on 22-Jan-2025 (Wed)

The Francisella genus includes the potential human pathogens F. tularensis, F. philomiragia, F. hispaniensis, and F. opportunistica [2-5]. There are also the fish pathogens F. noatunensis, F. orientalis, F. salimarina, and F. halioticida and many environmental isolates [4,6,7]. Some environmental species previously placed in the Francisella genus have been reclassified in a new genus, Allofrancisella [8]. Other Francisella species, some not officially named, have also rarely caused human infections [7,9,10].

The subspecies of F. tularensis are [1,11,12]:

● F. tularensis subspecies tularensis (also called F. tularensis type A). In addition, molecular typing methods have identified distinct clades or genotypes of F. tularensis subspecies tularensis that differ in their geographic locations and virulence [13,14].

● F. tularensis subspecies holarctica (also called F. tularensis type B).

● F. tularensis subspecies novicida (also called Francisella novicida since its classification as a subspecies of F. tularensis has been controversial).

● F. tularensis subspecies mediasiatica.

The majority of infections in humans and animals are caused by F. tularensis subspecies tularensis (the more virulent subspecies) and F. tularensis subspecies holarctica. F. tularensis subspecies tularensis is confined to North America, whereas subspecies holarctica is widely distributed over the Northern Hemisphere and has also been identified in Australia [13,15,16]

Within a single region, animal and human outbreaks may involve multiple F. tularensis types [17,18]

Human disease is rarely associated with the subspecies novicida, F. philomiragia, F. hispaniensis, or F. opportunistica [1,4,7,11,19].

Strains of subspecies mediasiatica are rare and have only been isolated from Central Asia and Siberia. No human infections caused by the subspecies have been reported.

Francisella spp are small, pale-staining, slow-growing, aerobic/microaerophilic, gram-negative coccobacilli. The organisms are fastidious and infrequently isolated from clinical specimens [11]. Most strains require cysteine or ϲyѕtinе for growth, but strains that are less fastidious or lack a cysteine requirement have been described [20]

F. tularensis grows on some commercial media, including chocolate agar, modified Thayer-Martin media, and buffered charcoal-yeast extract agar [11]. Culture material from nonsterile sites should be plated on supportive media supplemented with antibiotics. Growth in the laboratory is optimal at 35°C, with or without carbon dioxide supplementation

Cultures on solid media may require 2 to 5 days of incubation to observe growth, and blood cultures may require 5 to 10 days of incubation [11]. Iron supplementation of blood cultures can shorten the time to successful isolation of F. tularensis [21].

Isolates suspicious for F. tularensis can be characterized as tiny, poorly staining, gram-negative coccobacilli that are slow growing, grow better on chocolate agar than on blood agar, do not grow on routine gram-negative selective media, are oxidase negative, are catalase negative or weakly catalase positive, are beta-lactamase positive, and are satellite or XV test negative [22]. These features should alert laboratory personnel that an isolate may be F. tularensis

Specimens from patients or animals suspected of having tսlаrеmia may be processed initially using Biosafety Level 2 practices, containment equipment, and facilities [11,23,24]. Any isolate suspected of being F. tularensis should be handled using Biosafety Level 3 practices, containment equipment, and facilities and, in the United States, immediately referred to the state public health laboratory or another appropriate

In particular, automated identification systems should not be used because they may generate aerosols and misidentify F. tularensis. In addition, automated systems have misidentified Veillonella, Pseudomonas species, and Paenibacillus as F. tularensis [11,25,26].

Τսlаrеmia is very uncommon in Australia, England, and South American countries. Although tսlarеmiа has rarely been reported in African countries, there is increasing evidence of its presence in Africa [28-30]

In North America, F. tularensis has been described in the United States, Canada, and Mexico. In these regions, F. tularensis subspecies tularensis, the most virulent subspecies, accounts for approximately 90 percent of human infections

In the United States, almost 2500 cases were reported to the Centers for Disease Control and Prevention (CDC) between 2011 and 2022, with approximately 200 cases per year on average [31].

Over time, however, the southern border of tսlaremiа has shifted northward [33].

In the United States, tսlаrеmia can occur throughout the year but more commonly occurs in the summer months, from May to September (figure 2) (because of greater exposure to arthropods during this time) [32].

Europe and Asia — Τularemiа is known to be endemic in many European countries and in the former Soviet Union, Turkey, Iran, China, and Japan. Outside of North America, F. tularensis subspecies holarctica, a less virulent subspecies, is the predominant cause of infection.

The organism infects more than 100 species of wild and domestic vertebrates and over 100 species of invertebrates

The most important vertebrates associated with F. tularensis infection are lagomorphs (eg, rabbits and hares) and rodents.

● In the United States, important mammals include rabbits, beavers, muskrats, squirrels, and voles.

● In Europe, important mammals include hares, mice, rats, beavers, lemmings, and voles.

Transmission associated with necropsy of a harbor seal in Washington state suggests that marine mammals also may be a potential source of human infection [37]

Animals vary in their susceptibility and response to infection. Some lagomorphs, such as certain hare species, and rodents tend to develop a fatal illness when infected, while many other species develop infection but do not succumb to the illness

Francisella is hardy in nature (though it can be difficult to isolate in the laboratory). The organism can persist in animal carcasses, mud, or water for many weeks [38].

Humans can acquire infection by several routes (direct or indirect animal contact and arthropod bites) and manifest a variety of clinical syndromes depending on the route of transmission.

Human-to-human spread does not occur [ 39]. However, transmissions during an autopsy and from solid organ transplantation have been documented [40,41].

Overall, tick exposure during the summer months is the most commonly recognized mode of transmission in the United States, particularly in children [42]. Other arthropod vectors include mosquitoes, horse flies, fleas, and lice [43]

In Finland, Sweden, and areas of the former Soviet Union, mosquitoes constitute the most important vector [ 44]

F. tularensis may be passed transstadially (ie, across life stages) in ticks. However, studies attempting to demonstrate transovarial transmission have yielded conflicting results [45].

Transmission to humans can occur from contact with an infected animal, such as when hunting or skinning mammals. The trade in wild animals as pets is another means of exposure to infected animals [46]

An animal bite or scratch can also transmit infection; carnivores, such as wild animals, domestic cats, and, in at least one instance, birds can transmit tularemiа through carriage of the organism on claws or in the mouth [ 1,47,48].

Pet dogs may transmit infection by direct contact from bites, scratches, or face licking, or indirectly by facilitating exposure to contaminated carcasses or ticks [ 49,50]

Infection can also occur through splashing infected material from animals or ticks into the eyes or rubbing the eyes with contaminated fingers.

F. tularensis can survive in water for long duration, even if the water is brackish or frozen [1,38,54,55]. Francisella species survival is enhanced in the presence of amoeba, and this may contribute to its persistence in water [56].

As examples, transmission from unchlorinated drinking water occurred during several different outbreaks in Turkey, and an unusual transmission from a fish hook injury on a freshwater lake in the United States has been reported [51,52].

Transmission can occur from airborne spread of contaminated materials, including dust, hay, and water, as well as laboratory specimens. Activities that can increase the risk for airborne exposure to F. tularensis include lawn mowing or cutting brush.

In part because of this potential for airborne transmission, F. tularensis is a category A biοterrοrism agent (ie, of highest concern for biοtеrroriѕm use), as classified by the United States Centers for Disease Control and Prevention

Clustered cases (mainly of pneumonic tularеmiа but also of typhoidal and other forms) occurring without the expected epidemiologic exposures to animals, arthropods, or environmental activities should raise the possibility of a biοterrorism event.

The main risk factor for tulаremia is exposure to the organism. Given the routes of transmission, farmers, veterinarians, hunters, landscapers, national park service employees, meat handlers, and laboratory workers are among the occupations that have an increased risk for tulаremia.

Τսlаremia may occur at any age, but in the United States, it is most common in children under 15 years of age and in middle-aged adults (figure 3). In all age groups, males are infected more often than females; perhaps they are more likely to participate in at-risk activities

Cf graphique U2D

F. tularensis is a highly virulent organism; only 10 to 50 intradermal or inhaled organisms are required to produce clinical illness.

F. tularensis subspecies tularensis (type A) in general causes a more severe clinical infection than subspecies holarctica (type B).

After multiplying at the site of inoculation, F. tularensis spreads first to regional lymph nodes and may then spread systemically through a lymphohematogenous route. At sites of inoculation, there is an acute inflammatory reaction involving neutrophils, macrophages, and lymphocytes that results in tissue necrosis [59]. Granulomas may form and occasionally caseate, mimicking tuberculosis. Despite this host reaction, F. tularensis may remain alive in tissues for some time.

F. tularensis is a facultative intracellular pathogen that replicates primarily within host macrophages, but may also infect many other cell types [60-62].

Adherence of F. tularensis to host cells is mediated by type IV pili and/or other bacterial surface proteins [63].

Macrophage uptake of F. tularensis is a unique process called "looping phagocytosis" that involves asymmetric and spacious pseudopod loops [65].

Entry into macrophages is dependent upon complement and complement receptors; alternative pathways involve mannose receptors, class A scavenger receptors, Fc gamma receptors, and cell surface nucleolin

After being taken up, phagosome maturation and phagosome-lysosome fusion are impaired, and F. tularensis quickly escapes into the cytosol. Francisella bacteria multiply in the cytosol, induce macrophage cell death, and are released to further spread the infection. Similar events occur in neutrophils [67]. After phagocytosis, human neutrophils are unable to kill F. tularensis; the organism inhibits NADPH oxidase, the oxidative burst is suppressed, and the organism escapes into the neutrophil cytoplasm [66,67]

F. tularensis also can inhibit apoptosis of neutrophils and prolong survival of infected neutrophils

In a mouse model of F. tularensis infection, most organisms recovered from blood were not in leukocytes but, rather, free in plasma [68]. This was independent of the inoculum size, virulence, or route of administration. The organisms grew well in whole blood but not in plasma, suggesting a requirement for host cells. These observations suggest that F. tularensis in the blood may be taken up by leukocytes, in which it replicates, and then escapes into the plasma where it can begin a cycle of reinfection.

It has also been shown that F. tularensis can infect erythrocytes and persist within them. Intraerythrocytic organisms are protected from killing by gentamicin, perhaps contributing to relapse after inadequate treatment [69]. Furthermore, intraerythrocytic F. tularensis are protected from the acidic pH of the tick gut; this enhances tick colonization by the organism [70].

An early innate host response functions to contain infection prior to the development of acquired immunity, but is not sufficient to clear the infection.

Within the macrophage cytosol, organisms activate a host-protective multimolecular complex, the iոflаmmаsоmе, leading to the release of proinflammatory cytokines (interleukin-1 beta and interleukin-18) that trigger caspase-1 dependent cell death [ 71]. These effects require type I interferon signaling. Other mechanisms involved in the innate immune response include neutrophils, macrophages, dendritic cells, natural killer (NK) cells, lymphocytes, iոterleukiո-12, tumor necrosis factor-alpha, interferon gamma, and Toll-like receptor (TLR) 2 with either TLR1 or TLR6 [61,72].

Natural antibody responses to carbohydrate antigens occur after the development of cellular immunity but are insufficient to protect against infection [73].

In contrast, recovery from infection is dependent upon the development of cell-mediated immunity. Cell-mediated immunity is directed against protein antigens and is dependent upon CD4+ and CD8+ T-cells [ 61,74]. As a consequence, macrophages are activated to kill intracellular F. tularensis through a process that involves tumor necrosis factor-alpha, interferon gamma, and the production of reactive nitrogen and oxygen species [74].

Both humoral and cellular mechanisms are needed to optimize protective immunity in mice, but their relative importance in humans is unknown

The virulence of F. tularensis has been correlated with several of its phenotypic characteristics, including encapsulation, lipopolysaccharides (LPS), pili, and production of acid phosphatases and a siderophore, as well as a stealth-like behavior [61,75].

Subsequently, genomic studies have identified specific genetic correlates of virulence, including a cluster of genes known as the Francisella pathogenicity island (FPI) [61]. FPI genes are involved in animal virulence and phagosomal escape, and encode an atypical type VI secretion system, which is induced during intracellular growth. FPI genes are under transcriptional regulation responsive to environmental factors, including intracellular growth, iron limitation, and oxidative stress.

In addition to preventing phagosome-lysosome fusion and intracellular killing, F. tularensis is capable of delaying, suppressing, or avoiding many other host immune responses to its presence. These include resistance to complement-mediated lysis, poor recognition of F. tularensis LPS by TLR4, repression of the host cell iոflаmmаѕome, destabilization of host cell mRNA and impairment of proinflammatory cytokine production, the induction of prostaglandin E2 (PGE2) secretion that can inhibit interferon gamma production, and the alteration of host cell mitochondrial function [76-81].

F. tularensis (see Box 29-1) is the causative agent of tulare- mia (also called glandular fever, rabbit fever, tick fever, and deerfly fever) in animals and humans.

Three species of the genus Francisella are associated with human disease: Francisella tularensis, Francisella novicida, and Francisella philomiragia

F. novicida and F. philomiragia are uncommon, opportunistic pathogens that have a predilection for patients with immunologic deficiencies (i.e., chronic granulomatous disease, myeloproliferative diseases)

F. tularensis is a very small (0.2 × 0.2 to 0.7 μm), faintly staining, gram-negative coccobacillus (Figure 29-4).

is nonmotile, has a thin lipid capsule, and has fastidious growth requirements (i.e., most strains require cysteine for growth). It is strictly aerobic and requires 3 or more days before growth is detected in culture

Pathogenic strains possess an antiphagocytic, polysaccharide-rich capsule, and loss of the capsule is asso- ciated with decreased virulence. The capsule protects the bacteria from complement-mediated killing during the bac- teremia phase of disease.

This organism has an endotoxin, but it is considerably less active than the endotoxin found in other gram-negative rods.

A strong, innate immune response with production of interferon (IFN)-γ and tumor necrosis factor is important for controlling bacterial replication in macrophages in the early phase of infection. Specific T-cell immunity is required for activation of macrophages for intracellular killing in the late stages of disease. B cell–mediated immunity is less important for elimination of this facultative intracellular pathogen

F. tularensis subsp. tularensis (type A) is restricted to North America, whereas subsp. holarctica (type B) is endemic throughout the Northern Hemisphere. Type A strains are further subdivided into type A–west, which predominates in the arid region from the Rocky Mountains to the Sierra Nevada Mountains, and type A–east, which occurs in the central southeast states of Arkansas, Missouri, and Okla- homa and along the Atlantic Coast. Type B strains cluster along major waterways, such as the upper Mississippi River, and in areas with high rainfall, such as the Pacific Northwest.

Type A infections are most commonly associated with exposure to lagomorphs (rabbits, hares) and cats; type B infections are associated with rodents and cats, but not lagomorphs. Infections caused by biting arthropods (e.g., hard ticks [Ixodes, Dermacentor, Amblyomma spp.], deerflies) are more common with type A than with type B strains.

Type A–east infections are more commonly associated with disseminated disease and a high mortality rate when compared with disease caused by type A–west strains; the course of disease caused by type B stains is intermediate.

The spread to type A–east strains from the central southeast states to the Atlantic Coast states occurred when infected rabbits were imported from the central states to East Coast hunting clubs in the 1920s and 1930s.

The reported incidence of disease is low. In 2012, 149 cases were reported in the United States; however, the actual number of infections is likely to be much higher because tularemia is frequently unsuspected and is difficult to confirm by laboratory tests.

The incidence of disease increases dramatically when a relatively warm winter is followed by a wet summer, causing the tick population to proliferate.

In areas where the organism is endemic, it is said that if a rabbit is moving so slowly that it can be shot by a hunter or caught by a pet, the rabbit could be infected (Clinical Case 29-5).

Clinical Case 29-5 Cat-Associated Tularemia Capellan and Fong (Clin Infect Dis 16:472–475, 1993) described a 63-year- old man who developed ulceroglandular tularemia complicated by pneumo- nia after a cat bite. He initially presented with pain and localized swelling of his thumb 5 days after the bite. Oral penicillins were prescribed, but the patient’s condition worsened, with increased local pain, swelling and ery- thema at the wound site, and systemic signs (fever, malaise, vomiting). Incision of the wound was performed, but no abscess was found; culture of the wound was positive for a light growth of coagulase-negative staphy- lococci. Intravenous penicillins were prescribed, but the patient continued to deteriorate, with the development of tender axillary lymphadenopathy and pulmonary symptoms. A chest radiograph revealed pneumonic infiltrates in the right middle and lower lobes of the lung. The patient’s therapy was changed to clindamycin and gentamicin, which was followed by deferves- cence and improvement of his clinical status. After 3 days of incubation, tiny colonies of faintly staining gram-negative coccobacilli were observed on the original wound culture. The organism was referred to a national refer- ence laboratory, where it was identified as Francisella tularensis. A more complete history revealed the patient’s cat lived outdoors and fed on wild rodents. This case illustrates the difficulty in making the diagnosis of tula- remia and the lack of responsiveness to penicillins

Ulceroglandular tularemia is the most common manifes- tation. The skin lesion, which starts as a painful papule, develops at the site of the tick bite or direct inoculation of the organism into the skin (e.g., a laboratory accident). The papule then ulcerates and has a necrotic center and raised border. Localized lymphadenopathy and bacteremia are also typically present (although bacteremia may be difficult to document)

Oculoglandular tularemia (Figure 29-5) is a specialized form of the disease and results from direct contamination of the eye. The organism can be introduced into the eyes, for example, by contaminated fingers or through exposure to water or aerosols. Affected patients have a painful conjunc- tivitis and regional lymphadenopathy

Pneumonic tularemia (Figure 29-6) results from inhala- tion of infectious aerosols and is associated with high mor- bidity and mortality unless the organism is recovered rapidly in blood cultures (it is generally difficult to detect in respira- tory cultures)

Collection and processing of specimens for the isolation of F. tularensis are hazardous for both the physician and the laboratory worker. The organism, by virtue of its small size, can penetrate through unbroken skin and the mucous mem- branes during collection of the sample, or it can be inhaled if aerosols are produced (a particular concern during pro- cessing of specimens in the laboratory).

Detection of F. tularensis in Gram-stained aspirates from infected nodes or ulcers is almost always unsuccessful because the organism is extremely small and stains faintly (see Figure 29-4).

Blood cultures are generally negative for the organism unless the cultures are incubated for a week or longer.

Cultures of respiratory specimens will be positive if appropriate selective media are used to suppress the more rapidly growing bacteria from the upper respiratory tract

F. tularensis also grows on the selective media used for Legio- nella (e.g., BCYE agar). Aspirates of lymph nodes or draining sinuses are usually positive if the cultures are incubated for 3 days or longer.

Preliminary identification of F. tularensis is based on the slow growth of very small gram-negative coccobacilli on chocolate agar but not blood agar (blood agar is not supple- mented with cysteine). The identification is confirmed by demonstrating the reactivity of the bacteria with specific antiserum (i.e., agglutination of the organism with antibod- ies against Francisella)

Tularemia is diagnosed in most patients by the finding of a fourfold or greater increase in the titer of antibodies during the illness or a single titer of 1 : 160 or greater. However, antibodies (including immunoglobulin [Ig]G, IgM, and IgA) can persist for many years, making it difficult to differentiate between past and current disease

F. tularensis strains produce β-lactamase, which renders penicillins and cephalosporins ineffective.

The mor- tality rate is less than 1% if patients are treated promptly but is much higher in untreated patients, particularly those infected with type A–east strains

Because the organism is present in the arthropod’s feces and not saliva, the tick must feed for a prolonged time before the infection is transmitted. Prompt removal of the tick can therefore prevent infection