FOI researchers have now succeeded in mapping more recent and aggressive forms of tularaemia. By gradually altering its genetic make-up, the bacteria is able to grow inside the human body, which has created prerequisites for the pathogenic variant.

All living organisms want to spread their genes. This includes the tularaemia (rabbit fever) bacteria, which belongs to the Francisellaceae family. Pär Larsson, FOI, together with researchers from Umeå University and researchers in USA, examined 17 strains of the Francisellaceae family, to which Francisella tularensis belongs. By investigating genetic material, researchers have been able to show how rabbit fever bacteria have modified genes to grow inside host cells instead of on their own. When reproducing inside host cells, rabbit fever bacteria are pathogenic and virulent.

“The rabbit fever bacteria has done what we call a change of niche and we have used evolutionary theory to study what has prepared the way for this change of niche,” says Pär Larsson.


The family has pathogenic and virulent variants
In Sweden rabbit fever has been most common in Norrland, but appears to be moving south. People who are infected become very sick, but are cured with antibiotics and return to full health and become immune.

“It’s rather unfortunate that the disease is called rabbit fever. It really has nothing to do with rabbits, except that it is a lethal disease for rabbits. This is where we are observing the disease. We suspect that the source, the reservoir, is in water and that the bacteria is transmitted via mosquitoes,” says Pär Larsson.
The American variant is transmitted via the respiratory system and leads to pneumonia. The process is very quick, and if antibiotics are not administered in time there is a major risk of death due to blood poisoning. Pär Larsson says that since the Swedish and American varieties are so similar it is theoretically possible that the Swedish mutates and transforms into the American. The American also has a close relative that is not dangerous, which can mean that the latter was once transformed into the lethal variant. The rabbit fever bacteria is, however, fairly stable. There is a greater risk that the virus that causes bird flu mutates and infects human beings.

The threat of rabbit fever as a biological weapon reason for research

The Swedish form of rabbit fever has a darker and more dangerous side that explains researchers efforts to learn more about the bacteria and its history. Both the US and the Soviet developed biological weapons based upon rabbit fever during the Cold War.

“Rabbit fever is extremely contagious,” says Pär Larsson. If millions of anthrax bacteria are required to be effective, then only a few rabbit fever bacteria are needed. The mild Swedish variant and a very lethal American relative are genetically very similar. It is easier to understand how the American one works by studying the mild Swedish variant. That is why FOI has been studying rabbit fever for many years.

The darkest aspect of the rabbit fever bacteria is that it requires the laboratories of super powers to turn it into a weapon.

“It’s rather easy to collect and cultivate bacteria,” says Pär Larsson. An awareness of this threat is the strongest motive for research on the rabbit fever bacteria.