Paul Reiter on Global Warming and Mosquito-Borne Disease
Paul Reiter received his D.Phil. from the University of Sussex in 1978 for a dissertation on
the effects of insoluble surfactants on mosquito biology (not the title).[1] He is affiliated with
both the Dengue Branch of the Division of Vector-Borne Infectious Diseases, Centres for
Disease Control and Prevention and the Pasteur Institute.[2] His research areas are
mosquitoes (behaviour, ecology and physiology), mosquito-borne diseases (transmission
dynamics and epidemiology) and vector/disease control methods.[3] Paul Reiter is qualified
to comment on mosquito-borne diseases.
Reiter appears to have started writing on climate and vector-borne disease in response to
articles and talks in the 1990s predicting expansions in the ranges of vector-borne diseases
due to climate change.[3, 4, 5, 6] His writings on the subject are consistent. I present three
articles by Paul Reiter on this subject in chronological order.
Reiter’s article “From Shakespeare to Defoe: Malaria in England in the Little Ice Age” was
published early in 2000.[3] Discussions of the possible impacts of global warming at the time
included predictions (often model based) of the spread of malaria in Europe and North
America. Reiter found these predictions simplistic as they ignored both the history of malaria
and factors (other than temperature) involved in the transmission of malaria. He focuses on
the history of malaria starting with the Little Ice Age (as the coldest period in recent history)
in England (as a temperate country) and progressing to the present.[3]
The most effective of malaria vector in England was Anopheles atroparvus, which favours
brackish water for breeding. The distribution of malaria (called ague) reflected that of this
mosquito. While summer temperatures of more than 16oC increased the incidence of ague, it
was not limited to warmer years. Some clinical descriptions of ague from the Little Ice Age
are consistent with malaria. This evidence is supported by its cure - “an extract of cinchona
powder” which contains quinine. It is not possible to be certain which or how many species
of Plasmodium were present (the strain(s) is/are extinct) but Reiter notes that English marsh
parish mortality rates suggest that Plasmodium falciparum may have occurred there. The
incidence of malaria decreased over the 19th century though the temperature was rising.
Reiter lists possible reasons for this. England was clear of malaria by the 1950s.[3]
The World Health Organization made an effort to control malaria globally with some success
but this effort lost momentum and the disease has recovered much of its range. Probable
reasons for this return include deforestation, decline of health services, decline of vector
control programmes, ecological change, human migration, human population growth,
irrigation and other agricultural practices, resistance to insecticides and drugs, urbanisation
and the impacts of civil strife, war and natural disasters rather than climate change.[3]
The main point of this article was that malaria distribution is not solely controlled by
climate.[3]
The next article - “Temperatures without Fevers?” published in September 2000 – was coauthored by Chris Dye.[7] The article uses support for a model developed by Rogers and
Randolph[8] to predict the distribution of Plasmodium falciparum (the cause of the worst form
of malaria) to emphasise points concerning modelling and the complexity of malaria. Rogers
and Randolph’s model uses variables related to rainfall, saturation vapour pressure and
temperature. The model’s predicted 2050 distribution of Plasmodium falciparum does not
differ greatly from the present distribution.[7, 8]
The points the article seeks to make are that: single climate variable models should be treated
with caution; climate acts interactively on human host, parasite and vector; while models
may be used to identify high risk areas for disease outbreak, they do not remove the need for
detailed field studies; while climate models are improving, the quality of the disease data
available may not improve as such data remains difficult to collect and more research is
needed to establish what role climate change will have in the spread of malaria.[7]
Reiter published a review article, “Climate Change and Mosquito-Borne Disease,” in 2001.[1]
The review starts with a look at climate and weather (which are correctly distinguished) then
progresses to mosquito-borne diseases in general before looking at malaria, yellow fever and
dengue fever in particular.[1]
Of climate, it is noted that fluctuations on various time scales are natural but that human
activity has measurably increased the carbon dioxide (CO2) concentration which it is
generally agreed contributes to the present warming trend though the extent of this
contribution is uncertain.[1]
Mosquitoes occur wherever it is not permanently frozen though the majority of species are
found in the tropics and subtropics. The host specificity of mosquito species varies greatly.
Several life forms use the salivary secretion produced by mosquitoes to facilitate feeding to
move between vertebrate hosts. Most of these life forms require the mosquito as an
intermediate host.[1]
Climate affects both the mosquito and the parasite and parasite transmission. The most
important climate variables are temperature, rainfall and humidity. Other variables, including
day length (seasonality) and wind, also have an effect. Mosquitoes native in a given region
are adapted to surviving unfavourable seasons and weather conditions. Mosquitoes are
adapted to exploit microclimatic variations e.g. in buildings or forests – even in countries to
which they are not native. Aedes aegypti (a major dengue and yellow fever vector) survived
in temperate Memphis until displaced by another invasive alien mosquito. Climate variables
are thus not the only determinants of the range of mosquito-borne diseases.[1] This is
consistent with Graves and Reavey[9] who note that biotic interactions affect how much of a
species’ fundamental niche it is able to realise.
Disease transmission is affected by human activities. Deforestation and wetland drainage
alter habitat and thus result in changes in the species of mosquito present. Fertilizers and
herbicides affect larval populations. Water storage and drainage provide breeding sites as can
litter. Building design, construction and location in relation to breeding sites affect disease
transmission as do preventative measures. Human activity patterns also play a role.[1]
Reiter notes three ways of making predictions about the impacts of global warming: models,
past climate and the history of mosquito-borne diseases. Of models, he notes that standard
transmission models have only one variable that is influenced by climate, or more precisely,
temperature. The value of models in understanding transmission dynamics is acknowledged
but Reiter doubts their value for predicting the impact of climate change. He then notes the
sources of historical information on climate and weather events (and the sources of
prehistoric data) and mosquito-borne diseases.[1]
The historical distribution of malaria and the variables affecting its transmission (both
climatic and non-climatic) are reviewed. The cases of recent changes in the altitude at which
malaria was recorded are evaluated and compelling non-climate explanations are provided.
Malaria was found historically at higher altitudes than any of these outbreaks.[1]
The historical distribution and factors affecting the transmission of both yellow fever and
dengue are reviewed. As with malaria, climate has a role but the vulnerability of temperate
regions is largely determined by the patterns of human activity and the degree to which
mosquitoes are able gain access to buildings.[1]
The point of the article is that the use of climate to predict future mosquito-borne disease
occurrence is too simplistic and distracts from the urgent need to control these diseases
now.[1]
Reiter and others have continued to express concern about inaccurate publications on the
impacts of global warming on malaria.[10] Kovats and Haines (who both published on the
possibility of global warming affecting the range of mosquito-borne diseases in the
1990s[8, 11]) provide support in an article published 2005[12] by stating that there is not yet
convincing evidence that climate change is affecting the distribution mosquito-borne
diseases. The main focus of this article is the lack of preparedness to deal with extreme
weather events such as the 2003 heat wave in Europe.[12]
Reiter’s opinions on climate change appear to have progressed. In the first article, human
impacts on global warming are disputed.[3] In the review article, humans have measurably
increased CO2 but the extent to which this contributes to climate change is uncertain.[1] A
more recent article says anxiety about global warming is understandable but not an excuse for
bad science concerning its impact on diseases.[10] It is likely that Reiter remains sceptical
about the effects of human caused CO2 increases but he has read scientific literature on
climate change.[1, 3]
Reiter presents compelling arguments that global warming is not likely to greatly increase the
range of mosquito-borne diseases or to be the main driver of possible range expansions.
Kovats and Haines’[12] conclusion that there is no convincing evidence of the impacts of
climate change on mosquito-borne disease seems to reflect the current state of knowledge in
this area. Monitoring and research on this topic are required[11] but one of Reiter’s arguments
should be the focus of greatest concern: the lack of measures to prevent the spread of
mosquito-borne disease and the decline of public health services require attention and action
even without climate change.[1] Should these be attended to, the possible range expansions of
mosquito-borne disease due to climate change may be prevented or at least minimised.
References
1. Reiter P. 2001. Climate Change and Mosquito-Borne Disease. Environmental Health
Perspectives 109 (Suppl. 1): 141—161. Available from: http://links.jstor.org/sici?sici=00916765%28200103%29109%3C141%3ACCAMD%3E2.0.CO%3B2-S.
2. Hayes JM, et al. 2006. Risk factors for infection during a dengue-1 outbreak in Maui,
Hawaii, 2001. Transactions of the Royal Society of Tropical Medicine and Hygiene 100:
559—566. Available from: http://www.sciencedirect.com/ (Search Paul Reiter). (Reiter is the
third in a list of 13 authors)
3. Reiter P. 2000. From Shakespeare to Defoe: Malaria in England in the Little Ice Age.
Emerging Infectious Diseases 6 (1): 1—11. Available from:
http://www.cdc.gov/ncidod/eid/vol6no1/pdf/reiter.pdf.
4. Reiter P. 1996. Global warming and mosquito-borne disease in USA. Letter. The Lancet
348 (9027): 622. Available via ProQuest.
5. Reiter P. 1998. Global warming and vector-borne disease in temperate regions and at high
altitude. Letter. The Lancet 351 (9105): 839—840. Available via ProQuest.
6. Reiter P. 1998. Global warming and vector-borne disease. Letter. The Lancet 351 (9117):
1738. Available via ProQuest.
7. Dye C, Reiter P. 2000. Temperatures without Fevers? Science 289 (5485): 1697—1698.
Available from: http://links.jstor.org/sici?sici=00368075%2820000908%293%3A289%3A5485%3C1697%3ATWF%3E2.0.CO%3B2-Z.
8. Rogers DJ, Randolph SE. 2000. The Global Spread of Malaria in a Future, Warmer
World. Science 289 (5485): 1763—1766. Available from:
http://links.jstor.org/sici?sici=00368075%2820000908%293%3A289%3A5485%3C1763%3ATGSOMI%3E2.0.CO%3B2-9.
9. Graves J, Reavey D. 1996. Global Environmental Change: Plants, Animals &
Communities. Harlow: Longman. 226 p. 0-582-21873-X ISBN
10. Reiter P, Thomas CJ, Atkinson PM, Hay SI, Randolph SE, Rogers DJ, Shanks GD, Snow
RW, Spielman A. 2004. Global warming and malaria: a call for accuracy. The Lancet 4 (6):
323—324. Available from: http://www.sciencedirect.com/ (Search Paul Reiter).
11. Haines A. 1998. Global warming and vector-borne disease. Letter. The Lancet 351
(9117): 1737—1738. Available via ProQuest.
12. Kovats RS, Haines A. 2005. Global climate change and health: recent findings and
future steps. Canadian Medical Association Journal 172 (4): 501—502. Available from:
http://www.cmaj.ca/cgi/reprint/172/4/501.