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Lyme Disease and Climate Change by John Eoin Healy

Updated: Sep 27, 2023

Thanks to Siegied Tremel for use of this photo


Lyme Disease and Climate Change

Winter’s occasional icy grip might make us forget that Planet Earth is now experiencing a sustained period of climate change. What are the likely consequences for populations of Ixodes ricinus ticks, seasonal tick-biting activity and the ensuing Lyme disease risk to humans?

In the 12th book in the CABI Climate Change series (Climate, Ticks and Disease editor Pat Nuttall, University of Oxford, 2022), we wrote “Widening of the tick’s temporal distribution away from the historic ‘spring and autumn’ activity peaks in Ireland and the UK … may significantly alter the pattern and extent of infestation of passerine species and hence influence tick dispersal.” (Chap 15, Birds, Ticks and Climate Change, Tom Kelly, John Healy & Neil Coughlan).

Expanding on this, increasing ambient temperatures may allow ticks to be actively questing for hosts – whether they be birds or mammals – for an extending period of the year. Not only do active ticks feed and potentially transmit the Lyme-inducing Borrelia bacteria into their hosts, but active ticks also mate and reproduce. Since a single adult female tick can give rise to as many as 2,000 larvae, any extension of the days-in-the-year and hours-in-the-day when environmental temperatures favour tick activity will inevitably result in rapid amplification of tick populations. And movements of birds (migration and shorter journeys) will result in ticks being distributed more widely.

Back in 1935, John MacLeod carried out several detailed studies on conditions controlling the development and questing activity of ticks. Of course back then, ticks were only of interest as the vectors of louping ill virus to sheep and grouse as well as redwater fever (babesiosis) to cattle. It took another 45 years before Lyme borreliosis and its transmission by ticks was discovered. MacLeod’s laboratory work concluded that ticks began to actively quest for hosts when the air temperature reached 7–8 degrees C. Some researchers have claimed that ticks enter an obligatory phase of diapause (hibernation) in winter. But this must be reconciled with increasing reports across Europe of winter tick activity. MacLeod himself found no evidence that ticks are inevitably destined for a winter of “sleep” when he wrote “In no case has any indication been found of the overwintering torpor of unfed ticks ….”.

This has also been my experience. In Ireland, on sunny days I have found ticks active on vegetation when the air temperature was actually below the accepted 7-8 degrees C threshold.

On a few occasions in mid-winter I have sifted through leaf litter on a woodland floor and found supposedly dormant nymph and adult ticks. Placing them in the palm of my hand for no more than 20-30 seconds and exhaling warm breath onto them invariably resulted in a “re-awakening” and walking. It made me wonder about any kind of obligatory dormancy/diapause.

Through 2002-2004 I routinely collected monthly samples in a tick- infested woodland. Without difficulty, a fellow sampler and I could pick 100+ adult ticks from the vegetation in December, January and February, even when the air temperature was no higher than 8 degrees C.

There is clear evidence that climate change is bringing about conditions favourable for tick activity during periods of the year which previously might have been considered to be “tick free”. I have examined some data from the UK Met Office database.

Of the 11 regionals covered by the UK Met Office, for the sake of example I have considered just two - the furthest north (Scotland) and the most southerly (England South). From winter data(Dec, Jan & Feb), I have extracted separate subsets for these two regions on a) days with air frost, b) mean winter temperature, c) mean maximum daily air temperature and d) hours of bright sunshine. In each case I have compared mean values of these four parameters for the 15-year period 1971-1985 with the 15-year period 2007-2021.

The trend towards warmer conditions is clear in both regions – fewer days with air frost, higher mean winter temperature, higher mean maximum daily temperature and more hours of sunshine. Where the change from the 1971-1785 to the 2007-2021 period is statistically significant, the percentage change is shown in italics in green. All of the changes in the four climate variables shown above combine to create more favourable conditions for tick activity, for tick reproduction and for a consequent growth in the size of the tick population. This is inevitably leading to increasing human-tick contact and a growing incidence of Lyme disease among the human population.

The change in the “hours of bright sunshine” parameter is of particular interest. Since MacLeod established the threshold for tick activity at 7-8 degrees C back in the early 1930’s, his findings have been re-stated many times in research papers. But the attention has always been on air temperature, probably because ground temperature data is not usually available from weather stations. At any given time, the measured air temperature may not be the same as the temperature inside the body of a tick, or indeed any other animal or object. Air is a very poor absorber of solar radiation and has also has a poor capacity for heat retention. The Earth’s surface, and animals upon it, absorb solar radiation much more effectively and convert it to heat. Stand outdoors on a cold winter day in sunshine and turn your right shoulder to the sun. The left-hand side of your face will feel cold while your right cheek will feel warm even though the air temperature may be below 0 degrees C. Likewise, the body of a tick exposed to direct winter sunshine will have a temperature higher than the ambient air temperature. MacLeod’s 1935 study was carried out in a lab so solar radiation wasn’t considered. Even so, MacLeod was well aware of the potential effect of sunlight on ticks: “: “Sunshine will affect the ticks directly by insolation and indirectly by radiation of heat from the ground” and again “… air temperatures refer to the shade temperatures of the air and are not comparable to the temperature of an object which is exposed to the radiant heat of the sun.” (“Insolation” is the amount of solar radiation received by the surface of an object.)

The laws of physics tell us that the temperature within the bodies of these ticks would have been higher than the surrounding air temperature provided that the ticks were in direct sunlight. The bigger the animal the greater the heat absorption and retention and so we would expect adult ticks to benefit more from solar radiation than nymphs. This is most likely the explanation for tick activity when air temperatures are no higher than 8 degrees C – and also the reason for an adult tick being active on an icy frond of vegetation as shown in the image at the top of this article.

The change in climate variable between the 1971-1985 and the 2007-2021 periods shown in the tables above tell us that conditions for tick activity have improved - fewer days with frost, higher mean daily temperatures and more hours of sunshine. More hours of sunshine implies an extended period when ticks can be active, feeding and mating. Note in particular that in the England (South) data, the mean maximum daily temperature of 8.1 degrees C is above the threshold for activity so it is fair to say that ticks will, on average, be active on most winter days in this region.

Given the climate trends highlighted above, the public, GP’s and Public Health authorities should be aware that the risk periods for tick-bite are extending in time with tick activity continuing later into autumn, surging earlier in spring and becoming an intermittent threat during winter when conditions allow.

In known tick-infested areas, forestry and farm workers, walkers and other recreationists should always take the simple precautions of:

a) Using a tick repellent

b) Dressing appropriately (light coloured clothing, tucking trousers into socks etc

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