Tularemia: Epidemiology, Diagnosis, and Treatment

Abstract

Feature Nada S. Harik, MD Tularemia is a rare zoonosis caused by Francisella tularensis. Although many animals may be infected with tularemia, human infection most commonly occurs via an insect vector such as a tick or deer fly. In the US, most cases of tularemia occur in the summer in the south-central states, specifically Missouri, Arkansas, and Oklahoma. There are six major tularemia clinical syndromes each with different clinical presentations: ulceroglandular tularemia (42%–75% of all tularemia cases), glandular tularemia (15%–44% of all tularemia cases), oropharyngeal tularemia, oculoglandular tularemia, typhodial tularemia, and pneumonic tularemia. The diagnosis of tularemia is typically made clinically, taking into account exposure history and clinical manifestations and confirmed by serologic testing. Aminoglycosides are the drugs of choice for the treatment of tularemia. Tularemia prevention is best accomplished by keeping away from dead or infected animals and avoiding ticks. Tularemia is a zoonotic infection caused by F. tularensis, a small, fastidious, aerobic gram-negative coccobacillus. There are four distinct subspecies of F. tularensis; however, disease is mainly caused by F. tularensis subspecies tularensis (type A) and F. tularensis subspecies holarctica (type B).¹ Type A is more virulent and is primarily found in North America.¹ Type B is found throughout the Northern Hemisphere, mainly in Europe and Asia, and causes milder infection than type A.¹ F. tularensis is highly contagious; only a small inoculum is needed to produce disease.¹ More than 100 species of mammals have been noted to be infected with tularemia. This includes rabbits, hares, muskrats, prairie dogs, skunks, raccoons, rats, voles, squirrels, sheep, cattle, and cats.¹ Disease transmission can occur via handling the carcass of an infected animal, via the bite of a infected animal, or via ingestion of meat from a diseased animal. The bite of an insect vector such as a tick, deer fly, or flea can also transmit tularemia to humans. Insects become infected when they feed on an infected animal; ticks can also become infected by transovarian passage.² In the United States, ticks are the most common and important insect vector of tularemia.² Tick species that transmit tularemia to humans include Amblyomma americanum (lonestar tick), Dermacentor andersoni (wood tick), and Dermacentor variabilis (dog tick). Tularemia can also be caused by contact with aerosolized bacteria from mowing lawns, working on farms, or working in laboratories where F. tularensis is present. The disease can also be transmitted by drinking water contaminated with F. tularensis; this organism can survive in water and animal carcasses for long periods. Frozen rabbit meat has remained infective for greater than 3 years.³ Person-to-person transmission of tularemia does not occur. In the US, 90 to 154 cases of tularemia have been reported yearly to the Centers for Disease Control and Prevention (CDC) from 2001 to 2010.⁴  Tularemia has been reported by every state except Hawaii.⁴ Arkansas, Oklahoma, and Missouri account for approximately 50% of the cases of tularemia reported in the US each year.⁵ Figure 1 is a CDC map detailing the locations of reported cases of tularemia from 2001 to 2010.⁴  Figure 1. Reported cases of tularemia, United States, 2001 to 2010. One dot placed randomly within county of residence for each confirmed case. (From US Centers for Disease Control and Prevention⁴) Tularemia presents most commonly in the summer, due to high tick activity, in the south-central US and peaks in the winter, the primary hunting season, in the northeastern US.¹ Individuals at risk for developing infection include hunters, trappers, taxidermists, grounds maintenance workers, sheep herders/shearers, laboratory workers, those with tick exposure, and those living in or traveling to areas where tularemia is endemic. The highest incidence of tularemia occurs in children (and in adults older than age 75 years); boys have a higher incidence of infection than girls.⁵ The higher incidence in boys is most likely due to their greater participation in activities such as hunting that increase exposure to tularemia. Figure 2 shows the age and gender of reported tularemia cases from 2001 to 2010.⁴ Figure 2. Reported tularemia cases in the United States by age and gender, 2001 to 2010. (From US Centers for Disease Control and Prevention⁴) The incubation period of tularemia is 1 to 21 days, with an average of 2 to 5 days. There are six major tularemia clinical syndromes, which are classified by the portal of entry of the infection (see Table 1). Table 1. Common Characteristics of the Six Tularemia Clinical Syndromes The most common syndrome, accounting for between 42% and 75% of all cases of tularemia, is ulceroglandular tularemia.^6–8 This syndrome is characterized...