Ecology of Tularemia in Central US Endemic Region

Abstract

Tularemia is a zoonotic disease that occurs in the Northern Hemisphere caused by the gammabacterium Francisella tularensis. The most severe form of human tularemia occurs in the central USA and involves a rabbit enzootic cycle, ixodid tick vectors, and F. tularensis subspecies tularensis genotype A1. Enzootic tularemia is thought to have a spring-summer seasonality corresponding to the questing activity of its primary tick vectors. Domestic cats, another common incidental host, acquire the infection by preying on infected rabbits. The seasonality of tularemia in cats, which demonstrate a bimodal seasonal incidence curve with peaks in the spring and late summer-fall, may serve as a surrogate for the seasonality of the disease in its enzootic host. Human tularemia shows a unimodal late spring, early summer peak, which correlates to the seasonal questing activity of tick vectors of human tularemia. This difference in seasonality suggests that different tick species or tick life stages are involved in maintenance of the enzootic rabbit-tick cycle.

Francisella tularensis novicida infection competence differs in cell lines derived from United States populations of Dermacentor andersoni and Ixodes scapularis

“…In the United States, Dermacentor spp. as a whole, are a primary, epidemiologically relevant vector for F. tularensis ssp., though Amblyomma americanum , and Haemaphysallis leporispalustris are also important vectors in the south central United States 13 – 16 . The competence of United States populations of I. scapularis for F. tularensis ssp.…”

Frequent Exposure to Many Hunting Dogs Significantly Increases Tick Exposure

“…is why seropositivity to B. burgdorferi was similar in dog caretakers and environmental control groups, despite higher, but perhaps not more Ixodes, tick exposure in dog caretakers. Dog-based tick exposure may instead increase the risk of Rickettsia rickettsii or Francisella tularensis infection, both transmitted by D. variabilis (Ammerman et al 2004, Mani et al 2016. These diseases would be logical targets for future study within these cohorts.…”

Transstadial transmission and maintenance of Francisella tularensis in the Blacklegged Tick Ixodes scapularis.

“…The host preference of adult I. scapularis for large mammals including humans and the coincidence of questing activity with the fall-winter incidence of human tularemia supports its potential as a vector [14]. Domestic cats are highly susceptible to tularemia and the disease incidence in cats shows a bimodal pattern of springsummer and fall maxima [3]. Cats contract the disease by preying on tularemia infected small mammals and the disease during the spring-summer and fall peak incidence is indicative of concurrent tularemia infection cycle in small mammals [3].…”

“…Francisella tularensis is a highly infectious gram-negative coccobacillus which causes the disease tularemia in humans and a wide range of vertebrate and invertebrate animals [1]. There are two main subspecies of this bacterium, namely subspecies tularensis exclusively seen in the North America and subspecies holarctica seen in North America as well as in Eurasia [2,3]. Molecular subtyping have identified a number of genotypes for the two subspecies of which genotype A1b seems to be the most virulent with a case fatality rate of 25% and is associated with tick borne infections [4][5][6].…”

“…Domestic cats are highly susceptible to tularemia and the disease incidence in cats shows a bimodal pattern of springsummer and fall maxima [3]. Cats contract the disease by preying on tularemia infected small mammals and the disease during the spring-summer and fall peak incidence is indicative of concurrent tularemia infection cycle in small mammals [3]. Immature stages of I. scapularis seeks small mammals including rabbits and white footed mice as hosts; however, the host seeking activity of the immature I. scapularis is predominantly during summer months in the south-central tularemia endemic region [14].…”