eMedicine Specialties > Emergency Medicine > Infectious Diseases

Tick-Borne Diseases, Tularemia

Author: Jonathan A Edlow, MD, Associate Professor of Medicine, Department of Emergency Medicine, Harvard Medical School; Vice Chairman, Department of Emergency Medicine, Beth Israel Deaconess Medical Center
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Updated: Dec 9, 2008

Introduction

Background

First described in Japan in 1837, tularemia is an infectious disease caused by the gram-negative pleomorphic bacterium, Francisella tularensis. The disease name relates to the description in 1911 of a plaguelike illness in ground squirrels in Tulare County, California, and the subsequent work performed by Dr Edward Francis. In 1928, Francis described his personal experience with more than 800 cases.

F tularensis is found worldwide in more than 100 species of wild animals, birds, and insects. This occurs in both terrestrial (rabbits, hares, ticks, and flies) and aquatic animals (muskrats and beavers). Four major strains, which differ in both virulence and geographic range, exist. The "tularensis" strain, found primarily in North America, is the most virulent.

The organism produces an acute febrile illness in humans. The route of transmission and factors relating to the host and the organism influence the presentation.

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Pathophysiology

Categories of tularemia

Some authorities classify tularemia into 2 groups, which include the far more common ulceroglandular form (in which local or regional symptoms and signs predominate) and the more lethal typhoidal form (in which systemic symptoms dominate the clinical picture). More commonly, however, tularemia is divided into 6 forms:

• Ulceroglandular
• Glandular
• Oculoglandular
• Oropharyngeal
• Pneumonic
• Typhoidal

Each form reflects the mode of transmission. The organism gains access to the host by means of inoculation into skin or mucous membrane, inhalation, or ingestion. Although person-to-person transmission does not occur with F tularensis, the organism is extremely infectious, with as few as 10-50 inhaled organisms producing disease. It is therefore an organism that can infect laboratory technicians working with the organism, making it a candidate for use as a biological weapon.

Ulceroglandular form (70-80% of cases): The organism enters through a scratch, abrasion, or tick or insect bite and spreads via the proximal lymphatic system. Within the ulceroglandular form, more differentiation exists. A subcutaneous inoculum of as few as 10 organisms can cause disease.

Glandular form (rare): No ulcer is present, and the organism is presumed to have gained access to the lymphatic system and/or bloodstream through clinically unapparent abrasions.

Oculoglandular form (1% of cases): The organism enters through the conjunctiva from either a splash of infected blood or rubbing the eyes after contact with infectious materials (eg, blood from a rabbit carcass).

Oropharyngeal form (rare): This form occurs after ingestion of eating undercooked rabbit meat containing the organism.

Pneumonic form (uncommon): This form occurs when the organism is inhaled. This form is observed in laboratory workers and occasionally occurs naturally. Pneumonia also occurs in 10-15% of patients with ulceroglandular tularemia and in one half of those patients with typhoidal tularemia.

Typhoidal (or septicemic) form (10-15% of cases): This form is more severe than the others and often includes pneumonia. Ingestion may be the mode of transmission; however, in most cases, the portal of entry remains unknown.

Incubation

After an incubation period of 3-4 days (range, 1-14 d), a papule develops, accompanied by a high fever. The papule evolves into an ulcer associated with regional lymphadenopathy. Some patients infected by a second, less virulent strain (type B) have less dramatic presentations.

Carriers

Although numerous animals and insects can carry F tularensis, rabbits and ticks (especially Dermacentor and Amblyomma species) most commonly are implicated in human cases. The deer fly is another classic, although less common, vector.

Frequency

United States

A few hundred cases of tularemia are reported annually in the United States. As with most such diseases, most cases are likely unreported or misdiagnosed. Although sporadic cases occur in all states, those with highest prevalence are Arkansas, Illinois, Missouri, Texas, Oklahoma, Utah, Virginia, and Tennessee. Some occupations confer risk for tularemia; they include laboratory workers, landscapers, farmers, veterinarians, hunters, trappers, cooks, and meat handlers.

The frequency of tularemia has decreased markedly over the last 50 years, and a shift from winter disease (usually from rabbits) to summer disease (more likely from ticks) has occurred. Although this decrease led to the Centers for Disease Control and Prevention (CDC) removing tularemia from its list of reportable diseases in 1994, it was reinstated in 2000 due to concerns about tularemia being used as a biological weapon.

International

Tularemia is found worldwide, but the incidence is unknown.

Mortality/Morbidity

Untreated, tularemia has a mortality rate of 5-15%; this rate is even higher with the typhoidal form. Appropriate antibiotics lower this rate to about 1%.

Sex

Biologically, no gender bias exists; however, young–to–middle-aged men may be more likely to engage in activities (eg, associated with tick bites, rabbit, and wild game exposure) that predispose them to tularemia. Recently, tularemia has been associated with the bite of a pet hamster.¹

Clinical

History

• The general history for tularemia may include fever, chills, myalgias, and malaise. Occasionally, patients with tularemic meningitis, pericarditis, peritonitis, endocarditis, and osteomyelitis have symptoms that correspond to the organ system or systems involved. However, the usual manifestations correlate with the pathophysiological form outlined above.
• Ulceroglandular forms
  • Patients have ulcers at the site of inoculation.
  • In rabbit-associated cases, ulcers usually are on the fingers or hands.
  • In tick-associated cases, common sites include the groin, axillae, and trunk. Swollen regional glands reflect this same geographic pattern. Infected nodes are painful.
• Glandular form
  • This form is distinguished from the ulceroglandular form by the absence of an ulcer.
  • The bacterium presumably gains entry via microscopic abrasions or potentially through intact skin.
• Oculoglandular form
  • The patient has a painful, red eye, often with purulent exudate.
  • Swollen glands may occur in submandibular, preauricular, or cervical areas.
• Oropharyngeal form
  • Produced from eating undercooked infected meat, this form is associated with a sore throat, abdominal pain, nausea, vomiting and diarrhea, and occasionally, GI bleeding.
  • Abdominal pain is caused by mesenteric adenopathy, and bleeding results from intestinal ulcerations.
• Pneumonic form (Note: Considering tularemia in patients presenting with atypical pneumonia, especially with the epidemiologic profile as below, is important.
  • In this form, produced by inhalation of organisms or by hematogenous spread from ulceroglandular or typhoidal disease, patients have a dry cough, dyspnea, and pleuritic chest pain. Landscaping during the summer months, especially cutting grass with a power mower, which may aerosolize organisms, is another described risk.
  • Some patients with tularemic pneumonia have systemic symptoms without these respiratory complaints.
• Typhoidal (septicemic) form
  • F tularensis bacteremia causes this form and produces fevers, chills, myalgias, malaise, and weight loss.
  • The absence of an ulcer or lymphadenopathy makes diagnosis difficult.

Physical

Physical findings in tularemia vary with the mode of presentation.

• Findings common to most cases are fever, tender hepatosplenomegaly, and in about 20% of patients, a generalized maculopapular rash that occasionally becomes pustular.
• In one series, erythema nodosum occurred in 4 of 88 cases.²
• The ulcer forms at the site of skin entry of the organism. The location varies with the vector.
  • The lesion starts as a tender papule that evolves into an ulcer with sharply demarcated borders and exudate.
  • The base evolves from yellow to black.
  • Regional nodes are edematous and tender, can become fluctuant, and may drain spontaneously.
• Ocular findings may include unilateral intensely injected conjunctiva with purulent exudate, ulcerations and nodules on the palpebral conjunctiva, preauricular and cervical adenopathy, and corneal ulceration.
• Exudative and membranous pharyngitis with regional adenopathy may be observed with the oropharyngeal form.
• In the pneumonia form, rales are sometimes heard, but normal findings at lung examination are not uncommon.
• Physical findings associated with pericarditis, peritonitis, meningitis, and osteomyelitis can be observed.

Causes

Tularemia is caused by infection with the bacteria F tularensis. The 2 subspecies are A (tularensis) and B (holartica). In the western United States, type A infections may be less severe than type B infections.

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