cases are secondary to direct contact of some
kind, although there are infrequent cases of infection from inhalation.
No significant differences in infection patterns are seen by age
or by gender. Overall mortality rates prior to the advent of antibiotics
ran between 5% and 15%, and for pneumonic forms
between 30% and 60%. ...