# Climate change and infectious diseases

Global climate change has increased the occurrence of some infectious diseases. Infectious diseases whose transmission is impacted by climate change include, for example, vector-borne diseases like dengue fever, malaria, tick-borne diseases, leishmaniasis, zika fever, chikungunya and Ebola. One mechanism contributing to increased disease transmission is that climate change is altering the geographic range and seasonality of the insects (or disease vectors) that can carry the diseases. Scientists stated a clear observation in 2022: "The occurrence of climate-related food-borne and waterborne diseases has increased (very high confidence)."$^{:11}$
Infectious diseases that are sensitive to climate can be grouped into: vector-borne diseases (transmitted via mosquitos, ticks etc.), waterborne diseases (transmitted via viruses or bacteria through water), and food-borne diseases.(spread through pathogens via food)$^{:1107}$ Climate change affects the distribution of these diseases due to the expanding geographic range and seasonality of these diseases and their vectors.$^{:9}$ Like other ways climate change affects human health, climate change exacerbates existing inequalities and challenges in managing infectious disease.
Mosquito-borne diseases that are sensitive to climate include malaria, lymphatic filariasis, Rift Valley fever, yellow fever, dengue fever, Zika virus, and chikungunya. Scientists found in 2022 that rising temperatures are increasing the areas where dengue fever, malaria and other mosquito-carried diseases are able to spread.$^{:1062}$ Warmer temperatures are also advancing to higher elevations, allowing mosquitoes to survive in places that were previously in hospitable to them.$^{:1045}$ This risks malaria returning to areas where it was previously eradicated.
Ticks are changing their geographic range because of rising temperatures, and this puts new populations at risk. Ticks can spread lyme disease and tick-borne encephalitis. It is expected that climate change will increase the incidence of these diseases in the Northern Hemisphere.$^{:1094}$ For example, a review of the literature found that "In the USA, a 2°C warming could increase the number of lyme disease cases by over 20% over the coming decades and lead to an earlier onset and longer length of the annual Lyme disease season".$^{:1094}$
Waterborne diseases are transmitted through water. The symptoms of waterborne diseases typically include diarrhea, fever and other flu-like symptoms, neurological disorders, and liver damage. Climate changes have a large effect on the distribution of microbial species. These communities are very complex and can be extremely sensitive to external climate stimuli. There is a range of waterborne diseases and parasites that will pose greater health risks in the future. This will vary by region. For example, in Africa, Cryptosporidium spp. and Giardia duodenalis (protozoan parasites) will increase. This is due to increasing temperatures and drought.$^{:1095}$
Scientist also expect that disease outbreaks caused by vibrio (in particular the bacterium that causes cholera, called vibrio cholerae) are increasing in occurrence and intensity.$^{:1107}$ One reason is that the area of coastline with suitable conditions for vibrio bacteria has increased due to changes in sea surface temperature and sea surface salinity caused by climate change.$^{:12}$ These pathogens can cause gastroenteritis, cholera, wound infections, and sepsis. The increasing occurrence of higher temperature days, heavy rainfall events and flooding due to climate change could lead to an increase in cholera risks.$^{:1045}$

## Public health context

In 1988, little was known about the effects of climate change on human health. As of 2023, the evidence has grown significantly and is for example summarised in the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. The scientific understanding of potential health risks and observed health impacts caused by climate change is now better understood. One category of health risks is that of infectious diseases. A study concluded in 2022 that "58% (that is, 218 out of 375) of infectious diseases confronted by humanity worldwide have been at some point aggravated by climatic hazards". The World Health Organization considers climate change as one of the greatest threats to human health.
Infectious diseases have played a significant role in human history, impacting the rise and fall of civilizations and facilitating the conquest of new territories. During recent decades, there are significant regional changes in vector and pathogen distribution reported in temperate, peri‐Arctic, Arctic, and tropical highland regions.
Climate change is one of the factors that causes the spread of human diseases. Other key factors, include the mobility of people, animals, and goods; control measures in place; availability of effective drugs; quality of public health services; human behavior; and political stability and conflicts. The March 2022 report from the Intergovernmental Panel on Climate Change (IPCC) warned that without swift climate action we will see an escalation of infectious diseases. They will spread to new regions (may decline in some endemic areas) and surge in areas where they were previously under control. As a result, diseases that have never previously infected humans (Disease X) may 'spill over' from animals
Global warming, increased drought and flooding represent a significant threat to public health, likely leading to the escalation of vector, food and water-borne diseases The effects of climate change on health will impact most populations over the next few decades. However, Africa, and specifically, the African Highlands, are susceptible to being particularly negatively affected. For example, with regard to malaria, in 2010, 91% of the global burden due to malaria deaths occurred in Africa. Several spatiotemporal models have been studied to assess the potential effect of projected climate scenarios on malaria transmission in Africa. It is expected that the most significant climate change effects are confined to specific regions, including the African Highlands.
Climate change may lead to dramatic increases in the prevalence of a variety of infectious diseases. Beginning in the mid-'70s, an "emergence, resurgence and redistribution of infectious diseases" occurred. Reasons for this are likely multi-causal, dependent on a variety of social, environmental and climatic factors, however, many argue that the "volatility of infectious disease may be one of the earliest biological expressions of climate instability".

## Mechanisms and pathways

Infectious diseases (also called pathogenic diseases) depend on "a pathogen and a person coming into contact, and the extent to which peoples’ resistance is diminished, or the pathogen is strengthened, by a climatic hazard." Climatic hazards, which can be strengthened by climate change, include warming of land and oceans, heatwaves and marine heatwaves, floods, drought, storms, land cover change, fires and so forth.
Possible pathways that can increase the infectious disease occurrence and which are affected by climate change include:
* Climatic hazards bringing pathogens closer to people (e.g. shifts in the geographical range of species)
* Climatic hazards bringing people closer to pathogens (e.g. heatwaves bringing more people to recreational water activities)
* Pathogens strengthened by climatic hazards (e.g. "improved climate suitability for reproduction, acceleration of the life cycle, increasing seasons/length of likely exposure", for example ocean warming can lead to increased Vibriosis outbreaks)
* People impaired by climatic hazards (e.g. from malnutrition due to drought conditions)
Infectious diseases that are sensitive to climate can be grouped into:
* water-borne diseases (transmitted via viruses or bacteria, e.g.: E. Coli),
* vector-borne diseases (transmitted via mosquitos, ticks etc.), and
* food-borne diseases (e.g. Salmonella bacteria, causing Salmonellosis).$^{:1107}$
Climate change is affecting the distribution of these diseases due to the expanding geographic range and seasonality of these diseases and their vectors.$^{:9}$
Though many infectious diseases are affected by changes in climate, vector-borne diseases, such as malaria, dengue fever and leishmaniasis, present the strongest causal relationship. One reason for that is that temperature and rainfall play a key role in the distribution, magnitude, and viral capacity of mosquitoes, who are primary vectors for many vectors borne diseases. Observation and research detect a shift of pests and pathogens in the distribution away from the equator and towards Earth's poles.

### Changes to the distribution of vectors

Climate change affects vector-borne diseases by affecting the survival, distribution and behavior of vectors such as mosquitoes, ticks and rodents.$^{:29}$ The viruses, bacteria and protozoa are carried by these vectors transferring them from one carrier to another. Vectors and pathogens can adapt to the climate fluctuations by shifting and expanding their geographic ranges, which alter the rate of new cases of disease depending on vector-host interaction, host immunity and pathogen evolution. This means that climate change affects infectious diseases by changing the length of the transmission season and their geographical range.
Climate change is leading to latitudinal and altitudinal temperature increases. Global warming projections indicate that surface air warming for a "high scenario" is 4 C, with a likely range of 2.4–6.4 C by 2100. A temperature increase of this size would alter the biology and the ecology of many mosquito vectors and the dynamics of the diseases they transmit such as malaria.
Changes in climate and global warming have significant influences on the biology and distribution of vector-borne diseases, parasites, fungi, and their associated illnesses. Regional changes resulting from changing weather conditions and patterns within temperate climates will stimulate the reproduction of certain insect species that are vectors for disease.
One major disease-spreading insect is the mosquito, which can carry diseases like malaria, West Nile virus, and dengue fever. With regional temperatures changing due to climate change, the range of mosquitos will change as well. The range of mosquitoes will move farther north and south, and places will have a longer period of mosquito habitability than at present, leading to an increase in the mosquito population in these areas. This range shift has already been seen in highland Africa. Since 1970, the incidence of malaria in high-elevation areas in East Africa has increased greatly. This has been proven to be caused by the warming of regional climates.
The vectors of transmission are the major reason for the increased ranges and infection of these diseases. If the vector has a range shift, so do the associated diseases; if the vector increases in activity due to changes in climate, then there is an effect on the transmission of disease. However it will be hard to classify exactly why the range shifts or an increase in infection rates occurs as there are many other factors to consider besides climate change, such as human migration, poverty, infrastructure quality, and land usage; but climate change is still potentially a key factor.
Environmental changes, climate variability, and climate change are such factors that could affect biology and disease ecology of Anopheles vectors and their disease transmission potential.
Anopheles mosquitoes in highland areas are to experience a larger shift in their metabolic rate due to climate change. This climate change is due to the deforestation in the highland areas where these mosquitos' dwell. When the temperature rises, the larvae take a shorter time to mature and, consequently, a greater capacity to produce more offspring. In turn this could potentially lead to an increase in malaria transmission when infected humans are available.
Environmental changes such as deforestation could also increase local temperatures in the highlands thus could enhance the vector capacity of the anopheles. Anopheles mosquitoes are responsible for the transmission of a number of diseases in the world, such as, malaria, lymphatic filariasis and viruses that can cause such ailments, like the O'nyong'nyong virus.

### Increased water temperature

High temperatures can alter the survival, replication, and virulence of a pathogen. Higher temperatures can also increase the pathogen yields in animal reservoirs. During the warmer summer months an increase in yield of bacteria from drinking water delivery systems has been recorded. During times of warmer temperatures water consumption rates are also typically higher. These together increase the probability of pathogen ingestion and infection.
With an increase in not only temperature, but also higher nutrient concentrations due to runoff there will be an increase in cyanobacterial blooms.
The warming oceans and lakes are leading to more frequent harmful algal blooms. Also, during droughts, surface waters are even more susceptible to harmful algal blooms and microorganisms. Algal blooms increase water turbidity, suffocating aquatic plants, and can deplete oxygen, killing fish. Some kinds of blue-green algae (cyanobacteria) create neurotoxins, hepatoxins, cytotoxins or endotoxins that can cause serious and sometimes fatal neurological, liver and digestive diseases in humans. Cyanobacteria grow best in warmer temperatures (especially above 25 degrees Celsius), and so areas of the world that are experiencing general warming as a result of climate change are also experiencing harmful algal blooms more frequently and for longer periods of time.

### Changes in precipitation and water cycle

Climate change is forecast to have substantial effects on the water cycle, with an increase in both frequency and intensity of droughts and heavy precipitation events.
A literature review in 2016 found that generally there is an increase in diarrheal disease (except for viral diarrheal disease) during or after certain weather conditions: elevated ambient temperature, heavy rainfall, and flooding. These three weather conditions are predicted to increase (or to intensify) with climate change in future. There is already now a high current baseline rate of the diarrheal diseases in developing countries. Climate change therefore poses a real risk of an uptick in these diseases for those regions.

## Selected examples of relevant infectious diseases in humans
### Malaria

Increased rainfall could increase the number of mosquitos indirectly by expanding larval habitat and food supply. Malaria, which kills about 300,000 children (under age 5) annually, poses an imminent threat through temperature increase. Models suggest, conservatively, that the risk of malaria will increase 5–15% by 2100 due to climate change. In Africa alone, according to the MARA Project (Mapping Malaria Risk in Africa), there is a projected increase of 16–28% in person-month exposures to malaria by 2100.
Climate is an influential driving force of vector-borne diseases such as malaria. Malaria is especially susceptible to the effects of climate change because mosquitoes lack the mechanisms to regulate their internal temperature. This implies that there is a limited range of climatic conditions within which the pathogen (malaria) and vector (a mosquito) can survive, reproduce, and infect hosts. Vector-borne diseases, such as malaria, have distinctive characteristics that determine pathogenicity. These include the survival and reproduction rate of the vector, the level of vector activity (i.e. the biting or feeding rate), and the development and reproduction rate of the pathogen within the vector or host. Changes in climate factors substantially affect reproduction, development, distribution, and seasonal transmissions of malaria.
Malaria is a mosquito-borne parasitic disease that infects humans and other animals caused by microorganisms in the Plasmodium family. It begins with a bite from an infected female mosquito, which introduces the parasite through its saliva and into the infected host's circulatory system. It then travels through the bloodstream into the liver, where it can mature and reproduce.

### Dengue fever

Dengue fever is an infectious disease caused by dengue viruses known to be in the tropical regions. It is transmitted by the mosquito Aedes, or A. aegypti. Dengue incidence has increased in the last few decades and is projected to continue to do so with changing climate conditions. Dengue can be fatal. Dengue fever is spread by the bite of the female mosquito known as Aedes aegypti. The female mosquito is a highly effective vector of this disease.
The evidence for the spread of dengue fever is that climate change is altering the geographic range and seasonality of the mosquito that can carry dengue. Because there are multiple drivers of transmission, it is easier to model and project changes in the geographic range and seasonality. The drivers for the recent spread of this disease are globalization, trade, urbanization, population growth, increased international travel, and climate change. The same trends also led to the spread of different serotypes of the disease to new areas, and to the emergence of dengue hemorrhagic fever.
The World Health Organization (WHO) has reported an increase from a thousand to one million confirmed cases between 1955 and 2007. The presence and number of Aedes aegypti mosquitoes is strongly influenced by the amount of water-bearing containers or pockets of stagnant water in an area, daily temperature and variation in temperature, moisture, and solar radiation. While dengue fever is primarily considered a tropical and subtropical disease, the geographic ranges of the Aedes aegypti are expanding. The cases of dengue fever have increased dramatically since the 1970s and it continues to become more prevalent.
Dengue is ranked as the most important vector-borne viral disease in the world. An estimated 50–100 million dengue fever infections occur annually. In just the past 50 years, transmission has increased drastically with new cases of the disease (incidence) increasing 30-fold. The number of reported cases has continually increased along with dengue spreading to new areas.

### Tick borne disease

Tick-borne disease, which affect humans and other animals, are caused by infectious agents transmitted by tick bites. A high humidity of greater than 85% is ideal for a tick to start and finish its life cycle. Studies have indicated that temperature and vapor play a significant role in determining the range for tick population. More specifically, maximum temperature has been found to play the most influential variable in sustaining tick populations. Higher temperatures augment both hatching and developmental rates while hindering overall survival. Temperature is so important to overall survival that an average monthly minimum temperature of below -7 °C in the winter can prevent an area from maintaining established populations.
The effect of climate on the tick life cycle is one of the more difficult projections to make in relation to climate and vector-borne disease. Unlike other vectors, tick life cycles span multiple seasons as they mature from larva to nymph to adult. Further, infection and spread of diseases such as Lyme disease happen across the multiple stages and different classes of vertebrate hosts, adding additional variables to consider. Although it is a European species from the Lyme borreliosis spirochetes, Borrelia garinii was documented from infected ticks on seabirds in North America. Further research is needed to improve evolutionary models predicting distributional changes in this tick-borne system in the face of climate change. Infection of ticks happen in the larval/nymph stage (after the first blood meal) when they are exposed to Borrelia burgdorferi (the spirochete responsible for Lyme disease), but transmission to humans doesn't occur until the adult stages.
The expansion of tick populations is concurrent with global climatic change. Species distribution models of recent years indicate that the deer tick, known as I. scapularis, is pushing its distribution to higher latitudes of the Northeastern United States and Canada, as well as pushing and maintaining populations in the South Central and Northern Midwest regions of the United States. Climate models project further expansion of tick habit north into Canada as progressing Northwest from the Northeastern United States. Additionally, however, tick populations are expected to retreat from the Southeastern coast of the U.S., but this has not yet been observed. It's estimated that coinciding with this expansion, increased average temperatures may double tick populations by 2020 as well as bring an earlier start to the tick exposure season.
In the face of these expanding threats, strong collaboration between government officials and environmental scientists is necessary for advancing preventive and reactive response measures. Without acknowledging the climate changes that make environments more habitable for disease carriers, policy and infrastructure will lag behind vector borne disease spread.
In the United States, the Centers for Disease Control and Prevention (CDC) is conducting a grant program called Building Resilience Against Climate Effects (BRACE) which details a 5 step process for combating climate effects like tick borne disease spread.

### Leishmaniasis

As in other vector-borne diseases, one of the reasons climate changes can affect the incidence of leishmaniasis is the susceptibility of the sandfly vectors to changes in temperature, rainfall and humidity; these conditions will alter their range of distribution and seasonality. For example, modelling studies have predicted that climate change will increase suitable conditions for Phlebotomus vector species in Central Europe. Another model that looked at the distribution of Lutzomyia longipalpis, an important visceral leishmaniasis vector, suggested an increased range of this species in the Amazon Basin. A different study model that factored data on climate, policy and socio-economic changes of land use, found that the effects were different for cutaneous and visceral leishmaniasis, emphasizing the importance of considering each disease and region separately.
Parasite development inside the sand can also be affected by temperature changes. For instance, Leishmania peruviana infections were lost during sand defecation when the infected vector was kept at higher temperatures, whereas in the same experiment Leishmania infantum and Leishmania braziliensis temperature seemed to make no difference.
Leishmaniasis is a neglected tropical disease, caused by parasites of the genus Leishmania and transmitted by sandflies; it is distributed mostly in tropical and subtropical regions around the world, wherever the sand fly vector and reservoir hosts are present. The WHO estimates 12 million people around the world are living with leishmaniasis. Risk factors for the spread of this disease include poverty, urbanization, deforestation, and climate change.

### Ebola

The Ebola virus has been infecting people from time to time, leading to outbreaks in several African countries. The average case fatality rate of the Ebola virus is approximately 40% and there have been more than 28,600 cases with 11,310 deaths. Many researchers are linking deforestation to the disease, observing that change in the landscape increases wildlife contact with humans.
Recent studies are holding climate change indirectly liable for the uptick in Ebola: Seasonal droughts alongside strong winds, thunderstorms, heat waves, floods, landslides, and a change in rainfall patterns also impact wildlife migration. These conditions pull them away from their natural environment and closer to human proximity. One example of an Ebola outbreak caused by climate change or a shift in nature was seen during the drought of Central Africa. This ultimately amplified food insecurity leading West African communities to eat animals such as bats who were infected with the virus.

### Zika fever

Zika virus, a vector-borne virus was historically presented in cluster outbreaks in the tropical regions of Africa and Asia. Zika fever epidemics have affected larger populations including Micronesia and South Pacific Islands in 2007, and the Americas in 2013. Brazil has experienced one of the largest outbreaks of Zika virus with approximately 1.5 million cases reported in 2015. Pregnant women infected with Zika virus are at a higher risk of giving birth to children with congenital malformations, including microcephaly.
In the context of climate change and temperature rise, it is predicted that Zika virus will affect more than 1.3 billion people by 2050. This is largely due to the expansion of environments conducive to vector growth and life cycles, such as those with temperatures ranging from 23.9 °C to 34 °C. Mosquito behaviors are also affected by the change in temperature including increased breeding and biting rates. Furthermore, extreme climate patterns, including drought, floods and heatwaves are known to exacerbate the proliferation of mosquito breeding ground and as a result, escalate the rate of virus-borne diseases.

### COVID-19

There is no direct evidence that the spread of COVID-19 is worsened or is caused by climate change, although investigations continue. As of 2020, the World Health Organization summarized the current knowledge about the issue as follows: "There is no evidence of a direct connection between climate change and the emergence or transmission of COVID-19 disease. [...] However, climate change may indirectly affect the COVID-19 response, as it undermines environmental determinants of health, and places additional stress on health systems."
A 2021 study found possible links between climate change and transmission of COVID-19 by bats. The authors found that climate-driven changes in the distribution and richness of bat species increased the likelihood of bat-borne coronaviruses in the Yunnan province, Myanmar, and Laos. This region was also the habitat of Sunda pangolins and masked palm civits which were suspected as intermediate hosts of COVID-19 between bats and humans. The authors suggest, therefore, that climate change possibly contributed to some extent to the emergence of the pandemic.
Climate changed might induce changes to bat habitats which may have driven them closer to populated areas. Increased aridity and drought periods are predicted to push bats out of their endemic areas and into populated areas. This creates a knock-on effect of increasing their interactions with humans and hence the likelihood of zoonotic disease transfer.

### Vibrio infections

Scientist expect that disease outbreaks caused by vibrio (in particular the bacterium that causes cholera, called vibrio cholerae) are increasing in occurrence and intensity.$^{:1107}$ One reason is that the area of coastline with suitable conditions for vibrio bacteria has increased due to changes in sea surface temperature and sea surface salinity caused by climate change.$^{:12}$ These pathogens can cause gastroenteritis, cholera, wound infections, and sepsis. It has been observed that in the period of 2011–21, the "area of coastline suitable for Vibrio bacterial transmission has increased by 35% in the Baltics, 25% in the Atlantic Northeast, and 4% in the Pacific Northwest.$^{:12}$ Furthermore, the increasing occurrence of higher temperature days, heavy rainfall events and flooding due to climate change could lead to an increase in cholera risks.$^{:1045}$
Vibrio illnesses are waterborne disease and are increasing worldwide. Vibrio infections are recently being reported where historically it did not occur. The warming climate seems to be playing a substantial role in the increase in cases and area of occurrence.
Vibrio infections are caused by consuming raw or undercooked seafood, or by exposing an open wound to contaminated sea water. Vibrio infections are most likely to occur during the warm season, May through October.

### Skin Rashes

Climate change affects human health adversely and its impact on the skin is no exception. It is one of the greatest threats to our capacity to benefit in the context of “Skin Care for All.” In a study conducted in South Africa, the reduced work capacities and outputs were attributed to heat waves, which caused severe sunburns, sleeplessness, irritability, and exhaustion in workers. Risk assessments were conducted for extreme health impacts across African countries, especially Kenya, both at the regional and city scale. Rising temperature and humidity increase skin bacteria growth overall geographical distribution of other organisms that infect humans. The different organisms that form the skin microflora have variable optimal temperature for survival and growth. Staphylococcus aureus and Corynebacterium sp. amongst others are more tolerant to rising temperatures and higher salt conditions compared to other, non-commensal bacteria.

### Diarrhea diseases

One of the most commonly transmitted waterborne disease categories are the diarrhea diseases. These diseases are transmitted through unsafe drinking water or recreational water contact. Diarrheal diseases account for 10–12% of deaths in children under five, as the second leading cause of death in children this age. They are also the second leading cause of death in low and middle income countries. Diarrhea diseases account for an estimated 1.4–1.9 million deaths worldwide.

### Fungal infections

Fungal infections will also see an increase due to the warming of certain climates. For example, the fungus Cryptococcus gattii has been found in Canada but is normally found in warmer climates such as in Australia. There are now two strains of this fungus in the northwestern part of North America, affecting many terrestrial animals. The spread of this fungus is hypothesized to be linked to climate change.

### Emergence of new infectious diseases

There is concern about the emergence of new diseases from the fungal kingdom. Mammals have endothermy and homeothermy, which allows them to maintain elevated body temperature through life; but it can be defeated if the fungi were to adapt to higher temperatures and survive in the body. Fungi that are pathogenic for insects can be experimentally adapted to replicate at mammalian temperatures through cycles of progressive warming. This demonstrates that fungi are able to adapt rapidly to higher temperatures. The emergence of Candida auris on three continents is proposed to be as a result of global warming and has raised the danger that increased warmth by itself will trigger adaptations on certain microbes to make them pathogenic for humans.
It is projected that interspecies viral sharing, that can lead to novel viral spillovers, will increase due to ongoing climate change-caused geographic range-shifts of mammals (most importantly bats). Risk hotspots would mainly be located at "high elevations, in biodiversity hotspots, and in areas of high human population density in Asia and Africa".
Climate change may also lead to new infectious diseases due to changes in microbial and vector geographic range. Microbes that are harmful to humans can adapt to higher temperatures, which will allow them to build better tolerance against human endothermy defences.

## Infectious diseases in wild animals

Climate change and increasing temperatures will also impact the health of wildlife animals as well. Specifically, climate change will impact wildlife disease, specifically affecting "geographic range and distribution of wildlife diseases, plant and animal phenology, wildlife host-pathogen interactions, and disease patterns in wildlife".
The health of wild animals, particularly birds, is assumed to be a better indicator of early climate change effects because very little or no control measures are undertaken to protect them.

### Geographic range and distribution of wildlife diseases

Northern geographic shifts of disease vectors and parasitic disease in the Northern Hemisphere have likely been due to global warming. The geographic range of a lung parasite that impacts ungulates like caribou and mountain goats, Parelaphostrongylus odocoilei, has been shifting northward since 1995, and a tick vector for Lyme disease and other tick-borne zoonotic diseases known as Ixodes scapularis has been expanding its presence northward as well. It is also predicted that climate warming will also lead to changes in disease distribution at certain altitudes. At high elevation in the Hawaiian Islands, for example, it is expected that climate warming will allow for year-round transmission of avian malaria. This increased opportunity for transmission will likely be devastating to endangered native Hawaiian birds at those altitudes that have little or no resistance to the disease.

### Phenology and wildlife diseases

Phenology is the study of seasonal cycles, and with climate change the seasonal biologic cycles of many animals have already been affected. For example, the transmission of tick-borne encephalitis (TBE) is higher to humans when early spring temperatures are warmer. The warmer temperatures result in an overlap in feeding activity of ticks who are infected with the virus (nymphal) with ticks who aren't (larval). This overlapped feeding leads to more of the uninfected larval ticks acquiring the infection and therefore increases the risk of humans being infected with TBE. On the other hand, cooler spring temperatures would result in less overlapped feeding activity, and would therefore decrease the risk of zoonotic transmission of TBE.

### Wildlife host-to-pathogen interaction

The transmission of pathogens can be achieved through either direct contact from a diseased animal to another, or indirectly through a host like infected prey or a vector. Higher temperatures as a result of climate change results in an increased presence of disease producing agents in hosts and vectors, and also increases the "survival of animals that harbor disease". Survival of Parelaphostrongylus tenuis, a brain worm of white-tailed deer that affects moose, could be increased due to the higher temperatures and milder winters that are caused by climate change. In moose, this brain causes neurological disease and eventually ends up being fatal. Moose are already facing heat stress due to climate change, and may have increased susceptibility to parasitic and infectious diseases like the brain worm.

### Wildlife disease patterns

Predicting the impact climate change might have on disease patterns in different geographic regions can be difficult, because its effects likely have high variability. This has been more evident in marine ecosystems than terrestrial environments, where massive decline in coral reefs has been observed due to disease spread.

## Infectious diseases in domestic animals and livestock

Vector-borne diseases seriously affect the health of domestic animals and livestock (e.g., trypanosomiasis, Rift Valley Fever, and bluetongue). Therefore, climate change will also indirectly affect the health of humans through its multifaceted impacts on food security, including livestock and plant crops.
Mosquitoes also carry diseases like Dirofilaria immitis which affect dogs (dog heartworm). Therefore, tropical diseases will probably migrate and become endemic in many other ecosystems due to an increase in mosquito range.
While climate-induced heat stress can directly reduce domestic animals' immunity against all diseases, climatic factors also impact the distribution of many livestock pathogens themselves. For instance, Rift Valley fever outbreaks in East Africa are known to be more intense during the times of drought or when there is an El Nino. Another example is that of helminths in Europe which have now spread further towards the poles, with higher survival rate and higher reproductive capacity (fecundity).$^{:231}$ Detailed long-term records of both livestock diseases and various agricultural interventions in Europe mean that demonstrating the role of climate change in the increased helminth burden in livestock is actually easier than attributing the impact of climate change on diseases which affect humans.$^{:231}$
Temperature increases are also likely to benefit Culicoides imicola, a species of midge which spreads bluetongue virus. Without a significant improvement in epidemiological control measures, what is currently considered an once-in-20-years outbreak of bluetongue would occur as frequently as once in five or seven years by midcentury under all but the most optimistic warming scenario. Rift Valley Fever outbreaks in East African livestock are also expected to increase.$^{:747}$ Ixodes ricinus, a tick which spreads pathogens like Lyme disease and tick-borne encephalitis, is predicted to become 5–7% more prevalent on livestock farms in Great Britain, depending on the extent of future climate change.

## Responses

The policy implications of climate change and infectious diseases fall into two categories:
* Enacting policy that will reduce greenhouse gas emissions, thus slowing down climate change, and
* Mitigating problems that have already arisen, and will inevitably continue to develop, due to climate change.
Addressing both of these areas is of importance, as those in the poorest countries face the greatest burden. Additionally, when countries are forced to contend with a disease like malaria (for example), their prospects for economic growth are slowed. This contributes to continued and worsening global inequality.
Policies are required that will significantly increase investments in public health in developing countries. This achieves two goals, the first being better outcomes related to diseases like malaria in the affected area, and the second being an overall better health environment for populations. It is also important to focus on "one-health approaches." This means collaborating on an interdisciplinary level, across various geographic areas, to come up with workable solutions. As is the case when responding to the effects of climate change, vulnerable populations including children and the elderly will need to be prioritized by any intervention.
The United Nations Environment Programme states that: "The most fundamental way to protect ourselves from zoonotic diseases is to prevent destruction of nature. Where ecosystems are healthy and bio-diverse, they are resilient, adaptable and help to regulate diseases."

### Monitoring and research

Significant progress has been achieved in terms of surveillance systems, disease and vector control measures, vaccine development, diagnostic tests, and mathematical risk modeling/mapping in recent decades.
A tool that has been used to predict this distribution trend is the Dynamic Mosquito Simulation Process (DyMSiM). DyMSiM uses epidemiological and entomological data and practices to model future mosquito distributions based upon climate conditions and mosquitos living in the area. This modeling technique helps identify the distribution of specific species of mosquito, some of which are more susceptible to viral infection than others.
Scientists are carrying out attribution studies, to find the degree to which climate change affects the spread of infectious diseases. There is also a need for scenario modeling which can help further our understanding of future climate change consequences on infectious disease rates. Surveillance and monitoring of infectious diseases and their vectors is important to better understand these diseases. Governments should accurately model changes in vector populations as well as the burden of disease, educate the public on ways to mitigate infection and prepare health systems for the increasing disease load.