Disease will not be the greatest challenge that we will face in this century. As far as I am concerned disease, floods, rising sea levels, storms and heat waves are less important than drought and famine. Nevertheless the rest of these will present challenges and shouldn’t be ignored. They are just likely to cost fewer lives. With this in mind, I would like to offer six reasons why we can expect diseases to be a greater challenge to humanity under climate change.

Part I: Animal Migration

Climate change will cause species that have been isolated from us to leave their natureal habitat and increasingly bring them and their diseases in contact with us.

For one example, independent of flooding, increased rainfall in a warming world may expand the range of rodents that carry the hanta virus:

The caseload of HCPS increased 5-fold above baseline in the Four Corners states in 1998-1999. Regions that had received increased rainfall in 1998 were especially affected. A large majority of the 1998-1999 case patients reported indoor exposure to deer mice. Hantavirus outbreaks can occur in response to abiotic events, even in the face of extensive public education and awareness.

Brian Hjelle and Gregory E. Glass (2000) Outbreak of Hantavirus Infection in the Four Corners Region of the United States in the Wake of the 1997-1998 El Nino-Southern Oscillation, J Infect Dis. 181 (5): 1569-1573.

Other diseases spread by rodents are also likely to do expand their ranges with increased temperature and rainfall.

Interestingly, water- and food-borne diseases also seem to do better with higher temperatures:

In addition, most cases of water-and food-borne gastroenteritis, particularly illness related to Campylobacter and Salmonella, exhibit a distinct summertime pattern of occurrence. Although it is possible that seasonality is due to behavioural patterns (e.g., barbecuing or swimming in the summer), the association between warmer temperatures and disease suggests that rates of water-and food-borne illness are likely to increase with rising temperatures.

Brian Hjelle and Gregory E. Glass (2000) Outbreak of Hantavirus Infection in the Four Corners Region of the United States in the Wake of the 1997-1998 El Nino-Southern Oscillation, J Infect Dis. 181 (5): 1569-1573.

I would be remiss, however, if I didn’t mention how global warming makes it possible for mosquitos to survive through the winter by reducing the period over which freezing takes place, thereby making them endemic to a region year round, a drama which is being played out in Taiwan with respect to hemorrhagic dengue fever:

Furthermore, cosmopolitan genotypes of DENV-2, the causing agent of Taiwan’s largest-scale epidemic of dengue/DHF in last thirty years, had been gradually and effectively replacing Asian genotype 2 in the Philippines since 1998 and entered Taiwan in 2001 [40]. This cosmopolitan genotype of DENV-2 is different from the Asian 1 and Asian 2 genotypes of Taiwan’s DENV-2 isolates from 1981 to 1998 and the American/Asian genotype of Taiwan’s isolates in 2005, when the majority of dengue cases were dengue fever [41]. In other words, the more virulent genotypes/strains of the same serotype that have emerged during later years have resulted in more severe and/or larger-scale epidemics of dengue/DHF in many Asian countries [37], [39].

Shang C-S, Fang C-T, Liu C-M, Wen T-H, Tsai K-H, et al. 2010 The Role of Imported Cases and Favorable Meteorological Conditions in the Onset of Dengue Epidemics. PLoS Negl Trop Dis 4(8): e775. doi:10.1371/journal.pntd.0000775

Part II: Flooding

Regarding the West Nile Virus, we find that higher temperatures, elevated humidity and extreme precipitation events each contribute to the increased prevailence of the disease:

Results

Increasing weekly maximum temperature and weekly cumulative temperature were similarly and significantly associated with a 35-83% higher incidence of reported WNV infection over the next month. An increase in mean weekly dew point temperature was significantly associated with a 9-38% higher incidence over the subsequent 3 weeks. The presence of at least 1 day of heavy rainfall within a week was associated with a 29-66% higher incidence during the same week and over the subsequent 2 weeks. A 20-mm increase in cumulative weekly precipitation was significantly associated with a 4-8% increase in incidence of reported WNV infection over the subsequent 2 weeks.

Conclusions

Warmer temperatures, elevated humidity, and heavy precipitation increased the rate of human WNV infection in the United States independent of season and each others' effects.

Jonathan E. Soverow et al. (July 2009) Infectious Disease in a Warming World: How Weather Influenced West Nile Virus in the United States (2001-2005), Environ Health Perspect., 117(7): 1049-1052.

It has been projected that extreme precipitation events will result in the greater prevailence of waterborne diseases in the Great Lakes area:

Overall, the models project that extreme precipitation events will become 10% to 40% stronger in southern Wisconsin, resulting in greater potential for flooding, and for the waterborne diseases that often accompany high discharge into Lake Michigan.

Using 6.4 cm (2.5 in) of daily precipitation as the threshold for initiating combined sewer overflow into Lake Michigan, the frequency of these events is expected to rise by 50% to 120% by the end of this century.

Jonathan A. Patz, et al. (2008) Climate Change and Waterborne Disease Risk in the Great Lakes Region of the U.S., Am J Prev Med; 35(5)

Dengue and malaria are both associated with higher temperatures, elevated humidity and greater rainfall:

A diagnostic study of the effects of climate change in Mexico’s human population, which was published in September, 2006, by Mexico’s National Institute of Ecology and National Institute of Public Health stated, “In some states we found an increase in the incidence of diseases transmitted through vector [dengue and malaria] associated with the increase in temperatures and with rainfall as a co-variable.”

Horacio Riojas, head of the environmental-health unit at the National Institute of Public Health and the lead author of the study, says that the recent trend of higher temperatures and humidity permit the population of dengue mosquitoes to increase in endemic places like the southern state of Chiapas as well as in new regions like the northern state of Chihuahua. Riojas adds that the vector is finding new niches where it can survive.

Eliza Barclay (22 March 2008) Is climate change affecting dengue in the Americas?, The Lancet, Volume 371, Issue 9617, Pages 973 – 974

Water- and food-borne diseases (such as Campylobacter and Salmonella) are also expected to increase as the result of higher temperatures and extreme precipitation events:

Large water-borne disease outbreaks have been linked to extreme precipitation events, which are expected to increase in frequency in coming decades (Figure 3). In addition, most cases of water-and food-borne gastroenteritis, particularly illness related to Campylobacter and Salmonella, exhibit a distinct summertime pattern of occurrence. Although it is possible that seasonality is due to behavioural patterns (e.g., barbecuing or swimming in the summer), the association between warmer temperatures and disease suggests that rates of water-and food-borne illness are likely to increase with rising temperatures.

Brian Hjelle and Gregory E. Glass (2000) Outbreak of Hantavirus Infection in the Four Corners Region of the United States in the Wake of the 1997-1998 El Nino-Southern Oscillation, J Infect Dis. 181 (5): 1569-1573.

Part III: Drought

Drought concentrates contaminants and makes people more desperate for whatever water is available.

Kovatis states as much in relation to pathogens in a Bulletin of the World Health Organization:

Drought can also lead to increased concentrations of pathogens in surface water and to hygiene-related diseases.

Kovatis, R. Sari. El Niño and human health. Bull World Health Organ [online]. 2000, vol.78, n.9 [cited 2011-09-30], pp. 1127-1135

In particular, waterwashed diseases spread more easily under drought conditions :

Bradley showed that all the diseases that are commonly considered waterborne can also be transmitted by the waterwashed route, and that the latter was epidemiologically more important under conditions of water scarcity as, for example, in rural and periurban areas in developing countries. The water-washed transmission route is also likely to be important even in areas with an adequate water supply but where personal and/or domestic (including food) hygiene is poor-Operation Clean Hands, recently launched by the American Society for Microbiology (Cassell and Osterholm 1996), is an example of the recognition of the need to reduce water-washed disease transmission in a highly developed society.

D. D. Mara and R. G. A. Feachem (January 1999) Water- and Excreta-Related Diseases: Unitary Environmental Classification, Journal of Environmental Engineering, 125:334-339

Included among the water-washed diseases that may be exacerbated by drought conditions are the superficial
trachoma and scabies and intestinal shigella dysentery.

Part IV: Ocean Circulation and Lateral Gene Transfer

Land that warms more quickly than ocean will tend to change ocean circulation so that algae which would normally be carried out to sea will remain along the coasts – creating the giant algae blooms and deadzones as the blooms die off – and the same principle that keeps algae near the shore should keep sewage closer as well, helping to concentrate bacteria and strengthen the lateral gene transfer that leads to enhanced antibiotic resistance.

These changes in ocean circulation due to global warming have been observed:

Delayed early-season upwelling and stronger late-season upwelling are consistent with predictions of the influence of global warming on coastal upwelling regions.

John A. Barth et al. (March 6, 2007) Delayed upwelling alters nearshore coastal ocean ecosystems in the northern California current, PNAS vol. 104 no. 10, pp 3719-3724

They are resulting in deadzones along some coasts:

Eastern boundary current systems are among the most productive large marine ecosystems in the world. Their productivity arises from wind-driven upwelling of nutrient-rich water into the photic zones of coastal oceans and supports 20% of global fishery yield (1). Upwelling also transports oxygen poor waters onto productive continental shelves, where respiration can further reduce watercolumn dissolved oxygen (DO) content and thus subject coastal ecosystems to the risk of hypoxia or anoxia.

Chan, F., J. A. Barth, J. Lubchenco, A. Kirincich, H. Weeks, W. T. Peterson and B. A. Menge (2008), Emergence of anoxia in the California Current

The mechanism and its relation to global warming is explained in some detail in McGregor (2007):

20th-century intensification of coastal upwelling SST records for the 20th century suggests an influence of global warming on the temperature evolution and upwelling intensity at site GeoB6008. The rapid 20th-century cooling at Cape Ghir also coincides with the rise in atmospheric CO2 (Fig. 2C). This reflects the influence of CO2 on the land-sea thermal contrast in NW Africa and, in turn, on the alongshore winds driving the upwelling. According to the mechanism proposed by Bakun (1990), increased atmospheric CO2 concentration could lead to warmer surface air temperatures (SATs) over land relative to those over the ocean, particularly at night time when radiative cooling is suppressed by the blocking of outgoing longwave radiation by CO2. The increased SAT deepens the thermal low-pressure cell over land, while a higher-pressure center develops over the slower-warming ocean waters. The winds blow clockwise around the high and anticlockwise around the continental low. The coast represents the boundary separating the two centers. Therefore, along the coast, the wind is oriented alongshore and southward (equatorward), which thus drives the upwelling and negative SST anomalies.

McGregor, H.V., Dima, M., Fischer, H.W., Mulitza, S.(2007). Rapid 20th-Century Increase in Coastal Upwelling off Northwest Africa, Science, 315( 5812), 637-639.

Contaminants, such as radioactive material or toxins, tend to become more concentrated the further up the food chain you go. The same principle, it seems, applies to antibiotic resistance, and it is applicable in our oceans:

A total of 134 viable bacteria samples were isolated from the cloacal swabs of predatory fishes. Isolates were characterized by Gram-stain morphology and tested for resistance by using the Kirby-Bauer disc diffusion method. Thirteen drugs (penicillin G, piperacillin, ticarcillin, cefotaxime, ceftazidime, ceftiofur, amikacin, gentamicin, ciprofloxacin, enrofloxacin, doxycycline, chloramphenicol, and sulfamethoxazole) were selected for this study…. Sharks sampled in the Florida Keys exhibited the greatest resistance to a wide selection of drugs. Resistance to at least one drug was found in each of the six study sites and in all of the fish species sampled. Multidrug resistance was also documented in most of the study sites…. The findings of this study confirmed the presence of antibiotic-resistant bacteria in marine predatory fishes from multiple taxa and multiple geographic locations.

Jason K. Blackburn et al. (2010) Evidence of Antibiotic Resistance in Free-Swimming, Top-Level Marine Predatory Fishes, Journal of Zoo and Wildlife Medicine 41(1):7-16.

In at least one case a plasmid appeared to be a major vector for the transmission of antibiotic resistance in a marine environment:

The present study was aimed to detect the plasmid profile, the role of plasmid associated multiple antibiotic resistance of Vibrios isolated from coastal waters of Kerala. The isolated plasmids from antibiotic resistant Vibrios were tested for the presence of integrons using polymerase chain reaction (PCR) to elucidate the presence of plasmid borne integron, a key element in horizontal gene transfer. 100 isolates of Vibrios from water samples of shrimp farms and coastal landing sites were tested for the antibiogram profile to 22 antibiotics and the presence of the plasmids. Antibiotic resistance studies revealed that 78% were expressing multiple antibiotic resistance (MAR), defined as the isolates having resistant to more than three resistance determinants.

Manjusha, S. and Sarita, G. B (2011) Plasmid associated antibiotic resistance in Vibrios isolated from coastal waters of Kerala,
International Food Research Journal 18(3): 1171-118

As for lack of investment, we might eliminate a disease such as tuberculosis in the developed nations, but if we fail to do so in the developing nations where the use of antibiotics is less regulated, such diseases may come back to haunt us, and they may do so after acquiring antibiotic resistance. Tuberculosis has been making a comeback. Furthermore, if climate change hits our economies hard, there may not be the resources necessary to invest.

Part V: Dust Storms

In the area of dust storms fungi stand out:

One of the best examples of dust-borne pathogens is the small outbreaks of coccidiomycosis (caused by the fungal pathogen Coccidioides immitis) that occur annually in the Americas following dust events. One of the first links to be made between long-range transport of desert dust and ecosystem health was the isolation and identification of a terrestrial fungus (Aspergillus sydowii) as the causative agent of a Caribbean-wide sea fan disease from atmospheric samples collected in the US Virgin Islands. An outbreak of aspergillosis in caged desert locusts was documented following a dust event in Bikaner, India. Of those dust-associated isolates we have identified using DNA sequencing of the ribosomal gene ~20 % are species known to cause disease in a broad range of plant and animal life and ~10 % are known opportunistic human pathogens. Although dose is certainly an issue when determining risk from exposure, it should not be surprising that dust-borne pathogenic species capable of surviving atmospheric transport are capable of causing disease in downwind ecosystems.

Griffin, D.W. (2005) Clouds of desert dust and microbiology: A mechanism of global dispersion. Microbiology Today. 32:180-182.

You may want to look up other papers by Griffen if this is a topic that interests you.

Part VI: War and Human Migration

There are numerous ways in which war is likely to result in increased transmission of diseases. One of course is the likely decline in sanitation. But I have touched on that previously in relation to flooding. So at this point I will focus on human migration.

Regarding dengue, and more broadly, other vector-borne diseases, Eliza Barclay states:

Experts have cited the increased mobility of people within countries, from rural communities to cities and across international borders as another key contribution to the spread of dengue. You can reintroduce into a population a vector-borne disease if you have enough travel”, said Fauci.

Eliza Barclay (22 March 2008) Is climate change affecting dengue in the Americas?, The Lancet, Volume 371, Issue 9617, Pages 973 – 974

In any case, this was by no means meant to be comprehensive, but I would like to believe that for those who are interested it could serve as an introduction to a topic of considerable significance.