by Marina Brown
Tularemia, or Rabbit fever, is caused by the gram-negative coccobacillus Francisella tularensis. The disease mainly affects mammals, especially rodents, rabbits and hares, though it can also infect birds, reptiles and fish. Though a rare disease, Tularemia is highly infectious and can be fatal if not treated. F. tularensis is a facultative intracellular bacterium that is capable of infecting most cell types but primarily infects macrophages in the host organism, thus enabling it to evade the immune system.[i] F. tularensis enters the macrophage by phagocytosis and the bacterium is confiscated from the interior of the infected cell by a phagosome. F. tularensis then breaks out of this phagosome into the cytosol and rapidly proliferates. Eventually the infected cell undergoes apoptosis and the bacteria are released to initiate new rounds of infection.
Transmission of Tularemia between humans has not been reported, but the routes of entry for this highly infectious disease are the eyes, mouth, skin, throat or lungs. According to the WHO, “transmission can occur from tick bites, such as the dog tick (Dermacentor variabilis), the wood tick (Dermacentor andersoni), and the lone star tick (Amblyomma americanum). Deer flies (Chrysops spp.) have also been shown to transmit Tularemia in the western United States.”[ii] Also, handling of infected animal tissue usually encourages transmission of Tularemia to humans through skin contact. Usually this occurs when hunting or skinning infected rabbits, muskrats, prairie dogs and other rodents. Many other animals have also been known to become ill with Tularemia including but not limited to domestic cats. They are very susceptible to Tularemia and have been known to transmit the bacteria to humans. Also some cases have been reported in which hamsters bought from a pet store were infected with the disease, and those bitten by the hamsters subsequently contracted the disease. Tularemia can also be transmitted by eating undercooked meat of the infected animal. Although rare, humans can acquire Tularemia by inhaling dust or aerosols contaminated with F. tularensis bacteria. This can occur during farming or landscaping activities, especially when mowers or tractors run over infected animals or decaying carcasses. Transmission is found to be greater in Europe than in America.
Tularemia is found in natural reservoirs including small mammals such as voles, mice, water rats, squirrels, rabbits, and hares. Naturally acquired human infection occurs through inoculation into skin or mucous membrane; through bites of infected arthropods; handling infectious animal tissues or fluids; direct contact or ingestion of contaminated water, food, or soil; and inhalation of infective aerosols. The CDC calls Tularemia a “mostly rural disease and infections are usually limited to persons at risk due to occupational or recreational exposure to infected animals or their habitat, including rabbit hunters, trappers, persons exposed to ticks or biting insects, and laboratory technicians working with the bacterium.”[iii]
General characteristics of MO:
F. tularensis is a small, gram-negative aerobic coccobacillus; a fastidious organism that requires cysteine for best growth. The colonies grow poorly on sheep blood agar, do not grow at all on MacConkey agar, and grow best on (BCYE) agar, where you will see 1-3mm grey and white colonies after 48-72 hours. In a Thioglycollate broth the growth will be slow, with a denser band near top, diffusing with age. It is also a non-motile organism, and a facultative intracellular parasite of the macrophages, which helps it evade the immune system.
Key tests for identification (specific):
Doctors may check for F. tularensis in a blood or sputum sample that's cultured to encourage the growth of the bacteria. F. tularensis requires special media for cultivation such as buffered charcoal and yeast extract (BCYE). It cannot be isolated in the routine culture media because of the need for sulfhydryl group donors like cystein. According to the Mayo Clinic, “Serological tests (detection of antibodies in the serum of the patients) are available and widely used. Cross reactivity with Brucella can confuse interpretation of the results, and for this reason diagnosis should not rely only on serology.”[iv] Molecular methods of detecting Tularemia, such as PCR, are available in laboratories. A chest X-ray may be necessary to look for signs of pneumonia that are present along with the disease.
Signs and symptoms of disease:
The incubation period for tularemia is 1 to 14 days but most human infections become apparent after 3 to 5 days. In most susceptible mammals, the clinical signs include fever, lethargy, anorexia, signs of septicemia; and without treatment, possibly death. It is very contagious as only 10-50 bacteria are required to cause the illness. Animals only rarely develop the skin lesions commonly seen in people. According to the CDC, “Subclinical infections are common and animals often develop specific antibodies to the organism. Fever is moderate or very high and tularemia bacillus can be isolated from blood cultures often develop specific antibodies to the organism.”[v] The face and eyes redden and become inflamed due to the bacterial infection. Inflammation spreads to the lymph nodes, which enlarge and may look like the Bubonic plague. The inflammation of the lymph nodes causes a fever, and death occurs in less than 1% if therapy is initiated promptly.
In the early 20th century a Japanese physician
named Hachiro Ohara described a disease affecting those who hunted or ate
rabbit. The Japanese called it
meaning rabbit fever, which appeared endemic to the Abakuma Mountains.
According to the Texas State Health Department, “In
1910, a Utah physician, RA Pearse, described deer-fly fever. In 1911, George W.
McCoy and Charles W. Chapin, from the US Public Health Service, began examining
thousands of rats and ground squirrels in order to detect foci of suspected
plague infections in San Francisco.”[vi]
From the observed lesions and the symptoms of the Ground squirrels, plague was
suspected as being the culprit. A year later, however, the causative agent,
Bacterium tularense was isolated to be the microorganism responsible for the
disease in the Ground squirrels.
Dr. Edward Francis,
also from the US Public Health Service, established the cause of deer-fly fever
as Bacterium tularense in 1928, named for Tulare County, California where the
organism was first discovered. Because of his contributions to the discovery of
the bacterium and the disease, the organism was later renamed Francisella
tularensis. JAMA regards F. tularensis as “a potential bioweapons
agent because of its highly infectious nature and it has been included in the
biological warfare programs of the USA, USSR and Japan”[vii]
at various times because it is easily treatable if by chance it were to
mistakenly come in contact with civilians during war. In the mid 1970s the
United States was ordered to stop researching Tularemia as a potential bioweapon.
The virulence mechanisms for F. tularensis have not been well characterized. It is an intracellular bacterium that breaks out of phagosomal compartments to replicate in the cytosol. F. tularensis strains produce different hemolytic agents, which facilitate degradation of the phagosome. According to the Center for Infectious Disease Research and policy (CIDRAP), “A hemolysin protein NlyA with similarity to Pseudomonas aeruginosa HlyA was characterized in biovar novicida whereas an acid phospholipase C AcpA has been found in other strains to act as a hemolysin.”[viii] While F. tularensis does not contain virulence secretion systems typical of most better-characterized pathogenic bacteria, it does contain a number of ATP binding cassette (ABC) proteins that may be linked to the secretion of virulence factors. F. tularensis uses type IV pili to bind to the exterior of a host cell to become phagocytosed. According to CIDRAP, “Mutant strains lacking pili show severely attenuated pathogenicity. The expression of a 23-kD protein known as IglC is required for F. tularensis phagosomal breakout and intracellular replication; in its absence mutant F. tularensis die and are degraded by the macrophage. This protein is located in a putative pathogenicity island regulated by the transcription factor MglA. F. tularensis, in vitro, down regulates the immune response of infected cells, a tactic used by a significant number of pathogenic organisms to ensure that replication is unhindered by the host immune system by blocking the warning signals from the infected cells.”[ix]
If you have been infected with F. tularensis, then the CDC recommends antibiotics such as Streptomycin. Tularemia may also be treated with gentamicin for ten days, tetracycline-class drugs such as doxycycline for 2–3 weeks, chloramphenicol or fluoroquinolones. The best control for the organism is to spray heavily foliaged areas with DEET to kill the ticks that transmit the disease. Tularemia is easily treated with antibiotics, and very rarely fatal.
Prevention/Vaccine info, new trials:
CIDRAP acknowledges that “The attenuated F. tularensis live vaccine strain (LVS) has been used previously under investigational new drug status to vaccinate at-risk individuals. However the history of the strain and lack of knowledge regarding the basis of attenuation has so far prevented its licensing. Therefore the focus of current research is on producing a new vaccine against tularemia that would be suitable for licensing.”[x] Research trials are being performed by the U. S. Army Medical Research Institute of Infectious Diseases in Fort Detrick, Maryland. The trial undergoing investigation is the “Live Francisella Tularensis Vaccine (NDBR101)”[xi], according to the U.S. Army Medical Research Institute. The study started in January 2009, and is estimated to be complete sometime around May 2013. The best form of prevention is to stay away from infected rabbits and rodents, or any other reservoirs known to be infected. Wear long sleeves and long pants when going through highly wooded or grassy areas. It is a good idea to maintain your lawn, keep it mowed short to reduce the risk of flea and tick bites that most commonly transmit the bacterium.
Local cases or outbreaks (with incidence figures):
Because Tularemia is mostly a rural disease, local cases in the United States have occurred sporadically and in small clusters. According to the Journal of the American Medical Association, “In the United States, reported cases have dropped sharply from several thousand per year prior to 1950 to less than 200 per year in the 1990s. Between 1985 and 1992, 1409 cases and 20 deaths were reported in the United States, for a mean of 171 cases per year and a case-fatality rate of 1.4%.”[xii] In the United States, human cases have been reported from every state except Hawaii; however, most cases occur in south-central and western states such as Missouri, Arkansas, Oklahoma, South Dakota, and Montana.
Global cases or outbreaks (with incidence figures):
The worldwide incidence of tularemia is not known, and the disease is probably greatly under recognized and underreported. In April of 2000, Tularemia was identified in the country of Kosovo and confirmed in several laboratories across the region. The first reported case dated back to August 1999. According to the WHO, “The Institute of Public Health has now identified 250 suspected cases spread across almost 90 per cent of the territory, with most cases in the western area.”[xiii] 26 municipalities in Kosovo reported cases of Tularemia. In February of 2002, the WHO confirms that “The Institute of Public Health (IPH) has reported 715 cases of Tularemia since the outbreak began on 1 November 2001. 170 cases have been laboratory confirmed, while 404 suspected cases were found to be negative. An additional 141 suspected cases remain under laboratory and epidemiological investigation.”[xiv]
[iv] "Tularemia." Mayo Clinic. Mayo Clinic Staff, 18 Apr. 2008. Web. 7 Dec. 2009. http://www.mayoclinic.com/health/tularemia/DS00714.
[vi] Tularemia." Texas Department of State health Services. 30 Mar. 2007. Web. 8 Dec. 2009. http://www.dshs.state.tx.us/preparedness/bt_public_history_tularemia.shtm.
[vii] "Tularemia as a Biological Weapon." JAMA. 6 June 2001. Web. 8 Dec. 2009. http://jama.ama-assn.org/cgi/content/full/285/21/276.
[viii] "Tularemia: Current, comprehensive information on pathogenesis, microbiology, epidemiology, diagnosis, treatment, and prophylaxis." CIDRAP. University of Minnesota, 2 Apr. 2009. Web. 8 Dec. 2009. http://www.cidrap.umn.edu/cidrap/content/bt/tularemia/biofacts/tularemiafactsheet.html.
[ix] "Tularemia: Current, comprehensive information on pathogenesis, microbiology, epidemiology, diagnosis, treatment, and prophylaxis." CIDRAP. University of Minnesota, 2 Apr. 2009. Web. 8 Dec. 2009. http://www.cidrap.umn.edu/cidrap/content/bt/tularemia/biofacts/tularemiafactsheet.html.
[x] "Tularemia: Current, comprehensive information on pathogenesis, microbiology, epidemiology, diagnosis, treatment, and prophylaxis." CIDRAP. University of Minnesota, 2 Apr. 2009. Web. 8 Dec. 2009. http://www.cidrap.umn.edu/cidrap/content/bt/tularemia/biofacts/tularemiafactsheet.html.
[xi] "Continued Safety and Immunogenicity Study of a Live Francisella Tularensis Vaccine." Clinicaltrials.gov. U.S. Army Research Institute, 6 Nov. 2008. Web. 8 Dec. 2009. http://clinicaltrials.gov/ct2/show/NCT00787826.
[xii] "Tularemia as a Biological Weapon." JAMA. 6 June 2001. Web. 8 Dec. 2009. http://jama.ama-assn.org/cgi/content/full/285/21/2763.