Insects may carry new infectious diseases

• Insects and other invertebrates are the arena for the evolution of new infectious diseases in humans, new research shows.
• Scientists now believe that not only are insects the carriers of some existing diseases but they are also the vehicle where recently emerging highly infectious diseases, such as the plague that killed millions in the 14th and 17th centuries, evolve.
• Writing in the October edition of Nature Reviews: Microbiology, the researchers point to the large reservoir of diseases in invertebrates, such as fleas and nematode worms, which are currently harmless to humans, but which could evolve quickly into a range of new diseases.
• Their study looked at the bioluminescent bacterium, Photorhabdus asymbiotica, which cause pustulant sores to appear on parts of sufferers' bodies.
• The researchers suspect that this new bacterium evolved recently from a well-known bacterium, Photorhabdus luminescens, which kills insects with the help of nematode worms.
• The bodies of insects killed by Photorhabdus luminescens infection are left luminous.
• Evidence suggests that the plague could have evolved from a close insect-pathogenic ancestor as little as 1,500 years ago, an eye blink in evolutionary time, suggesting that similar scenarios are possible for other insect-associated microbes.
• The researchers suggest the reason that unusual infections, such as those caused by Photorhabdus asymbiotica, are not currently a large problem is that there are a range of antibiotics to treat them and insecticides that help suppress invertebrate numbers.
• However, they warn it could be only a short time before the parallel problems of insect resistance to insecticides and bacterial resistance to antibiotics unite to throw up new diseases that doctors will have difficulty controlling.
• "Understanding the mechanism that the bacteria use to change their disease-causing ability is important if we are to successfully treat emerging infectious diseases before they get out of control and become epidemics.