Microbes Vs Insects Vs humans

Allison Cartwright is our ECS Publications Officer. She loves a bit of hiking, but doesn’t enjoy some of the insects she meets on her journeys. 

While exploring the countryside with kids completing their expedition for the Duke of Edinburgh Award, there’s one uniting experience for all participants. As with other walkers, we’re all being eaten alive by insects.

Sure, we have insect repellent, but it doesn’t seem to be working to an acceptable level. Did I miss a patch during application? Or are the nippers immune to the repellent?

repellent

Midge misery

Attention soon turns to the next question – what is eating us? There seem to be 3 main culprits in the Northern Irish mountain: midges, horse flies (or clegs as we call them here) and the odd black fly.

Midges surround us as we pitch tents while horse flies and black flies munch on us during the day.

With all of these insects, it’s only the females of the species that are blood suckers. Their final feast of blood is needed for the next generation. I’m just thankful we don’t have mosquitoes!

Midges

Cleg spittle

Other than the irritation caused by the insect’s saliva as its mouth parts break my skin, do I have anything to worry about from these bites? With any bite and the desire to scratch, skin can be broken.

This will allow the entrance of microbes and potential infection. Thankfully, this doesn’t seem to be common. I don’t know anyone who’s become ill as a result of these bites.

Instead, infection potential could be related to the insect that bit you. Black flies are not known to transmit disease to humans, but they can pass on a parasitic nematode, Onchocerca volvulus which causes onchocerciasis, or “river blindness”. It serves as the larval host for the nematode and acts as the vector by which the disease is spread.

The annoying insects even inspired an enduring classic of Canadian folk music. ‘The Black Fly Song’ by Wade Hemsworth was written in 1949, about being tormented by black flies while working in the wilds of Northern Ontario.

Bluetongue blues

Midges aren’t a concern for us, as they don’t carry human pathogens, however, they can affect livestock, especially sheep.

The most common pathogen from midges is the bluetongue virus, where infected animals develop swelling to the tongue and lips giving them a blue colour. This virus has an incubation time of up to 20 days and can kill an animal within a week. However, animals can recover after a few months of contracting the virus.

Bluetongue virus has spread north with global warming, but it’s not a big problem in the UK or Ireland (yet?) as our cold winters kill the virus and its host. There has been concern that global warming may add mosquitoes to the list of our biting, flying insects.

Bluetongue virus

Bluetongue virus

Malaria area

If mosquitos colonise, we could be at risk from many viruses including West Nile virus, dengue, chikungunya virus and equine encephalitis virus. Not to mention the Plasmodium sp. parasite that causes malaria!

If insects are vectors for microbial diseases, how can we control them? Control methods include the use of insecticides and biological pest control. The latter can include the use of microbes, both bacteria and fungi.

As mosquitoes are vectors of many diseases which affect humans, there is more research into controlling this flying insect than others. With many, the aim is to prevent the survival of the water dwelling insect larvae.

mosquito-1016254

Ground control

As a child growing up in Zimbabwe, I remember doing this basic control. After heavy rain, we would walk the garden tipping over movable objects with puddles to ensure mosquitoes didn’t lay their eggs.

With any control method that kills the larvae, there will be fewer adults to feed on us. It often aims to affect the gut microbiome to either alter the insect’s behaviour or kill them.

The bacterium Bacillus thuringiensis has been used to control both mosquito and blackfly larvae. These bacteria kill the larvae by producing protoxins once it has been eaten. As the larvae are filter feeders, the introduction of these bacteria to the waterbody will result in death of the larvae that ingest it.

mosquito-on water

Fighting fungi

Some biological control also focuses on the adult insects. The control of mosquitoes includes their infection with the bacterium Wolbachia. Successful colonisation of the mosquito with these bacteria doesn’t kill them, but instead it reduces the prevalence of the dengue virus, thus reducing disease potential from a bite.

In addition to bacteria, fungi can also be used to control pest insects. The pathogenic fungi Beauveria bassiana has been used to infect mosquitoes. Once the fungi enter the insect’s exoskeleton it moves to the gut where it can interact with the bacteria in the gut including Serratia marcescens. Combined, the interaction between fungi and bacteria causes death to the mosquitoes. With all of these controls, it’s fascinating to note that our best method of preventing the spread of harmful microbes is by using other microbes!

Although the insect bites are annoying, on reflection I can’t complain as the common biting insects in the UK and Ireland present little health risk. More worrying are the insects one may encounter while abroad. Thankfully, there’s a lot of useful information on how to protect oneself from harmful insects while travelling.

References:

Arora, A.K., Douglas, A.E. (2017) Hype or opportunity? Using microbial symbionts in novel strategies for insect pest control. Journal of Insect Physiology, 103: 10-17.

Carpenter, S., McArthur, C., Selby, R., Ward, R., Nolan, D.V., Mordue Luntz, A.J., Dallas, J.F., Tripet, F., Mellor, P.S. (2008) Experimental infection studies of UK Culicoides species midges with bluetongue virus serotype 8 and 9. Veterinary Record, 163: 589-592.

Medical Entomology (2008) Insects, ticks and other arthropods [online]. Available from: https://extension.entm.purdue.edu/publichealth/arthropods.html

Schneider, S., Tajrin, T., Lundström, J.O., Hendriksen, N.B., Melin, P., Sundh, I. (2017). Do multi-year applications of Bacillus thuringiensis subsp. Israelensis for control of mosquito larvae affect the abundance of B. cereus group populations in riparian wetland soils? Microbial Ecology, 74: 901-909.

Wei, G., Lai, Y., Wang, G., Chen, H., Li, F., Wang, S. (2017) Insect pathogenic fungus interacts with the gut microbiota to accelerate mosquito mortality. PNAS, 114 (23): 5994-5999.

 



Categories: Feature Articles

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