How new diseases from insects hit people like the plague

Scientists have traced the first steps in the way some new diseases emerge, and how harmless bacteria living in insects become dangerous disease-causing bugs which can affect humans, like the plague or anthrax. Researchers from the University of Bath are presenting their results today (Wednesday, 08 September 2004) at the Society for General Microbiology’s 155th Meeting at Trinity College Dublin.

The scientists believe that because of the similarities between human and insect immune systems, any bacteria that have successfully evolved to infect insects already have a head start if they attack people.

“There are millions of bacteria in the environment, and sometimes they cause ‘emerging diseases’ by attacking people – in other words a new type of disease which we haven’t seen before. But this is not a completely new phenomenon, the bacteria may have been around for centuries, it is just that a new strain is suddenly able to infect humans as well as other animals,” says Dr Nick Waterfield of Bath University. “We need to understand the mechanism that the bacteria are using to change their disease-causing ability if we are to successfully treat emerging diseases before they get out of control and become epidemics.”

A very infectious bacterium that attacks insects, Bacillus thuringiensis, is closely related to the microbe that causes anthrax in humans, a bacterium called Bacillus anthracis. Similarly the plague bacterium, Yersinia pestis, evolved relatively recently from an ancestor, Yersinia psuedotuberculosis, by passing through the fleas which carried it, and moved across rats to humans.

“We believe that these interactions between bacteria and insects may significantly contribute to the evolution of human diseases,” says Dr Waterfield. “There are two main ways insects help the bacteria: by passing microbes directly into our bloodstream when they bite us – like fleas or ticks; and by acting as a reservoir to cook up future human diseases.”

“The picture is further complicated by climate change, which seems to be altering the range of places insects can survive and breed, bringing new insects which can carry ancient diseases into the Northern hemisphere,” says Dr Waterfield.

The scientists have been studying a newly recognised but non-lethal bacterium called Photorhabdus asymbiotica that has been identified by hospitals in both the United States and Australia, which provides a safe system to study the problem. This bacterium seems to have evolved from the well known insect-disease causing bacteria, Photorhabdus luminescens and Photorhabdus temperata, which attack insects with the help of their nematode worm partners.

The researchers hope that a better understanding of the role of insects in the evolution of diseases will allow scientists to identify situations that could act as a source of new infection.