FRANCISELLA TULARENSIS (a.k.a. Pasteurella tularensis)^ref

Table of contents :

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Epidemiology Genomics Transmission Pathogenesis Symptoms & signs Laboratory examinations Therapy Prognosis Prevention

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• Francisella tularensis subsp. holarctica (a.k.a. type B; biogroup palearctica; formerly referred to as palaearctica or palearctica in some sources): less virulent; does not demonstrate citrulline ureidase activity; 3 biovars have been identified :
• biovar I : erythromycin sensitive; primarily found in North America, Europe, Siberia, the Far East, and Kazakhstan
• biovar II : erythromycin resistant; primarily found in Eurasia
• biovar japonica : found in Japan
Epidemiology : dominant strain in Eurasia; co-dominant in USA; in Sweden, human outbreaks have been registered every 5 to 10 years during this century. In the intervening years there may be none or only a handful of cases reported. The largest recorded outbreak happened in 1966-67 when over 2800 people (mainly farmers) got the pneumonic form of tularemia^ref
• Francisella tularensis subsp. mediasiatica : found in the Central Asian republics of the former Soviet Union (virulence is similar to F. tularensis, subspecies holarctica)
• Francisella tularensis subsp. novicida : considered to be of low virulence and generally causes illness only in immunocompromised hosts. F. tularensis and F. novicida traditionally have been considered separate species; however, the current approach is to consider F. novicida as a
subspecies of F. tularensis
• Francisella tularensis subsp. tularensis (a.k.a. type A; biogroup tularensis; also referred to as subspecies nearctica by investigators in the former Soviet Union): demonstrates citrulline ureidase activity; highly infectious and virulent). Type A is said to be more virulent than type B

Epidemiology : dominant strain in Eurasia; co-dominant in North America; 2 distinct genetic clades have been identified^ref
• Francisella tularensis strain Schu 4 : lacks pOM1 and pNFL10 plasmids found in low virulence strains, and has genes for enzymes in the shikimate, purine biosynthetic and salvage pathways

Epidemiology : this infectious disease was first described in Japan in 1837. The name of the disease is related to 1911 description of a plague-like illness in ground squirrels in Tulare County, California, and work done by Dr. Edward Francis. Dr. Francis described his personal experience with more than 800 cases in 1928.

• USA: the disease seems to be more endemic in the midwestern USA as well as Martha's Vineyard, an 100-square-mile island off the coast of Massachusetts, USA, with 37 cases affected from 2000 to 2005. In August 2000, a Chilmark resident died of the pneumonic form of tularemia and 14 other cases were successfully treated that summer. In 2001, 3 people, including a 4-year-old Newton boy, who was bitten by a tick apparently on Martha's Vineyard came down with the disease, and, later recovered. 2 cases were reported in 2002, 4 in 2003, 1 in 2004, and 8 in 2005. Prior to 2000, Martha's Vineyard had not had an outbreak since 1978, when 12 people were stricken. The sudden appearance of tularemia on Cape Cod, MA, was associated with the importation into the mainland state and Martha's Vineyard of 29 689 cottontail rabbits from the midwestern states of Missouri and Kansas from 1937 to 1940 by various Massachusetts game clubs (Belding DL, Merrill: Tularemia in imported rabits in Massachusetts. N Engl J Med 1941; 1085-87). 340 of these rabbits were released on Nantucket Island. Prior to 1937, only 1 case of the infection was recognized, related to contaminated rabbit meat from the midwestern USA^ref. Useful information on the disease in cats and the association of certain types of ticks, the potential for tick exposure, and the occurrence of disease in animals and humans^{ref1, ref2}. The genetic diversity of F. tularensis obtained from Martha's Vineyard ticks was examined : between 2001 and 2003, 0.7% of 4246 dog ticks (Dermacentor variabilis) harbored the tularemia bacillus, and there was a degree of genotypic diversity suggestive of long-standing enzootic transmission of the infection on the island. USA averaged 124 cases of tularemia in 1999 and 2000, usually < 2% of which are fatal. Since 1965, there have been 13 other cases in New York city; the last occurred in 1998. Tularemia in humans is generally a rural disease, and occurs naturally throughout much of North America and Eurasia. Natural outbreaks of tularemia are relatively rare^ref. Documented outbreaks under wartime conditions, however, including an outbreak in 1942 during the Battle of Stalingrad^ref and the 2002 report of hundreds of cases in Kosovo^ref, indicate the heightened risk associated with susceptible environments, ie, areas in which agriculture, water, and overall sanitation have been severely disrupted. 3 Boston University researchers became infected with tularemia (2 in May 2004 and 1 in Sep 2004), when they thought they were working with a harmless form of the germ supplied by a lab on the South End medical campus of University of Nebraska in Lincoln (USA) because naturally occurring lethal type A strain contaminated pooled rabbit blood from a farm in South Carolina used to promote growth of the bacteria^ref. Several government environmental air monitors in the Mall area detected low levels of Francisella tularensis bacteria that cause tularemia, on 24-25 Sep 2005, as thousands of protesters marched against the Iraq War. Public health agencies had no reports of any related human or animal illnesses caused by the bacteria. Since the bacterium is ubiquitous, unlike plague, the BioWatch system may detect its nucleic acid if, for instance, material containing the organism is aerosolized by landscaping or yard work. Were any findings of investigations into the similar situation in Houston in Oct 2003 made public? Soil itself is not a reservoir -- e.g., tularemia does not persist in soil in spore form like anthrax or tetanus. Basically if you want a new strain of F.tularensis, go to your nearest stream or other body of water! The disease is uncommon in DC, though it was common in the 1920s and 1930s among market men handling wild rabbits (from Jellison's text "Tularemia in North America 1930-1974").  Indeed, Jellison stated that by 1929, the District ranked 4th in North America in total cases of the disease diagnosed, following Ohio, Montana, and Tennessee. [Given relative sizes, DC seemed to be the highest in cases per square mile]. From 1948-1974, there were only 6 cases of tularemia in DC. From 2000 to 2004, there were no cases reported in DC (from CDC "Summary of Notifiable Diseases"). Back in 1978 there was an outbreak in DC of 3 cases^ref: in June 1978, 3 cases of tularemia pneumonia occurred in persons residing in the Washington, DC, area. The patients, all men, became ill 3 to 4 days after a brief session training their hunting dogs in an undeveloped wooded area adjacent to a housing complex. One of the dogs, which later died, had captured a wild rabbit during the training session. All 3 men had handled the rabbit while familiarizing their dogs with the rabbit's scent. The men had no other common exposure that was a likely source of infection. One might also begin to look at the resident squirrel population in the DC mall area to see whether they are indeed serving as reservoirs for F. tularensis. There certainly is a large population of them, as any visitor can attest, and this would be consistent with theories expressed concerning tularemia bacillus presence in the environment -- that is, through carcasses decaying in the soil, airborne dissemination of contaminated dust/dirt, and possible presence short term in local water pools. We have seen similar sampling and detection systems based on sensitive PCR techniques show up positive in the past out here in Utah, prior to the 2002 Olympic Games. These were generally explained away, given the endemic nature of F. tularensis in the local rabbit population, the time of year, and environmental climate changes (fall season is cooler, cloudier with more humidity), etc. For DC, however, we need to test the idea of indigenous host reservoirs to address the question of source rather than to just say that it is not associated with any terrorist activity. The National Park Service can start by rounding up some squirrels for testing. The incidence of Lyme disease in the United States is greater than 5-fold that of all other tick-borne infections combined^ref. If we eliminate Lyme disease, we find that tularemia was the most common disease in this group until 1965, but has since been superseded by Rocky Mountain spotted fever -- and more recently Ehrlichiosis^ref --. Most significantly, although deaths due to other tick-borne diseases have declined in recent years, the number dying from tularemia has remained surprisingly constant for over 4 decades^ref.
• Russia : outbreaks occurred in central Russia since March 2005 (334 people with symptoms of tularemia - laboratory-confirmed in 128 persons -, including 54 cases among children under 14 years of age, have been reported in the Nizhny Novgorod (99), Voronezh (28), Moscow (135), and Vladimir (82) regions)^ref
• Germany : 3 cases in 2004, 5 cases in 2005

Genomics : the complete genome sequence of a highly virulent isolate of F. tularensis (1,892,819 bp) uncovers previously uncharacterized genes encoding type IV pili, a surface polysaccharide and iron-acquisition systems. Several virulence-associated genes were located in a putative pathogenicity island, which was duplicated in the genome. > 10% of the putative coding sequences contained insertion-deletion or substitution mutations and seemed to be deteriorating. The genome is rich in IS elements, including IS630 Tc-1 mariner family transposons, which are not expected in a prokaryote. A computational method was used for predicting metabolic pathways and found an unexpectedly high proportion of disrupted pathways, explaining the fastidious nutritional requirements of the bacterium. The loss of biosynthetic pathways indicates that F. tularensis is an obligate host-dependent bacterium in its natural life cycle^ref

bites from ... :
• 125 types of infected vertebrates (Rodentia as Cynomys parvidens, Oryctolagus cuniculus (die within 7 days of getting tularemia; also from dogs after they capture infected wild rabbit and then lick humans on the face ), Lepus europeus, black-tailed jack rabbit (Lepus californicus), squirrels, Ondatra zibethicus and Castoridae)
• dogs are thought to be resistant to natural infection. Clinical disease, when it occurs, typically consists of fever, anorexia, and listlessness. Additional symptoms may include skin or mouth lesions, draining abscesses, enlarged lymph nodes, and nasal and ocular discharge.
• cats are thought to be more susceptible to tularemia than dogs, but still considered fairly resistant to natural infection. Clinical disease, when it occurs, may consist of fever, anorexia, listlessness, skin or mouth lesions, draining abscesses, and enlarged lymph nodes.
• livestock, particularly sheep, pigs, and horses, are variably susceptible to tularemia infection. Large epidemics with high mortality have been seen in range sheep. Clinical signs are not specific, and usually include fever, anorexia, and depression. Cough, rapid respiration, diarrhea, and stiffness and edema of the limbs may also occur.
• cattle appear to be resistant
• little is known of the true incidence and spectrum of clinical disease in domestic animals. Important wild animal hosts include cottontail and jackrabbits, beaver, muskrat, meadow voles, and sheep in North America, and other voles, field mice, and lemmings in Europe and Asia.
The incubation period is 1-10 days. In sheep and most mammals, the disease is characterized by sudden onset of high fever, lethargy, anorexia, stiffness, reduced mobility, or other signs associated with septicemic disease. Pulse and respiratory rates are increased. Coughing, diarrhea, and pollakiuria may develop. Prostration and death may occur in a few hours or days. Sporadic cases are best recognized by signs of septicemia. Outbreaks in untreated lambs may have up to 15 percent mortality. Subclinical cases may be common. The most consistent lesions are miliary, white to off-white foci of necrosis in the liver and sometimes in the spleen and lymph nodes. Enlargement of the liver, spleen, and lymph nodes is common. Organisms can be readily isolated from necropsy specimens by use of special media. Risk of infection during necropsy or to laboratory personnel is significant; special procedures and facilities are essential. Tularemia must be differentiated from other septicemic diseases (especially plague) or acute pneumonia. When large numbers of sheep show typical signs during periods of heavy tick infestation, tularemia or tick paralysis should be suspected. Tularemia should be considered in cats with signs of acute lymphadenopathy, malaise, oral ulcers, and history of recent ingestion of wild prey. Diagnosis of acute infection is confirmed by culture and identification of the bacterium, direct or indirect fluorescent antibody test, or a 4-fold increase in antibody titer between acute and convalescent serum specimens. A single titer of =1:80 by the tube agglutination test is presumptive evidence of prior infection.
• infected vectors (Chrysops discalis, Aedes cinereus, Dermacentor andersoni and Dermacentor variabilis)
=> ulceroglandular tularemia : regional lymphadenitis and ulcer at entry point (absent in glandular form), malaise, headache, chills, fever. F. tularensis is capable of surviving for weeks at low temperatures in water, moist soil, or decaying plant and animal matter. Although hundreds of differing vertebrates and invertebrates can be infected with the tularemia bacillus, < 12 are important in its ecology. In Oct 2004 hamsters from a Canadian pet distributor were found to be infected with type B tularemia as well. No human cases were reported. Tularemia has been associated with hamster hunting in Russia and in Hungary (Szekelyfoldi, J. 1970. Tularemia in hamster hunters. Del Med J 42:305-6). In 1966, 18 cases were diagnosed in which the source of infection was hamsters. These infections occurred in 4 villages in Hungary along the Berettyo River. Of note, tularemia has also been found in bordering Rumania, where an epidemic among hamster hunters was reported in 1955. In addition to Russia, there has also been tularemia in hamster hunters in Ukraine. In April-May 1934, an outbreak occurred in connection with the hunting of water rats, hamsters, and hares
conjunctival infection from handling carcasses of infected animals => oculoglandular tularemia : conjunctivitis, chemosis, palpebral edema, Parinaud's oculoglandular syndrome, epiphora and photophobia associated to preauricolar, submandibular and laterocervical lymphadenitis
ingestion of contaminated food or water => oropharyngeal tularemia : ulceronecrotic angina (sometimes including laterocervical or sub-mandibular lymphadenitis) => typhoidal or gastroenteric tularemia : malaise, fever, weight loss, no focal signs or symptoms. Several large outbreaks of typhoidal tularemia with pneumonia have occurred in the USA, Finland, and Sweden. Water-related outbreaks include approximately 30 outbreaks in the then Soviet Union during the period 1936 to 1963, 64 cases in Dagestan (2001), Russia associated with a flood-plain swamp^ref, 49 cases in Tuscany, Italy (1982) linked to an unchlorinated water system^ref, and an outbreak in the Smolensk province of Russia (1997) also attributed to contamination of a water supply^ref
inhalation of aerosols of the organism (although it is associated with biological terrorism, it may also occur naturally in a setting such as contaminated moving lawns, as is exemplified by the sporadic case described in New York City and the outbreak among gardeners on Martha's Vineyard, dust from from fodder, grain or wool contaminated with rodent excreta or dead rodents, or farmers working with hay) => primary pneumonic tularemia (mortality if untreated : 40%) =septicemia=> secondary alveolar pneumonia, endocarditis, pericarditis, peritonitis, appendicitis, osteomyelitis and meningitis. Fever, cough, focal pulmonary infiltrates, hilar lymphadenopathy with or without pharyngitis or bronchiolitis
non-animate environmental sources of F. tularensis : investigations into the cluster of pneumonic tularemia on Martha's Vineyard^ref, showed that primary pneumonic tularemia had been reported in persons who disturbed the carcasses of infected rabbits, in European farmers who worked with contaminated hay, in 2 boys who mowed over a rabbit, and in a man who used a brush cutter to clear a lot where many rabbits lived. In addition, they comment that lawn mowing has been epidemiologically implicated in an outbreak of psittacosis, in which patients were no more likely than controls to

keep, handle, or feed birds but were more likely to have mowed lawns and lawn mowing may have aerosolized Chlamydia psittaci shed by sick birds. Rodents can excrete viable F. tularensis in both urine and feces, and Feldman, et al. propose that on Martha's Vineyard, F. tularensis was shed in animal excreta, persisted in the environment, and infected people after being mechanically aerosolized and inhaled. The authors felt that the outbreak indicates that people can acquire primary pneumonic tularemia from mowing in the absence of any obvious exposure to infected animal tissue.
environmental survival : although F. tularensis does not form spores, it can survive in water, soil, and decaying animal carcasses. Feldman et al.^ref reported that the organism persists in water and mud for as long as 14 weeks, in oats for 4 months and in straw for 6 months. Although they were unsuccessful in culturing F. tularensis from environmental samples, the optimal methods for collecting and isolating the organism from grass and air have not been determined. In addition, they may have collected samples under environmental conditions that differed from those existing when the patients were exposed. I am unaware of any primary literature that documents this for Type A (Francisella tularensis tularensis). Indeed, the North American reports of persistence of F. tularensis in water and mud (classic studies by Jellison and others) need to be reexamined, because it is unclear which subspecies was present. I would bet that it was all Type B. There is no question that Type B (F. tularensis palaearctica) persists in water and mud, and large outbreaks have been recorded in Eurasia associated with arvicoline living in the fields or countryside, rodent excreta contaminating hay or other environmental materials. Type B is a very different bug from Type A. Type A depends on ticks for its enzootic cycle. Type B may have multiple modes of perpetuation, including emerging evidence for an association with free-living amebae, which might explain prolonged persistence in wet environments. Naked bacteria of either type seem extremely fastidious and labile in vitro, a fact that stands in contrast to a perception that the agent of tularemia is environmentally persistent. Environmental material may certainly be contaminated by excreta or animal carcasses and serve as fomites, but the duration of infectivity of such materials is extremely poorly defined except for Type B. The suggestion that Type A (which is most likely the one detected around DC) is stable in soil (by soil I mean dry earth, as opposed to mud, which has a large aqueous fraction) for any length of time needs to be backed up by finding primary peer-reviewed publications, and not by referring to discussion within reviews or reports of outbreaks wherein arguments are being made about the most likely mode of transmission. Facts get altered when they are passed from review to review, and from recent discussions referencing such reviews. In addition, it is not correct to equate the physiologic attributes of one F. tularensis subspecies with another. As for urban tularemia, dog ticks (Dermacentor variabilis) are important vectors of Type A. Dog ticks certainly occur in urban areas, as do their main reproductive hosts, raccoons and skunks. The potential for zoonoses transmitted by urban dog ticks was underscored by the cluster of Rocky Mountain spotted fever cases in a Manhattan park^ref. Cottontail rabbits and their ticks (Ixodes dentatus, Haemaphysalis leporispalustris) are also fairly common in urban peridomestic settings.

Although the ID₅₀ of the bacillus in the laboratory is < 100 organisms, the lung infection does not transmit from person-to-person.
Laboratory examinations : PCR, intradermoreaction
Therapy : the drugs of choice are streptomycin (1 g intramuscularly [IM] twice daily) or gentamicin (5 mg/kg IM or intravenously [IV] once daily). Alternatives are ciprofloxacin (400 mg IV twice daily) or the bacteriostatic drugs doxycycline (100 mg IV twice daily) or chloramphenicol (15 mg/kg IV 4 times daily). (The dosages listed are for adults.) Suggested duration of treatment is 10 days for streptomycin, gentamicin, or ciprofloxacin and 14 days for doxycycline and chloramphenicol. When clinically indicated, a change to oral therapy can be made with the latter 3 medications. It should be noted that gentamicin, chloramphenicol, and ciprofloxacin, although quite active, are not FDA-approved for the treatment of tularemia. b-lactam antimicrobials and macrolides are not recommended for treatment. For postexposure prophylaxis in the setting of biowarfare, the Working Group recommends the use of oral doxycycline (adult dosage of 100 mg orally twice daily) or ciprofloxacin (adult dosage of 500 mg orally twice daily) for 14 days.
Prognosis : in the pre-antimicrobial era, the mortality from type A infections was between 5-15% of cases but as high as 30-60% for untreated pneumonic and typhoidal forms^ref
Prevention : attenuated vaccine
Although not generally transferable from person to person, the infectious dose of F. tularensis is quite low, and the organism is considered by CDC as a category A biological weapon