June 2017 was the 390th consecutive month with average global temperatures higher than the average for the 20th century, according to the US National Oceanic and Atmospheric Administration. The six-month period from January to June 2017 was the second warmest in the 138-year record, behind 2016 and just ahead of 2015.
A large part of the land and ocean surfaces had warmer or much warmer than average temperatures in the six-month period, with many areas having record highs (dark red in the map) and only a few areas being colder than average.
Global warming is normally thought of as creating greater numbers of extreme events. Droughts, floods and storms damage crops and can cause food shortages that result in malnutrition and starvation in developing countries and push up prices on world markets. Floods and high winds destroy homes and buildings, leaving people without shelter, food and clean water. The recent floods in India, Nepal and Bangladesh and Nigeria, and the trio of hurricanes — Harvey, Irma and Jose — impacting the Caribbean and the southern US, are all examples of the devastating impact these events can have.
Vectors and diseases
Flooding and other major climatic events are the highly visible effects of extremes in weather that are likely to increase with global warming. Slowly but steadily, however, the rising temperatures are also affecting survivability and breeding ability of living organisms, from viruses to the largest land and sea creatures.
Even slight changes in temperatures can have a dramatic effect on the geographical area where there are suitable conditions for survival. These changes could either open new areas to pests and diseases or make the environment too harsh by restricting breeding and survivability of different life stages.
The interaction between physical factors, such as temperature and rainfall, and biological factors, such as competitors and predators could result in negative or positive consequences depending on the geographic area and other local conditions.
Arthropods have no physiological mechanism to control their body temperature and neither do the microorganisms that cause human diseases. Consequently, their temperature is determined by the ambient temperature and their moisture levels are affected by the local climate or indoor conditions.
Incubation times of microorganisms in a host vector species are very sensitive to temperature. They usually have an exponential relationship, so a small change in temperature can have a very large effect on survival.
The infectious microorganisms and their arthropod hosts are also affected by rainfall, elevation, wind and duration of sunlight. These all create a limited climate ‘envelope’ in which both an infectious microorganism and the vector can survive, according to the WHO.
Added to the climatic factors are increasing urbanisation worldwide, changes in land use, variable governmental infrastructure such as healthcare, and international travel and trade. These make the predictions of the impact of climate change on pests and their impact on us extremely complex.
Currently 4 billion people live in urban areas — 54% of the global population. It is forecast that by 2045 this number will increase by 2 billion. Most of this growth will occur in developing countries where urbanisation is likely to be rapid, unplanned and unsustainable. This will make many of the new urban areas havens for urban pests and focal points for the spread of existing and new vector-borne diseases.
Urban areas create a complex mix of microhabitats and a “heat island” that has temperatures up to 12°C above surrounding areas. Human activities can provide a ready abundance of food and water in addition to the protected shelter free from many natural predators that restrict populations in pests’ original habitats.
A study conducted for the UK government on insect species that could cause a “statutory nuisance” as a result of climate change, found some pest species would increase and some existing pests would not be affected.
The study found that insect species that were unlikely to be affected by higher temperatures in the UK include the German cockroach (Blattella germanica), bed bug (Cimex lectularius), Pharaoh ant (Monomorium pharaonis), woodworm (Anobium punctatum), cat flea (Ctenocephalides felis) and common clothes moth (Tineola bisselliella). These are largely pests inside buildings that provide protection from the fluctuating weather outdoors:
The species most likely to be affected and increase populations in the UK with climate warming included several species of mosquito inhabiting rural and urban areas, the invasive garden ant (Lasius neglectus), Mediterranean termite (Reticulitermes grassei) — which had a colony in Devon, southern England in 2000, and the Argentine ant (Linepithema humile), which is a major invasive species worldwide.
Two species, Musca domestica and a sand fly, Phlebotomus mascittii, are also likely to thrive in the UK with increased rainfall.
Many urban pest species will be affected by climate change worldwide. Some examples of mosquitoes, ticks, rodents and triatomine bugs are described below.
The Asian tiger mosquito, Aedes albopictus, is classified as one of the world’s worst invasive species. Originating in tropical Asia, it has been carried around the world through international trade. Both Ae. albopictus and Ae. aegypti, the yellow fever mosquito which originated in tropical Africa, are ideally adapted to the urban environment.
Us humans by both design and neglect replicate their natural breeding sites in large numbers in urban areas — small quantities of still water, around homes and factories and in discarded cans, bottles and other refuse. Both these Aedes species are vectors of a number of dangerous viruses including dengue, Chikungunya, Zika and yellow fever.
Ae albopictus eggs are cold and drought resistant so can survive in cooler climates than Ae aegypti (which is more efficient at transmitting disease). The climatic envelope is thought to be a mean winter temperature of above 0°C for egg overwintering and a mean annual temperature above 11°C for adult survival and breeding, with sufficient rainfall in summer to maintain egg-laying habitats.
With global warming, this zone will spread northwards and to higher altitudes. Once limited by temperature to approximately 1000 m in elevation in the tropics, Ae albopictus has recently been found at 1700m in Mexico and 2200m in the Colombian Andes.
The Asian tiger mosquito was first reported in Texas in the US in 1985, since when it has spread northward and eastwards to over 25 states. In Europe, it was first detected in Albania in 1979 and became established in the country. It was found in Genoa, Italy in 1990 has become established in most areas below 600m above sea level. It has since spread east and west around the Mediterranean. The mosquito has also been detected in many other European countries as far north as the Netherlands (see map below), but has not established populations, so far.
The eggs of Ae albopictus were detected in a mosquito trap near a motorway service station in the UK in 2016, by a Public Health England monitoring programme. No adults were detected, but a control programme was carried out in a 300m radius of the trap to make sure none would survive (Vaux AGC et al, International Conference on Urban Pests, 2017).
Both Aedes species have been discovered in Russia near Sochi on the eastern Black Sea coast since 2001. Ae albopictus has spread rapidly north and south (Roslavtseva & Alekseev, International Conference on Urban Pests 2017).
Climate change predictions show that Ae albopictus could establish in most of Europe due to warmer and wetter conditions, with winter temperatures the most significant. Hotter and drier summers in southern Europe, however, will reduce the suitable areas there. Increasing urbanisation is also likely to provide the species with a competitive breeding advantage over local mosquitoes.
Many mosquito species pose a risk to health worldwide and need constant monitoring to study their distribution as climate changes, including:
- Aedes aegypti, Ae. albopictus, and other Aedes spp:
Diseases: Chikungunya virus, dengue virus, West Nile virus, yellow fever virus, Zika virus (and many other viruses), lymphatic filariasis/ elephantiasis (parasitic roundworm)
- Anopheles species (60+ species transmit diseases):
Diseases: malaria (parasitic protozoan), lymphatic filariasis (parasitic roundworm), West Nile virus
- Culex species:
Diseases: West Nile virus, lymphatic filariasis/ elephantiasis (parasitic roundworm)
- Haemagogus species; Sabethes species:
Disease: yellow fever virus
Ticks can transmit bacterial, protozoan and viral pathogens, causing diseases such as Lyme disease, Rocky Mountain spotted fever, anaplasmosis and babesiosis. Lyme disease is the most common vector-borne disease in North America.
In the 40 years since Lyme disease was first discovered the number of cases and the geographical range where it occurs have greatly increased in both North America and Eurasia.
Predicting how the risk of tick-borne diseases will be affected by climate change is difficult, however, because of the number of variables:
- the complex life-cycle of ticks
- seasonal behaviour of ticks, such as period of feeding
- interactions between host immune systems and pathogens
- abundance of ticks
- multiple host species
- seasonal human behaviour
All these can be affected in different ways by warming temperatures and changes in rainfall patterns.
In North America, Ixodes ticks are highly dependent on the white-footed mouse (Peromyscus leucopus) for survival. A study in Quebec found that the mouse had spread northwards between 1975 and the present, coincident with climate warming, helping the northward expansion of the ticks.
Transmission of pathogens to humans is mainly by the nymphs because the younger larval stage needs to feed on an infected host first to become infected with a pathogen. The feeding activity of the nymphs coincides with human outdoor activity in spring and early summer, so they are both more likely to be carrying a pathogen and pass it on to a human.
Nymphs and larvae of tick species may feed at overlapping times or different seasons. Hosts infected by nymphs are more likely to pass on a pathogen to the larvae feeding on the same hosts. This is complicated by varying infectious phases of pathogens. The bacterium Anaplasma phagocytophilum, for example, is infectious in a host for only two weeks, while Lyme disease, caused by the bacterium Borrelia burgdorferi, is infectious for months, therefore Lyme disease has a much longer window of infection when the next generation larvae can become infected.
Rodents are major urban pests, both around buildings and in agricultural areas. They are vectors for many diseases by being reservoirs themselves and from the ectoparasites that they carry, such as ticks and fleas, which also carry a number of human diseases. They are responsible for significant losses of food worldwide, all the way through the supply chain from farm to consumer.
Hantaviruses are lesser known viruses that are carried by rodents and can be passed on to humans through inhaling the dust from their dried droppings.
Rodent populations in wild areas benefit from warm wet winters and springs, therefore in temperate zones, populations can increase if these conditions become more common. They are then more likely to encounter humans. Flooding can also drive rodents into buildings to seek shelter and drought to seek moist food and water. The problems caused by rats and mice are well known
In central Asia great gerbil populations, which are the main vectors for the plague, have been monitored in their desert habitats since the 1940s. Fleas that infest the gerbil burrows are the main vectors of the plague bacteria Yersina pestis in the gerbils. When gerbil populations rise above a certain threshold the plague is more likely to spread in the human population. Modelling of their populations and the predicted climate change in central Asia shows that plague outbreaks are likely to become more frequent over the next century.
Bank vole populations in Europe increase following high tree seed production, which occurs after high summer and autumn temperatures. Deer mouse populations in the US increase following higher rainfall that increases grass seed production.
Triatomine bugs are one of the most important disease vectors in Latin America, responsible for Chagas disease, caused by the protozoan parasite Trypanosoma cruzi. Around 6-7 million people worldwide are infected with the parasite, mostly in Central and South America where it is endemic. It has spread to the US, Canada, Europe and some Western Pacific countries through travel.
Triatomine bugs live mostly in cracks and small spaces in the walls and roofs of poorly constructed houses, coming out at night to seek a blood meal. The infection is spread by the faeces when the bug defecates close to a bite and the host rubs the bite, spreading the parasite into the bite wound or then from their hands into their eyes, mouth or other broken skin.
Triatomines are present across a wide range of habitats, mainly in tropical areas, but species also occur in temperate areas north and south; in Patagonia, Southern Argentina, where there are cold winters and Indiana and Maryland in the US.
A study of the two most important vector species, Rhodnius prolixus (in tropical Central America and northern South America) and Triatoma infestans (in the temperate Southern Cone region of South America) found climatic warming would negatively affected both species.
As the study notes, much of the research on climate change focuses on change in geographic range of pests and disease vectors. Many processes are affected by temperature, however, including: host finding, feeding, egg production, hatching rate, immature development time, cessation of moulting, and metabolic rate processes.
Optimum conditions can produce an exponential change in numbers of a disease vector, and conversely, small changes can produce a dramatic reduction. Using a measure called force of infection (FOI) that combines a number of variables, climatic projections for 2050 forecast a decreasing trend of infections in both areas that have a moderate to high risk of transmission of the disease.