Tularemia
The Agent
Courtesy: CDC/ Larry Stauffer, Oregon State Public Health Laboratory
Tularemia is caused by the bacterium Francisella tularensis (F. tularensis), one of the most infectious agents known. The bacterium is naturally found in small mammals such as rabbits, rodents, and hares, and in the insects that feed on these animals (for this reason, the disease is sometimes called rabbit fever or deerfly fever). People can become infected with this bacterium as a result of bites by infected flies, ticks, or other insects, handling of infected animal material, eating or drinking contaminated water, or inhaling the bacterium. Tularemia is not known to spread from one person to another.
Cases of infection in the United States are quite rare, with only about 200 cases reported each year, but experts think that some cases of tularemia may not be recognized or reported. Less than two percent of reported cases end in death. The symptoms vary depending on the route of infection and may range from mild illness to severe respiratory illness, including pneumonia and a serious systemic infection. The most dangerous form of the illness is caused by inhalation of the bacterium. Tularemia responds to antibiotics if treated early enough.
The Problem
Francisella tularensis is classified as a highest risk, Category A potential bioterrorism agent. Research programs directed toward the development of F. tularensis as a biological weapon previously existed in Japan, the former Soviet Union, and the United States. This research included the creation of strains resistant to existing antibiotics and vaccines as well as strains harboring virulence genes from other pathogens.
F. tularensis is a formidable threat as a possible bioterrorism agent for the following reasons.
- It is very infectious - as few as 10 organisms can cause disease.
- Inhalation of the bacterium produces severe respiratory and systemic disease that can result in a mortality rate of 30% or more if not treated with antibiotics.
- It is found widely in nature so that it would not be extremely difficult to obtain the bacterium and grow it under relatively simple laboratory conditions.
- It could be easily disseminated in an aerosol form, although creating this form of the bacteria requires sophisticated technology.
- There is no vaccine currently available for general use.
The Research
Following the terrorist attacks of 2001, research on Category A pathogens has been made a high priority. Because of the potential risk of F. tularensis as a bioweapon, there is a need to develop vaccines and diagnostic tools for this agent. To accomplish this most effectively, researchers need to understand more fully the basic biology of F. tularensis to know (1) how it causes disease, (2) which bacterial factors are responsible for its virulence, (3) its growth cycle, and (4) its natural host(s) . Currently, there is little information on many aspects of F. tularensis biology, in part because it is such a severe health threat. Studies of virulent strains of this organism need to be conducted in Biosafety Level 3 (BSL-3) laboratory facilities and are subject to strict regulations and laboratory practices. Furthermore, basic genetic systems to study and alter Francisella strains have only recently been developed.
Investigators in the Department of Molecular Virology and Microbiology ( MVM) at Baylor College of Medicine ( BCM) are currently in the process of evaluating a candidate tularemia vaccine. Drs. Hanaa El Sahli and Wendy Keitel are conducting a Phase I clinical trial to assess the safety and tolerability of a replicating F. tularensis Live Vaccine Strain (LVS).
The major goals of their study are to
- Test the safety and tolerability of increasing doses of LVS among healthy human subjects given by two different routes – under the skin (subcutaneous) or into the skin (scarification).
- Determine the dose of LVS required to infect humans when administered by the two different routes.
- Determine whether the bacterium can be grown from clinical samples collected at various times after administration of LVS by the different dosages and routes.
- Assess selected aspects of the effects of infection with LVS.
- Analyze immune responses (both humoral and cell-mediated) to LVS.
- Identify which F. tularensis proteins elicit the best immune responses. This information can then be used to improve tularemia vaccine development and diagnostics.
Drs. Joseph Petrosino, George Weinstock, and Timothy Palzkill are addressing critical areas of F. tularensis research using an approach called functional genomics. First, they are obtaining the genome sequences of several important strains of F. tularensis. The DNA sequences are being determined in the BCM Human Genome Sequencing Center ( BCM-HGSC, Dr. George Weinstock, Co-Director). With this data, they can compare the genomes of different strains to identify F. tularensis virulence factors and potential targets for drug, vaccine, and immunodiagnostic development.
These MVM researchers are also using the genome sequence data to isolate every gene and produce every protein from F. tularensis. These proteins will be used in screens to identify the comprehensive set of proteins recognized by the human immune response to F. tularensis (in collaboration with Drs. El Sahli and Keitel). Proteins identified in these screens will be included in future studies aimed at the development of protective subunit vaccine and immunodiagnostic reagents.
In addition, Drs. Petrosino, Weinstock, and Palzkill are using genome comparisons of virulent F. tularensis strains to LVS to identify the mutations that attenuate or weaken LVS. Mutations believed to be attenuating will be introduced into a virulent Francisella strain and tested for their ability to confer an attenuated phenotype in a mouse tularemia model.
Drs. Petrosino, Weinstock, and Palzkill are also collaborating with researchers at the Molecular Sciences Institute (Berkeley, CA) to construct novel protein-DNA chimeras called “tadpoles” that can be used as diagnostic reagents in real-time PCR applications. These tadpole reagents have been able to detect biological targets at attomolar concentrations, and in some applications have been shown to be over a million-fold more sensitive than ELISA-based detection assays. The goal of this project is be able to detect the presence of F. tularensis, and/or an immune response to F. tularensis, in the extremely limited quantities that may be present in clinical samples.
Together, these approaches taken by MVM faculty represent a comprehensive F. tularensis program targeted toward vaccine and immunodiagnostic development.