Lesson 7 of 7 / Climate, biodiversity and disease ecology
Warming and disease-range shifts
What else is needed besides a warmer climate for transmission to occur?
I can explain changing dengue and malaria risks beyond the tropics.
I can explain insect temperature responses and Aedes aegypti life cycles.Syllabus
Syllabus 9477, Extension B(h). Explain insect temperature responses and Aedes aegypti life cycles.
I can explain changing geographical risk of dengue and malaria with warming.Syllabus
Syllabus 9477, Extension B(i). Explain changing geographical risk of dengue and malaria with warming.
About 7 min
Make a guess. You are not marked.
A country warms enough for Aedes mosquitoes to survive there. Will dengue automatically spread?
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It also needs the virus, hosts and contact.
Transmission needs the right vector, an infected host, the virus and bites. Climate is only one condition.
The key idea
Climate can alter vector suitability and season length, but transmission also needs the correct vector, pathogen, hosts and contact opportunities.
Trace the biological links
Which links are still missing after a vector range expands?
Dengue is viral and mainly uses Aedes mosquitoes. This hypothetical site is initially too cool for sustained vector establishment. Without the pathogen, vector presence alone does not create this disease.
A conditional scenario, not a map or forecast for any real place. Excessive heat or drought can reduce suitability elsewhere. Imported cases and established vectors are different evidence from sustained locally acquired transmission.
Explanation
Dengue is caused by dengue viruses and is transmitted mainly by infected Aedes mosquitoes, including Aedes aegypti. Malaria is caused by Plasmodium parasites and is transmitted by infected female Anopheles mosquitoes. The mosquito is a vector, not the pathogen; these two diseases do not share the same principal vector genus.
Warming can make previously cool regions or higher elevations more suitable for vector survival and development, or lengthen a transmission season. Faster pathogen development within a vector can also increase the chance that it becomes infectious before dying, within suitable temperature limits.
However, excessive heat or drought can reduce suitability. Rainfall, breeding-water availability, housing, vector control, travel and introduction of the pathogen also affect transmission. A suitable vector population alone does not guarantee local disease, and an imported case is not automatically evidence of sustained local transmission.
Interpret projections as conditional outcomes with uncertainty. Compare vector records, temperature, rainfall and locally acquired cases over time, while considering surveillance changes. Climate is an important driver among interacting factors, not a complete explanation for every case increase.
Step by step
- 1
Name pathogen and vector separately
Avoid calling the mosquito the disease-causing microorganism.
- 2
Explain the climate mechanism
Survival, development or season length may change.
- 3
List necessary additional links
Pathogen presence, hosts and effective contact complete the chain.
Worked example
Work through the evidence
A temperate city now has Aedes mosquitoes but no locally acquired dengue. Does this refute a climate-related change in vector suitability?
One way to explain it
No. Vector suitability can increase without local transmission if the pathogen is absent or other links in the transmission chain are missing.
Why this answer works
- Vector presence and disease incidence are different measurements.
- The observation limits a disease claim, not necessarily a habitat-suitability claim.
Watch out for this
A student says: "Malaria is a dengue virus spread by Aedes." What is wrong with this?
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Malaria involves Plasmodium parasites and Anopheles vectors; dengue involves a virus and Aedes vectors.
In the real world
Singapore releases male mosquitoes against dengue
Singapore's National Environment Agency releases male Aedes aegypti mosquitoes that carry Wolbachia bacteria. Males do not bite. When they mate with wild females, the eggs do not hatch, so the next generation is smaller. NEA reports that at release sites the Aedes aegypti population fell by about 80 to 90 per cent, and residents were more than 70 per cent less likely to be infected with dengue. The mosquitoes are not genetically modified.
Source: National Environment Agency (NEA), Project Wolbachia: frequently asked questions
Check your understanding
Which observation most directly indicates local transmission rather than importation alone?
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Locally acquired infections linked to local vector exposure
This supports a transmission chain within the setting.
Quick recall
What else is needed besides a warmer climate for transmission to occur?