With the advent of antibiotics, vaccines and sanitation, it looked as if we had conquered infectious disease. Some tropical diseases, such as malaria, remained in certain places, but in the industrialised world, we were entering what promised to be a healthy new era.
However, as the decades have passed, old diseases have resurfaced, existing ones have spread, and new infections are emerging. Lyme disease is one that has risen to prominence over the last few years and now affects more than 100,000 people each year in temperate regions. In the United States, the reported incidence increased from just under 9,000 in 19921 to over 30,000 in 2014. In the United Kingdom, cases went up nearly four-fold between 2001 and 20112. The disease is also found throughout Europe, Asia and parts of Africa, and over the last decade it has been reported in countries where there had been no previous records of its existence, such as Australia.
In 2012, I became one of these cases. It has been a deeply personal experience of the negative consequences of how we’ve altered the natural world. Although we don’t know for sure why the disease is spreading and increasing, many scientists believe that environmental factors are to blame.
Lyme disease is caused by bacteria in the genus Borrelia. More than 30 species have been described, but most human cases appear to be due to B. burgdorferi, the dominant strain in the US. Borrelia are found in a range of wild animals, known as ‘reservoir species’ as they maintain an active pool of the bacteria from which it can be transmitted to humans. These include deer, mice and other rodents, as well as migratory birds. The bacteria are picked up from these species by Ixodid ticks, often called ‘deer ticks’, during feeding. The ticks feed three times during their life, dropping off to metamorphose after each blood meal, then seeking a new host and transmitting the bacteria they ingested from the previous one.
Once inside a human, the bacteria can produce a range of symptoms, from an acute, flu-like illness, to persistent, multisystemic disease, with neurological, musculoskeletal and dermatological manifestations. In rare cases, where it has caused inflammation of the brain or heart, it has been fatal.
Since both ticks and bacteria have multiple hosts, there are a number of ways that disease prevalence can be influenced by environmental factors. It is still unclear exactly how, and studies vary in their findings; however, changes in land use, biodiversity loss, the introduction of non-native species, and climate change have all been implicated.
Land Use Change
Some scientists studying land use change have suggested3 that the increase in cases of Lyme is partly due to agricultural land reverting to forest and people spending more time in nature, increasing their chances of coming into contact with ticks. However, the problem is likely more complex than simple reforestation, as research4 in New York state has found that tick numbers are higher in fragmented than intact forests. In other places, forests appear to have little to do with the disease, such as in Virginia5, where the highest incidence was correlated with herbaceous land cover, including lawns, parks and fields.
Since the spread of the disease is partly reliant on growing populations of reservoir species, changes in biodiversity6 may have a role to play. When predators decrease or are eliminated, reservoir hosts may increase, enabling faster, uninterrupted transmission of the disease. One fascinating modelling experiment7 in Minnesota found that removing wolves led to in an influx of coyotes. This reduced fox numbers and, in turn, caused an increase in the small rodents they prey upon and that also act as reservoir hosts. However, this doesn’t explain the situation everywhere because in most regions where Lyme has recently increased, wolves were eradicated a long time ago.
Invasive and exotic species also appear to offer improved opportunities for tick survival and reproduction. It was found in Connecticut that both ticks and reservoir species were more numerous where Japanese barberry8, a garden escapee, was abundant. A study in Calfornia9 found that the density of nymphal ticks (the second life stage) was higher in forests affected by Sudden Oak Death, caused by a non-native plant pathogen.
As one of the most environmentally disruptive forces, it may seem obvious that climate change would have something to do with Lyme disease, but research findings vary wildly10. While some laboratory experiments have shown that low temperatures can be lethal to ticks, in field experiments11, temperature did not appear to determine overall survival rates, and other studies have shown that tick numbers are instead correlated with moisture12. Whatever the weather, ticks appear pretty tough, and models13 predict they will spread as the climate warms and weather patterns shift.
It may be decades before we determine why Lyme disease cases are spreading and increasing. However, with its unique ecological profile, few doubt that environmental disruption is playing a key role. It is tempting to conclude that halting climate change, reintroducing wolves, and eradicating invasive species would curtail the disease. That may be true, but for now, we will have to stick to tucking our trousers into our socks and doing thorough ‘tick checks’ every time we go outdoors.
1. CDC MMWR (2008) Surveillance for Lyme Disease – United States, 1992—2006 [online].
2. CDC (2015) Lyme disease graphs [online].
3. Ostfeld, R and Keesing, F (2013) Straw men don’t get Lyme disease: response to Wood and Lafferty. Trends in Ecology & Evolution [online], 28(9) pp 502 – 503
4. Allan, B, Keesing, F, Ostfeld, R (2003) Effect of forest fragmentation on Lyme disease risk. Conservation Biology [online], 17(1) pp 267-272
5. Seukep, S, et al (2015) http://link.springer.com/article/10.1007/s10393-015-1034-3. EcoHealth [online], 12(4) pp 634-644.
6. Ostfeld, R and Keesing, F (2001) Biodiversity and disease risk: the case of Lyme disease. Conservation Biology [online], 14(3) pp 722-728
7. Wilmers, C and Levi, T (2012) Wolves–coyotes–foxes: a cascade among carnivores. Ecology [online], 939(4) pp 921-929
8. Stafford, K, Worthley, T, Ward, J and Williams, S (2009) Managing Japanese Barberry (Ranunculales: Berberidaceae) infestations reduces blacklegged tick (Acari: Ixodidae) abundance and infection prevalence with Borrelia burgdorferi (Spirochaetales: Spirochaetaceae) Environmental Entomology [online] 9. Swei, A, Ostfeld, R, Lane, R, Briggs, C (2010) Effects of an invasive forest pathogen on abundance of ticks and their vertebrate hosts in a California Lyme disease focus. Oecologia [online], 166(1) pp 91-100
10. Estrada-Pena, A (2009) Tick-borne pathogens, transmission rates and climate change. Frontiers in Bioscience [online], 14 pp 2674-2687
11. Ostfeld, R and Brunner, J (2015) Climate change and Ixodes tick-borne diseases of humans. Philosophical Transaction of the Royal Society [online] 12. Berger, K, Ginsberg, H, Dugas, K, Hamel, L, Mather, T (2004) Adverse moisture events predict seasonal abundance of Lyme disease vector ticks (Ixodes scapularis). Parasites & Vectors [online], 7(181) pp 1-8
13. Brownstein, J, Holford, TR and Fish, D (2005) Effect of climate change on Lyme disease risk in North America. EcoHealth, [online], 2 pp 38–46