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The impact of climate change on pestpopulations and public health

May 2012



Introduction

The World Health Organization (WHO)book, Public health significance of urban

pests, tells us that to protect public healthwe must improve pest and pest-relateddisease management.

The second half of the 20th century andthe beginning of the 21st has witnessedimportant changes in ecology, climate

and human behaviour that favour thedevelopment of pests.

“Climate change is particularly relevantbecause it is expected to alter not only thenatural environment as a result of floodingor drought but also the urban environmentas a result of changes in land use.”(Bonnefoy et al, 2008)

Warmer temperatures, changingprecipitation patterns, higher sea levelsand more extreme weather events such asflooding will become more frequent and

will impact on pest populations.

These changes make crucial the need tocarefully assess the potential threat of pests to public and environmental health.

Pest management must be recognisedas fundamental to public health and aproactive approach adopted by thoseworking in the profession.



The effects of climate change on rodent populations

The current trend of a warmer and wetterclimate will impact on rodents and theirinteraction with humans.

Data suggests that temperature is themajor factor influencing the reproductivepotential of rodents and that thisreproductive potential is increased duringthe warmer months or during rainy

seasons, as described by Meehan in 1984,in his book, ‘Rats and Mice’.

Researchers Brown and Singletonpublished work in 1999 in the Journalof Applied Ecology, showing that otherclimatic factors such as precipitation, canalso increase rodent populations.

Urban rodents can be infected with a widerange of pathogens, such as Salmonella,as confirmed by Hilton in 2002 in theInternational Journal of EnvironmentalHealth Research and Toxoplasma gondii,

as presented by Murphy in 2008 at the 6thInternational Conference on Urban Pests.The transmission of pathogens to humansis also influenced by climatic factors.

Floods may cause rodents to be displacedfrom their habitats and as a result,bring them into even closer contact withhumans, increasing the likelihood of disease transmission. Flooding has beenassociated with increases in the incidenceof Leptospirosis (also known as Weil’sdisease).

Outbreaks of rodent-borne Hantavirushave been associated with increases inmouse populations, driven by food supplyas a result of prolonged precipitation, asreviewed by Epstein in his 2001 paper‘climate change and emerging infectiousdiseases’.

Rats can also damage electrical cables,

gas and water pipes by gnawing onthem and this has been known to causefires and floods. In addition there couldbe an increased economic expense asrodents consume and contaminate scarcefoodstuffs.

Changes in the climate may also result inchanges in human lifestyles, such as anincrease in outdoor activities. This in turnaffects rodent populations where morediscarded food is available.

The increased availability of alternative

food sources may impact the effectivenessof treatments, such as rodenticide baitacceptance, the type of bait and baitingtechniques used.

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The effects of climate change on tick populations

According to WHO, ticks are responsiblefor transmission of more cases of humandisease than any other arthropod vectorsin North America and Europe. Tickstransmit the most common vector-bornediseases, for example Lyme Disease.

Regional studies show that tickpopulations are increasing. Climate

change may increase tick numbers andcases of tick-borne diseases. Milder wintertemperatures in particular have importanteffects on tick distribution, fostering shiftsto higher altitudes.

Ticks dwell predominantly in woodlandsand meadows on hosts such as deer, smallmammals, rodents and birds. Most ticksthat can impact human health are rare inurbanised environments but increasinglythey occur closer to home where a moistmicrohabitat is provided by high grass,gardens and rough forest edges.

Seasonal patterns of plants are changing.Studies indicate that flowering periodsand leaf unfurling are occurring earlier,while autumn leaf fall is increasinglydelayed. Since the early 1960s the overallgrowing season has extended by 10.5days and, according to predictions by theNational Farmer's Union (NFU), it willhave extended by 45-50 days by 2080.

The early and prolonged abundance of plants may aid earlier births, better birthweights, increased juvenile survival, andlower adult mortality of host animals.This in turn would support higher numbersof ticks.

Milder winters aid ticks to actively feedand continue their life cycles. A lack of

harsh winters also reduces the mortalityrate among host species.

Milder weather all year round is likelyto encourage people to enjoy outdoorsmore frequently and for longer periods,increasing public exposure to ticks.

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The effects of climate change on mosquito populations

Increases in international travel and tradeallow accidental importation of mosquitovectors and hosts. As our climate andenvironment change, mosquitoes willspread to new areas, bringing emergingand resurging vector-borne diseases.

Mosquitoes are highly responsive tochanges in climate and increased

temperature is associated with increasedabundance, assuming there are sufficientnumbers of water-filled sites as habitat.

The predicted warmer summers andmilder winter temperatures will favourmosquito development and extend thebiting season of some species.

Wetter winters will provide moretemporary and underground aquatic sitesfor some species during winter and spring.Drier summers could however reducepossible aquatic sites for other species.

Mosquitoes change behaviour, adaptingto new breeding habitats, such as slurrypits and rainwater pools in used tyres. In awarmer climate, the increased use of waterbutts by households is likely to increase thenumber of biting and non-human bitingspecies around homes. Intense summerrainfall resulting in localised floodingcould also increase the abundance of flood-water species.

Rises in sea levels and increased stormsurges, particularly on the east coast,could eliminate some mosquito sites forexample saltmarshes, but create new siteswhere there is inundation of salt-water.

There are more than 25 common speciesof mosquito in the UK and each willrespond slightly differently to climate

change. Climate change is likely toincrease the abundance of mosquitoesin the UK but may also increase theabundance of their predators.

A further important factor is thatincreased movement of people and goodsmay result in resevoirs of disease such asdengue being formed in areas that havepreviously been disease free.

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The effects of climate change on fly populations

The public health significance of flies iswell established. For example the housefly,Musca domestica, has been implicatedin the spread of many diseases byresearchers such as Greenberg, in his 1973book, Flies and disease.

The potential increases in fly populationsdue to climate change are of concern, in

terms of both the risk to public health andthe considerable nuisance value.

In many countries the presence of fliesin food premises is in contravention of food regulations. In some countries thepresence of nuisance flies is also anoffence.

Goulson and colleagues developedpredictive models to forecast flypopulations, published in the Journal of Applied Ecology in 2005. Modelswere produced for the housefly, Musca

domestica, and blowflies, Calliphora spp.Predictions based on climatic factors onlywere strongly correlated with observeddata. This observation suggests that flypopulation changes are largely driven bythe weather rather than biotic factors.

The models predict that under likelyscenarios of UK climate change, flypopulations could increase substantially,with increases of up to 244 percent by

2080 compared with current levels.If these predictions hold true, it is possibleincreases in fly-borne diseases will occur.

The effect on other fly populations is lessclear. The influence of climate changeon the stable fly, Stomoxys calcitrans, ablood-feeding, biting pest of livestock andhumans, was examined in an international

study by Gilles and colleagues inthe journal Medical and Veterinary Entomology in 2008. The main conclusionis that stable fly infestations are unlikely toworsen in response to global warming.

The study noted that althoughinfestations are unlikely to worsen a shiftin the infestation period could occur, whichis still of importance. Stable flies breed inlivestock manure and rotting vegetation.Sanitation/husbandry practices can have amajor impact on populations and are likelyto compensate for the effects of climate

change on these flies.

Studies such as these are essential. Theyenable the forecasting of fly populationlevels ensuring that control measures canbe focussed as part of an integrated pestmanagement programme, reducing publichealth risk and fly nuisance.

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The National Pest Advisory PanelChartered Institute of Environmental Health

Chadwick Court, 15 Hatfields, London SE1 8DJTelephone 020 7928 6006 Fax 020 7827 5831Email [email protected] Web www.cieh-npap.org

This report is printed on totally chlorine-free and recyclable paper, produced from sustainable forests.

The CIEH would like to thank Killgerm Group, Health Protection Agency, Borreliosis and Associated Diseases

Awareness UK and BASF for their assistance in producing this booklet.

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