Insects Implicated In Evolution Of New Human Infectious Diseases

[Guest guest]

Insect-pathogenic nematodes harboring Photorhabdus spp are used as

biopesticides in a number of countries, including the United States

and Australia.

 

 

 

 

 

http://www.spacedaily.com/news/life-04zzzze.html

 

 

 

Insects Implicated In Evolution Of New Human Infectious Diseases

 

 

Bath, United Kingdom (SPX) Oct 26, 2004

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.

 

As part of their research, Dr Nick Waterfield and Professor Richard

ffrench-Constant [correct] from the University of Bath, and Professor

Brendan Wren, from the London School of Hygiene and Tropical Medicine,

are studying a new disease-causing (pathogenic) bacterium that has

been identified in about a dozen people in the USA and Australia.

 

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.

 

This family of bacteria are known as Photorhabdus (glowing rods)

because they are the only terrestrial bioluminescent bacteria. The

bodies of insects killed by Photorhabdus luminescens infection are

left luminous.

 

Genome studies have found close similarities between the

plague-causing bacterium, Yersinia pestis, and its ancestor, Yersinia

psuedotuberculosis, which has been shown to be capable of causing

disease insects, at least in the laboratory.

 

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.

 

In-built similarities between human and insect immune systems also

mean that any diseases that have successfully evolved to infect

insects already have a head start if they attack people.

 

" Most scientists are looking at diseases of farm animals as the

biggest threat to humans. Insects are numerous and reside in close

proximity to man, yet they have been generally ignored as a potential

source of microbes that could be harmful for man, " said Dr Waterfield.

 

" As well as passing microbes directly into our bloodstream when they

bite us, insects can also act as a reservoir to `cook up' future human

diseases.

 

" 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.

 

" There are countless species of bacteria in insects, and sometimes

they cause emerging infectious diseases by becoming harmful to people

- in other words by evolving into a new type of disease which we

haven't seen before.

 

" The species of bacteria may have been around for centuries, but it is

just that a new strain evolves that is suddenly able to infect humans

as well as other animals.

 

" 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 devastating diseases such as

malaria into the Northern hemisphere. "

 

In the case of Photorhabdus luminescens, the infection is naturally

resistant to several antibiotics but the relapsing infection can be

combated using repeated administration of certain drugs.

 

The researchers predict that the failure of automated hospital

diagnostic machines to recognise this unusual bacterium means that

many more cases of this new infection may exist. This is likely to be

particularly true for the developing world, where doctors may diagnose

the sore as a bad ulcer and treat immediately with antibiotics.

 

Now that researchers are actively looking for Photorhabdus asymbiotica

infection, they are beginning to find cases in countries in Asia.