Increased Infectious Disease Distribution and Prevalence due to Climate Change

Author & Editor: Emilyann Ashford

Temperatures across the world have increased by an average of 0.18 degrees Celsius per decade since 1981, according to the NOAA 2020 Annual Report. Not only is species diversity across the globe being affected by this change in climate, so is human health. Distributions of infectious diseases are expected to alter increasing their affected areas based on various future climate model projections. The Center of Disease Control (CDC) has reported on two ways that infectious diseases could spread and how they may be impacted due to climate change: vector-borne and food or water-borne.

Vector-Borne Diseases

Diseases caused by parasites, viruses, and bacteria which utilize vectors for dispersal methods, are considered Vector-Borne. Vectors are any living organism that transmits a pathogen from one host to another. Mosquitos and Ticks are the main transmitters, as they are blood-sucking insects which allows for easier infection of the host, and thus they contribute to more diseases as shown in Table 1 (7). More than 700,000 human deaths can be attributed to vector-borne diseases annually, while some survivors are left with life-long disabilities (10). For example, Lymphatic filariasis is a parasitic disease caused by filariasis worms and transmitted by mosquitos, which can cause elephantiasis. Elephantiasis is characterized by extreme swelling of legs or arms which may cause permanent disability if left untreated (See Figure 1). A list of common vector-borne diseases is shown in Table 1. 

Figure 1. Photo of Elephantiasis on right leg, with illustration of filial worm causing this disease in bubble.

Figure 1. Photo of Elephantiasis on right leg, with illustration of filial worm causing this disease in bubble.

Table 1. List of some Vector-borne diseases, with their vector and type of pathogen shown (9).

Table 1. List of some Vector-borne diseases, with their vector and type of pathogen shown (9).

Climate Change is expected to indirectly affect infectious vector borne diseases via an increase in geographical distribution of vectors due to increased precipitation and global warming (7). Two diseases that have already altered their geographic distribution are Malaria and Dengue Fever. Both of these diseases being mosquito-borne, many studies have shown how seasonality, distribution and transmission intensity have been responsive to global change (1). 

The Impact of Climate on Malaria Transmission

Digging deeper into the story relating malaria and climate change, we see various factors coming into play. The transmission of Malaria depends on a multitude of factors ranging from environmental threshold conditions for the mosquito, to extrinsic parasitic survival and abundance (1). Dr. M. Craig and his team studied thirty years of malaria case data from South Africa to understand if there was a correlation between specific climate factors and malaria incidences (1). They found that seasonal changes in case numbers were significantly affected by daily temperatures and total rainfall (1). Higher abundances of healthy, biting mosquitos were seen in warmer moister climates. Thus, a general correlation is seen with more cases of malaria during warmer season, as shown in Figure 2 (1). Heavy precipitation had a similar effect on cases, with cases increasing as precipitation increased (1). They believed that the more rainfall, the larger the breeding pools will be for reproduction, and the earlier the pools may be available. This will lead to greater mosquito populations, and thus greater case numbers of malaria the following season, also seen in Figure 1 (1). Thus, the earlier the rainy seasons begin, and the earlier the temperature rises, the more cases of malaria can be expected that year (1).

Figure 2. Scatterplot with Linear regressions outlining the effects of: a) total rainfall from November to March on delta log case numbers of Malaria and b) mean average temperature from January to October on delta log case numbers of Malaria (1).

Figure 2. Scatterplot with Linear regressions outlining the effects of: a) total rainfall from November to March on delta log case numbers of Malaria and b) mean average temperature from January to October on delta log case numbers of Malaria (1).

The Impact of Climate on Dengue Fever 

Dengue fever is a viral infectious disease caused by the Dengue Virus and transmitted by mosquitoes as mentioned above (2). Around 40 percent of the world's population lives in the geographical distribution of the disease and thus are at risk of contracting Dengue fever (2). Currently there are various Dengue outbreaks, for example there are cases on the rise in the Caribbean (10). A few studies suggest climate change may be playing a role in such increases (3,4). Hales et al., created a model based on previous outbreaks and collected climate data to understand changes in geographical distribution of Dengue in response to climatic factors (4). This model created allowed for a distribution modeling to 89% accuracy which projects that by 2085, if climate change continues as predicted, upwards of 50% of the global population could be at risk of Dengue transmission due to an increase in its geographical distribution (4). This increase in geographical distribution is shown in Figures 3 and 4, showing current, past and projected distributions respectively (4).

Figure 3. World map of current distribution of Dengue fever, with affected areas shown in green (4).

Figure 3. World map of current distribution of Dengue fever, with affected areas shown in green (4).

Figure 3. Two world maps, in A) 1990, and B) 2085 showing projected probability of Dengue Transmission, with colors ranging from least likely in purple, to most likely in red (4).

Figure 3. Two world maps, in A) 1990, and B) 2085 showing projected probability of Dengue Transmission, with colors ranging from least likely in purple, to most likely in red (4).

Food/Water Borne Diseases

Infectious diseases are also transmitted through food and water ingested. Almost 50 million people will be infected by a food or water borne pathogen each year. Illnesses resulting from food and water borne exposure range from gastrointestinal issues to cancers or even death (9). Many of these illnesses are caused by harmful bacteria like, Salmonella, Shigella dysenteriae, Escherichia coli, Cryptosporidium parvum, and Vibrio cholerae which are labeled to be the most threatening to human health by the CDC (8). Increased rainfalls and higher temperatures will also create warmer waters, serving as ideal habitats for these types of harmful bacteria. Also, important to note is the availability of social infrastructure affects how prevalent infectious diseases will become as climate change occurs; areas with greater infrastructure will be less affected than those lacking this infrastructure due to their ability to implement cleaning and filtering processes.

Impact of Climate on Vibrio cholerae

It has already been seen that cholera cases, caused by Vibrio cholerae, vary based on seasonality. Some areas, such as Bangladesh, show biannual increases in cases, while others show only one case peak (5). In order to understand this seasonality in response to climatic factors such as temperature and rainfall, Hasizume et al., studied the correlation between these and number of cholera cases every week for a year, while also incorporating river level as a factor (5). This study found that the number of cholera cases seemed to increase with both higher rainfalls at lag (5). You can see how rainfalls at lag work by looking at Figure 5, around week 25 rainfall increases, which then causes an increase in river level around week 31, as well as an increase in Cholera cases around week 39. Hasizume et al., hypothesized that an increase in rainfall, would cause the rise in river levels to promote contamination of river water due to flooding, and therefore produce the lagged increase in Cholera cases (5).

Figure 5. Number of Cholera cases per week compared to rainfall, temperature, and river levels (5).

Figure 5. Number of Cholera cases per week compared to rainfall, temperature, and river levels (5).

What is Being Done?

Many people are modeling ways to understand how the geographical distributions of infectious diseases will be altered in the future, but only recently are they considering how climate change can play a part. Water and food borne diseases will become an issue, only if we are not implementing programs to process and clean water and potentially infected food. This will likely occur in lower socioeconomic countries that aren’t able to fund these programs. High socioeconomic countries will not be as affected due to their ability to distribute funds towards these cleaning and filtering processes. Some supporting programs, in lower socioeconomic areas, are being implemented by the CDC and can be found here. As for Vector-borne diseases, we currently have two vaccines available and approved by the World Health Organization for malaria (7). While other vaccines are in production or in clinical trial stages, we are seeing low efficacies such as 36% after four doses of a trial vaccine for malaria (7).

What Can We Do?

It is vital to understand how each factor partakes in the spread of common infectious diseases, as well as their interactions—especially for those vector-borne. For vector-borne diseases it is important to continue funding the research of such diseases to produce vaccines or treatments as these infections are projected to become more prevalent over larger geographical distributions. In the response to Water or food borne diseases it is important that we remember to report the common “food poisoning” illnesses to the CDC via NORS, which stands for National Outbreak Reporting System. This allows the CDC to identify infected foods or water and inform the public before the outbreak continues and harms more human health.

Citations

  1. Craig, M. H., Kleinschmidt, I., Nawn, J. B., Le Sueur, D., Sharp, B. L. Exploring 30 years of malaria case data in KwaZulu-Natal South Africa: Part I. The impact of climatic factors. 2004. 9(12);1247-1257 

  2. Dengue Fever. https://www.cdc.gov/dengue/index.html 

  3. Hales, Simon., Wienstein, Phil., Souares, Yvan., Woodward, Alistair. El niño and the dynamics of vector-borne disease transmission. 1999. Environmental Health Perspectives. 102(2). 

  4. Hales, Simon.,  de Wet, Neil., Maindonald, John., Woodward, Alistair. Potential effect of population and climate changes on global distribution of dengue fever: an empirical model.  2002. The Lancet. 360(9336): 830-834.

  5. Hashizume M., Armstrong, B., Hajat, S., Wagatsuma, Y., Faruque, AS., Hayashi, T., Sack, DA., The effect of rainfall on the incidence of cholera in Bangladesh. Epidemiology., 2008 Jan; 19(1):103-10. 

  6. Kurane, Ichiro., The Effect of Global Warming on Infectious Diseases. Osong Public Health Res Perspective. 2010. 1 (1); 4-9 

  7. Manning JE, Cantaert T. Time to Micromanage the Pathogen-Host-Vector Interface: Considerations for Vaccine Development. Vaccines (Basel). 2019. 7(1):10. 

  8. Rauner, Bruce. Emergency Preparedness: Food and Water-Borne Illnesses. Illinois Department of Public Health. 

Emilyann Autumn