The Effects of Climate Change on Vector-Borne Diseases

By Beth Pedersen

Fall 1997

Abstract

An Overview of Climate Change

Climate Change and Disease

Focus: The Resurgence of Malaria

Mathematical Modeling of Malaria Transmission

Conclusions

References

The earth's climate is changing. It is being altered anthropogenically--a direct result of the increased emissions of greenhouse gases into the atmosphere over the last two hundred years. Other contributing factors include de-forestation and biomass burning, which can also be related to the development process. The most significant parameters of climate change which influence infectious disease patterns are temperature, precipitation and humidity. Increasing levels of all three parameters have favored a corresponding increase in the transmission of vector-borne disease. Response to climate alterations is exhibited by a northward movement of the geographic distribution of disease vectors, along with alterations in the life cycle and transmission potential of both vectors and infectious parasites. This has caused a corresponding increase in the dynamics of transmission of infectious disease between humans and the vector species. These changes are especially evident in the increased number of malaria outbreaks that have occurred in recent years. Computer models predict that transmission rates will continue to increase, mirroring the increasing surface temperature of the earth, unless drastic action is taken.

The earth's climate is continuously changing. During the Cretaceous period, an era which ended approximately 65 million years ago, the overall global temperature was believed to be 10-15 ^oC warmer than it is today. And during the most recent ice age, a period defined as between 90,000 and 12,000 years ago, the Earth's mean temperature was 5 ^oC lower than it is at present. But for the past 10,000 years, informally referred to as the "current interglacial period," the surface temperature has remained fairly constant. That is, up until the last two hundred years...

Ever since the beginning of the Industrial Revolution, the emissions of green house gases--those gases that trap heat within the earth's atmosphere--have been rising at an alarming rate. GHGs include compounds such as carbon dioxide, carbon monoxide, sulfur oxide compound (SO_x), nitrogen oxide compounds (NO_x) and methane. Carbon dioxide levels alone have increased by over 30%. The net increase of GHGs in the stratosphere has resulted in an enhancement of the greenhouse effect. This "global warming", in turn, impacts all areas of life on this planet. In addition to changes in the physical environment of the earth, effects have been observed in the socio-economic status and the general health of the entire human population.

Variations in climate can have a profound effect on the earth's biota, manifested by alterations in the frequency and severity of storms, hurricanes, floods and droughts. These in turn have a huge impact on societies, productivity and the rate of development in both a regional and a global sense. Climate change affects humans via three interconnected pathways:

1. Distribution and quality of surface water
2. Life cycle of disease vectors and host/vector relationships
3. Ecosystem dynamics of predator/prey relationships which control populations

The remainder of this paper will focus on the second pathway.

There are many types of infectious diseases that affect the human population. Agents or carriers of these diseases can take the form of parasites, viruses, bacteria, protozoa, or multi-celled organisms such as flukes. Vector-borne diseases involve a third party, or "vector", which transmits the infective agent between humans or other animals. Vectors are typically cold-blooded organisms, and include insects, ticks, snails and crustaceans. Two factors play key roles in the transmission of vector-borne diseases: geographical distribution of vectors and regional vectorial capacity. For vectors with long life spans such as tsetse flies and ticks, vector abundance is also an issue.

A summary of the most common vector-borne diseases in the table below:

Temperature: It has been shown that increases in temperature cause a corresponding acceleration of vectors' metabolic processes. This then affects their nutritional requirements. Blood-feeding vectors such as the mosquito need to feed more frequently to sustain themselves. This causes an increase in biting rates, which are a direct measurement of transmission potential. Temperature changes also relegate the geographic distribution of vectors, which are limited by minimum and maximum temperatures.

Humidity: High relative humidity favors almost all metabolic processes in vector organisms. It also serves to prolong the lifetime of these carriers, and may cause an acceleration in their reproductive rates.

Precipitation: Common vector organisms, such as the mosquito and the blackfly, have aquatic larvae and pupae stages. The amount of precipitation received by a certain region determines the availability of breeding sites for these vectors. The relative impact of precipitation depends on local evaporation rates, soil percolation rates, the geographic layout of the region and the proximity of large bodies of water. Breeding rates typically increase proportionately with the amount of rainfall received in a given region. However, extremely heavy rainfall will wash vector larvae away or kill them directly.

Wind: Although only a secondary or sub-parameter of climate change, winds contribute to the passive dispersal of flying insects. Wind speed and direction can therefore display noticeable effects on the distribution of disease vector on a localized level.

There are also many indirect effects of climate change on the transmission of infectious diseases. One that is often overlooked is the alteration of human behavior patterns within a given region. These changes can make the population more susceptible to infection. This is especially true with regards to fecal-oral infections, food-borne diseases, and those communicable diseases that are easily spread from person to person. Competition between vector species can also cause increases in the outbreak of disease. When two species must fight over the same natural food source, one or the other will be driven to alternate feeding grounds. And for blood-feeding vector species, this target is usually humans or livestock.

Changes in temperature, precipitation, humidity and storm patterns are often related to the El Nino-Southern Oscillation (ENSO) phenomenon. These are associated with upsurges of water-borne diseases as well as vector-borne pathogens such as malaria, dengue, yellow fever, encephalitis, plague and the hantavirus. A paper written by Paul Epstein details the disruption of ecosystem dynamics and the resulting increase in outbreaks of vector-borne diseases associated with the ENSO phenomenon.

Few diseases have had as great an impact on the social and economic development of societies as has malaria. Almost half of the population of the world (5.3 billion) is classified as having a significant risk of contracting this disease. Approximately 270 million people are actually infected each year. Before World War II, malaria was a relatively common disease in many temperate areas of the world. But in the years after 1945, the use of DDT led to a significant decrease in the incident rate. Occurrences of malaria in temperate climates were virtually eliminated, along with a drastic reduction of infection in tropical regions. Another reason for the decreasing rate of contraction was the development of chloroquinone, a synthetic form of the compound quinone that had traditionally been used for treatment. This new form was less toxic and far more effective than the naturally occurring extract, which had to be harvested from the bark of the cinchona tree.

Malaria is caused by one or more of four species of parasites of the genus Plasmodium. The vector associated with malaria transmissions is the mosquito of the genus Anopheles. The life cycle of the parasite involves transmission both from mosquito to man and from man to mosquito.

The parasite is carried by the female mosquito host and reproduces within her body. After two or three days of incubation the parasites can be found in the salivary glands. When the mosquito bites a human, saliva is sub-cutaneously injected and the parasites are transferred. The parasites quickly retreat to the liver where they mature and multiply. It is not until they re-emerge in the bloodstream and invade the blood cells that symptoms of the disease appear. But by this time the parasites have reproduced thousands of times. They thrash about, popping blood cells, clogging blood vessels, debilitating their host and in some cases killing within hours.

In recent years a resurgence of malaria outbreaks has occurred. The new wave of outbreaks is most pronounced in the coastal regions of Africa, the northern third of South America and the entire region of South-East Asia. The development of vector resistance to long-term use of insecticides is considered to be the primary reason for this phenomenon, although climate change is also considered a major factor.

Recent emergence of a malaria cycle has been observed in Venezuela. While unsure as to the exact cause of each outbreak, speculation has been made that the people's immunity to th e disease may be lowered during the drought year following an ENSO occurrence. This is because during the drought, much of the marshy wetland that provides the optimum breeding grounds for Anopheles dry up. The number of infected insects is reduced drasti cally. Unfortunately, the mosquito populations generally recover more quickly than do those of their predators. This combination of decreased immunity in humans and increased number of mosquitoes then leads to malaria infections on epidemic levels.