Climate Change and Vector-Borne Diseases

Climate change creates new risks, particularly in the United States, for human exposure to vector-borne diseases (VBDs) — diseases which are transmitted to humans through the bites of insects (referred to as vectors) that carry the disease-causing pathogens. Common vectors include mosquitoes, ticks, and flies.

Climate change creates new uncertainties about the spread of VBDs such as the Zika virus, dengue fever, malaria, and Lyme disease by altering conditions that affect the development and dynamics of the disease vectors and the pathogens they carry.

Rising global temperatures can lengthen the season and increase the geographic range of disease-carrying insects. As temperatures warm, mosquitoes and other warm-weather vectors can move into higher altitudes and new regions farther from the equator. For instance, in some regions in the United States, warming is lengthening the the season for Zika-carrying mosquitoes.

Increased rainfall, flooding and humidity creates more viable areas for vector breeding and allows breeding to occur more quickly, as eggs hatch faster in hotter climates. For example, officials braced for an increase in risk for Zika and West Nile virus infections after the massive flooding event in Louisiana in August 2016, which increased the breeding habitats for Aedes mosquitoes.

Human migration exposes people to viruses to which they are not immune. As populations migrate in response to climate change, they bring disease to new regions and urban areas. Infectious diseases spread more quickly in overcrowded urban areas.

The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) stated that VBDs are some of the best-studied diseases associated with climate change, due to their widespread occurrence and the vectors’ sensitivities to their environments. While the direction of change is difficult to predict, there is mounting evidence that certain vectors, such as Aedes mosquitos, are mostly changing their range. For example, dengue fever, chikungunya and West Nile virus are emerging in areas where they were previously unknown and there is mounting evidence that this is due, in part, to increasing temperatures, along with other factors, such as increasing global travel and trade. While there is less research on Zika, the same vectors facilitate its spread, which scientists say points to increased risk in some regions.


Vector-Borne Diseases (VBDs) in a Warming World

Observed Impacts and Risks

In the Unites States, mosquito season has grown in 76 percent of major cities since the 1980s due to increases in hot and humid weather conditions. Globally, the World Health Organization (WHO) reports that there are more than 1 billion cases and more than 1 million deaths from VBDs annually. VBDs including malaria, dengue, Lyme disease, schistosomiasis, leishmaniasis, Chagas disease, and yellow fever, account for more than 17 percent of all infectious diseases.

VBDs overwhelmingly and disproportionately impact people living in tropical and subtropical developing countries, though warmer temperatures, migration, travel, and trade increases the risk of these diseases spreading to more temperate climates.

According to the WHO, there are three key components that determine the occurrence of VBDs:

  • vector and host abundance;

  • local prevalence of disease-causing parasites and pathogens;

  • and human population behavior and disease resilience.

Climate change affects these three key components through changes in temperature, precipitation, humidity, and other factors that influence the reproduction, development, behavior, and population dynamics of insects, pathogens, and people.

Insect vectors have several physical traits that help them take advantage of climate impacts like flooding, increased precipitation, and warmer weather.

Body Temperature: Insects cannot regulate their body temperature and are dependent on external warmth to survive. Rising temperatures may cause vector range patterns to shift, increasing the risk to new populations.

Breeding: Humidity and water is crucial for vector breeding, so more insects can hatch in areas with standing water and high precipitation.

Pathogen Incubation: The incubation period of pathogens within vectors is also temperature-dependent, and becomes shorter in warmer conditions.

Infographics sourced from the Earth Institute at Columbia University.


Future Projections

VBD cycles are complex because of constantly changing interactions between pathogen, insects, and people. Changes in climate make these interactions less predictable, multiplying the risks of the disease.

In the United States, projected increases in spring, summer and fall temperatures will likely increase the total number of days each year when temperature fall between 50 to 95°F—the temperature window in which mosquitoes thrive. Projections for more heavy downpours may also increase the threat from VBDs.

The future risk to humans of VBDs depends on many factors. The American Journal of Preventative Medicine reports that predicting the impact of climate change on VBDs requires long-term studies that look at “other agents of global change, such as increased trade and travel, demographic shifts, civil unrest, changes in land use, [and] water availability,” many of which intersect heavily with climate change.

WHO estimates that rising global temperatures, as well as altered precipitation and humidity linked to climate change, could significantly alter VBDs and their effect on human populations—making epidemics more difficult to predict and control. The changes in VBDs would likely occur as both short-term epidemics and long-term gradual changes in disease trends.


Zika Virus

Zika virus is a tropical disease spread by Aedes mosquitoes, which also carry West Nile virus, as well as yellow and dengue fever. Since its discovery in 1947 in the Zika forest of Uganda, Zika virus has spread to many other parts of the world, where it is becoming a serious global pandemic. The virus, which can be transmitted from mother to fetus during pregnancy, has been linked to an increase in miscarriages, and deaths in newborns, and birth defects, especially a condition known as microcephaly, in which the brain does not fully develop and babies are born with abnormally small heads. Initial research published on August of 2016 suggests that certain adult brain cells, that replace lost or damaged neurons, are also vulnerable to the Zika virus infection.

The Aedes vectors breed in containers where fresh water is collected, and their eggs can survive for long periods in a dormant state. But temperatures play a vital role in the vector’s survival, viral replication and infective periods. Higher temperatures from climate change could expand the geographic range, decrease the incubation period of the pathogen, and increase the biting rate of the mosquitoes. More precipitation could provide additional habitat for larvae, and changes in human behavior, such as deforestation, dam construction, the extinction of natural predators, and changes in biodiversity, can also accelerate the spread of Zika.

An April 2016 study found that the potential habitat range for Aedes aegypti could increase up to 13 percent under the RCP 8.5 high greenhouse gas emissions scenario by 2061-2080. Up to 460 million additional people could be exposed under this scenario when translated in terms of today’s population.

Since 2015, international organizations and national governments have become increasingly concerned with the spread of the Zika virus. Health officials in El Salvador, Brazil, Columbia, Ecuador and Jamaica recommended in January  2016 that women delay pregnancy until the rate of Zika transmission is down. The WHO declared the disease a Public Health Emergency of International Concern in February 2016.

In the U.S., the Centers for Disease Control and Prevention (CDC) issued warnings to pregnant women traveling to countries where the Zika virus transmission is occurring. A March 2016 study shows meteorological conditions are suitable for Aedes mosquitoes in the southern half of the U.S. during peak summer months, and in Southern Florida and Texas during winter months. Thus, Zika transmission can be expected in more parts of the U.S. In June of 2016, the first baby with Zika-related microcephaly was born in New Jersey.  In August 2016, the CDC warned pregnant women against traveling to Miami-Dade County in Florida, where more than 14 cases of local transmission of Zika virus has been confirmed.


Malaria

The WHO reports that in 2015, approximately 3.2 billion people – nearly half of the world's population – were at risk of malaria. Sub-Saharan Africa carries a disproportionately high share of malaria cases and deaths. Malaria is transmitted through the bites of female Anopheles mosquitoes, which lay eggs in the water and thrive during rainy seasons of tropical countries.

Rainfall patterns, humidity and temperature greatly affect the transmission rate of malaria. Due to climate change, equatorial countries and semi-arid parts of southern Africa are expected to experience increased precipitation and warmth of an estimated 1.4 to 1.6°C by 2050. WHO predicts that two to three degrees warming could put up to 7 percent more people at risk of malaria.

The Environmental Health Perspectives reported that the U.S. has become more vulnerable to tropical diseases as climate change expands vector ranges. Furthermore, other non-environmental factors, such as cheap airfares and the rise of tourism in tropical cities, allow for almost 2,000 Americans to return from overseas with malaria annually.


Lyme Disease

 

Lyme disease is the most commonly reported VBD in the United States. In 2014, it was the fifth most common Nationally Notifiable disease, according to the CDC. Lyme disease is caused by the bite of an infected blacklegged tick. However, Lyme disease is not nationwide and is concentrated heavily in the Northeast and upper Midwest.

Today, these ticks live in 44 percent more counties than they did in 1996, spread over 43 states. The most dramatic changes were visible in the northern U.S., where climate change is predicted to increase their habitats. Since 1990, average daily temperatures in U.S. have increased by approximately 0.4°C, with most occurring over the past 30 years. The hydrological cycle is also changing, with increases in cloud cover and precipitation, which also helps the tick population to grow.