Description

• Tularemia is an infection caused by Francisella tularensis, often presenting with fever, myalgia, and headache 3 to 6 days after initial exposure. The extent of the illness depends on infecting dose, subspecies, and route of entry.
• Six clinical forms are typically described:
  • Ulceroglandular tularemia
    • Constitutes 75% of all cases
    • A papule, which ruptures and ulcerates, occurs at the site of entry.
  • Glandular tularemia
    • Identical to the ulceroglandular form however does not have an identified primary skin lesion
  • Oculoglandular tularemia
    • Occurs when the organism gains access via the conjunctival sac
    • Usually from the patient rubbing the eyes with contaminated fingers
    • Yellow nodules and ulcers may appear on the palpebral conjunctiva associated with enlarged preauricular nodes.
  • Oropharyngeal tularemia
    • Occurs after the ingestion of contaminated food or water
    • An ulcerative or membranous tonsillitis accompanies a painful sore throat.
    • Lower GI tract involvement with vomiting, diarrhea, and abdominal pain may be associated.
  • Typhoidal tularemia
    • Presents with fever of unknown origin, without localizing lymphadenopathy or skin findings
    • Shock, pleuropulmonary findings, odynophagia, diarrhea, and bowel necrosis are often associated.
  • Pneumonic tularemia
    • Occurs after inhalation of the organism
    • It can also be present in association with ulceroglandular and typhoidal tularemia.
    • Pulmonary tularemia is the most fulminant and lethal form.
    • Symptoms include fever, dry cough, and pleuritic chest pain.
    • Tularemia in this form is a feared potential biologic weapon because an exposure to only 1 to 10 colony-forming units can result in infection. Although not transmitted person to person, laboratory workers working with organism on an agar plate are at risk for this form of disease.

Epidemiology

• F. tularensis is found primarily in the Northern hemisphere from the 30- to 70-degree latitudes. Wild and domestic mammals (e.g., cats, rabbits, hares, squirrels, boars, beavers, deer, and rodents) may be infected as well as invertebrates (e.g., ticks, deerflies, horseflies, and mosquitoes).
• Humans acquire tularemia after a bite by an infected arthropod or through contact with tissues or body fluids of an infected animal. The subspecies holarctica has been shown to persist in various water sources, and waterborne transmission to humans has been reported.
• Inhalational exposure can happen in the laboratory setting or after the organism is aerosolized during meat preparation or certain outdoor activities.
• Most commonly reported during the summer months in children between 5 and 9 years of age and adults >55 years old with a male preponderance
• Annual incidence is 0.041 cases per 100,000 persons.
• U.S. states with an increased incidence in recent years include Wyoming, Nebraska, Kansas, South Dakota, Colorado, Missouri, and Arkansas.

Risk Factors

• Most frequently infected groups include hunters, trappers, farmers, and veterinarians.
• Activities involving wild animals or exposure to various arthropod vectors
• Tick exposure is a common mode of transmission in children in the United States.
• Infection has been linked to landscapers using lawn mowers and brush cutters.
• Laboratory personnel working with samples known to be or potentially infected with Francisella

General Prevention

• Isolation of the hospitalized patient
  • Standard precautions are recommended for protection against secretions. Human-to-human transmission has not been reported.
• Control measures
  • Protective clothing and insect repellent should be used to minimize insect bites.
  • Inspection for ticks and their immediate removal should be routine after outdoor activity in endemic areas.
  • Rubber gloves should be worn while handling or cooking wild animals (e.g., rabbits, lemmings) possibly contaminated with Francisella.
  • Game meat should be cooked thoroughly.
  • Laboratory workers should wear rubber gloves and masks in a biosafety level 3 environment when working with specimens potentially containing Francisella.
• Vaccine
  • Significant research into various vaccine techniques continues to evolve given concerns of F. tularensis as an agent of bioterrorism.

Pathophysiology

• Human infection can result from various modes of entry:
  • Skin contact with infected animals
  • Vector-borne infection described after the bite of a tick (dog tick, wood tick, lone star tick), mosquito, horsefly, or deerfly
  • Inhalation of aerosolized organism seen in laboratory workers, crop harvesting, disturbance of contaminated hay, and grass cutting
  • Ingestion of contaminated food products or water
• A primary lesion develops at the site of exposure.
• Local tender lymph node swelling ensues.
• After skin inoculation or inhalation, the organism can spread via the bloodstream to various organs.

Etiology

Tularemia is caused by a small, fastidious, nonmotile, gram-negative coccobacillus; four distinct subspecies have been described:

• Tularensis (type A): found primarily in North America; causes the most severe cases of tularemia in humans
• Holarctica (type B): subspecies found primarily in Europe and Asia; less virulent than tularensis
• Novicida: rarely isolated but can be found worldwide
• Mediasiatica: recovered from ticks and animals in Central Asia; not associated with disease in immunocompetent humans
• An additional species, Francisella philomiragia (formerly Yersinia philomiragia), has also been reported. This is a rare cause of human disease and is possibly associated with salt water exposure.