What is the epidemiology of tularemia?

Tularemia Epidemiology

Tularemia is a zoonotic disease with complex epidemiology involving over 250 animal species as reservoirs, multiple transmission routes, and distinct geographic and seasonal patterns that clinicians must recognize to identify at-risk populations and prevent outbreaks.

Geographic Distribution

• Tularemia occurs throughout the Northern Hemisphere between 30° and 71° N latitude, with endemic presence in North America, Europe, and Asia 1, 2
• The disease occurs both sporadically and in epidemic form across most European countries, with the notable exception of the United Kingdom where it has not been reported 1
• In North America, F. tularensis subsp. tularensis (biovar A) predominates and is the most virulent subspecies, while F. tularensis subsp. palaearctica (biovar B) is less virulent and occurs in Europe, Asia, and North America 1
• Finland and Sweden experience outbreaks comprising hundreds of cases at least once per decade, representing some of the highest incidence areas in Europe 3

Reservoir and Vectors

• Evidence of infection has been documented in more than 250 animal species, including mammals (particularly hares, rabbits, and rodents), birds, fish, amphibians, arthropods, and parasites 1
• The natural reservoir between outbreaks remains unknown, though the organism replicates intracellularly in protozoans and shows association with natural water sources 3
• Primary arthropod vectors include Dermacentor spp. and Ixodes spp. ticks, deer flies, and mosquitoes 1, 4
• Small animals, particularly rabbits, hares, and muskrats serve as the main reservoir hosts for human transmission 4

Transmission Routes

Multiple transmission pathways to humans have been documented 1:

• Tick bites (by Dermacentor and Ixodes species)
• Bites from infected mammals, deer flies, or mosquitoes
• Direct contact with infected animals, even through intact skin 1
• Inhalation of contaminated aerosols 1
• Ingestion of contaminated water or food 1, 5
• Occupational exposure in laboratory and agricultural settings 1

High-Risk Populations

Individuals with increased risk of tularemia include 1:

• Laboratory workers (due to high infectivity via aerosol route)
• Farmers and agricultural workers
• Veterinarians
• Hunters and trappers
• Cooks and meat handlers
• Travelers visiting rural and agricultural areas in endemic countries 2

Seasonal and Temporal Patterns

• 65% of cases occur from May to July and are associated with tick exposure 1
• Cases associated with animal exposure (rabbits, rodents) occur in two peaks: 33% in November-December and 22% in May-July 1
• In the United States, fewer than 200 infections occur annually, with the last naturally occurring inhalation case in 1976 1
• During World War II, hundreds of thousands of infections occurred in Europe, highlighting the epidemic potential under certain conditions 1

Waterborne Epidemiology

• Waterborne outbreaks have been documented, with contaminated local water supply systems serving as the source 5
• Water samples from outbreak investigations have tested positive for F. tularensis by PCR, establishing water as a significant transmission route 5
• The association of tularemia with natural water may be significant in locating the reservoir between epidemic periods 3

Bioterrorism Considerations

• F. tularensis is classified as a Category A agent for bioterrorism due to ease of airborne transmission 6
• The US biological weapons program in the 1960s substantiated its potential as a biological weapon through extensive studies 1
• Any case of inhalation anthrax should prompt immediate notification of public health authorities, as it always indicates bioterrorism 1
• After aerosol exposure, "typhoidal" or "pneumonic" tularemia is the most common bioterrorism-related form 1

Mortality and Disease Burden

• Mortality rates can reach 60% if not promptly diagnosed and treated, with the poorest outcomes in pneumonic and typhoidal forms 6
• Historical mortality rates for naturally occurring disease range from 1.4% in recent US surveillance (1985-1992) to 45-80% for inhalation cases 1
• The disease is re-emerging in Europe in recent years, posing renewed diagnostic and therapeutic challenges 6

Common Pitfalls in Epidemiologic Recognition

• The epidemiology of tularemia remains poorly understood despite its widespread distribution, complicating outbreak detection 1
• Clinicians may fail to consider tularemia in travelers returning from endemic areas with compatible symptoms 2
• Between outbreaks, surveillance may miss sporadic cases due to low clinical suspicion in non-endemic periods 3
• Laboratory personnel face significant occupational risk and require biosafety level 3 precautions, which may not be universally implemented 1