6. A mosquito-borne disease was known to be prevalent in the foothills of a mountain in
Africa. However, over the last decade, the disease has increasingly been found
among people living higher up in the mountain. The possible reason for this
phenomenon is:
a. The disease is now transmitted by a different vector in those living in higher
altitudes.
b. Infected people tend to move higher up as it is cooler.
c. People living higher up in the mountain have become susceptible to the
disease
d. The altitudinal range of the mosquitoes has expanded due to global warming

Correct option: d. The altitudinal range of the mosquitoes has expanded due to global warming.​​

Introduction

Mosquito-borne diseases that were previously confined to warm African foothills are now increasingly reported in cooler, high-altitude regions. This shift is strongly linked to global warming, which raises temperatures at higher elevations, allowing mosquito vectors such as Anopheles to survive and breed in areas that were once too cold.​

Explanation of the scenario

In the question, a mosquito-borne disease was historically prevalent only in the foothills, but over the last decade it has spread to people living higher up the mountain. This pattern matches documented observations from African highlands, where warming temperatures have allowed malaria vectors to move hundreds of meters uphill over recent decades. As climate change raises minimum temperatures and alters rainfall, high-altitude regions become suitable for mosquito survival and pathogen development, leading to new transmission zones.​

Option-wise analysis

Option a: Different vector at higher altitudes

Statement: “The disease is now transmitted by a different vector in those living in higher altitudes.”

• Most classic exam questions on this theme are built around the effect of climate change on the same mosquito vector expanding its range, not on a complete switch to a new vector species.​

• While in theory some diseases can be transmitted by more than one vector, there is no information in the stem suggesting a change in vector; it only indicates that the disease has appeared at higher altitude, which can be fully explained by range expansion of the original mosquito species.​
  ➡ Conclusion: Not the best answer; it adds an assumption (new vector) that is unnecessary and unsupported.

Option b: Infected people move higher because it is cooler

Statement: “Infected people tend to move higher up as it is cooler.”

• Human movement can spread pathogens, but the key ecological constraint for mosquito-borne disease is whether the vector can survive and reproduce in the new environment.​

• Even if infected people relocate, sustained transmission at higher altitude still requires suitable climatic conditions for mosquitoes; historically, low temperatures in African highlands limited mosquito survival and parasite development, so human movement alone does not explain long-term establishment of disease.​
  ➡ Conclusion: Human migration may contribute, but it does not adequately explain the new stable presence of the disease at higher altitude; therefore this is not the correct primary reason.

Option c: People higher up have become susceptible

Statement: “People living higher up in the mountain have become susceptible to the disease.”

• Population susceptibility may influence outbreak size, but susceptibility alone cannot create transmission in the absence of the mosquito vector and suitable climate.​

• High-altitude populations may indeed lack prior immunity, but the question asks for the reason the disease is now found at higher altitude, which is primarily an ecological/vector issue, not a change in host susceptibility.​
  ➡ Conclusion: Susceptibility may increase impact, but it does not explain the initial appearance and spread; hence this is not the best choice.

Option d: Altitudinal range of mosquitoes has expanded due to global warming (Correct)

Statement: “The altitudinal range of the mosquitoes has expanded due to global warming.”

• Studies from East African highlands show that Anopheles mosquitoes have shifted their range uphill by roughly several meters of elevation per year over the last century, closely tracking rising temperatures.​

• Climate change raises average and minimum temperatures at higher elevations, making previously cold highlands warm enough for mosquito survival, faster development, and successful transmission of pathogens like malaria.​

• This directly matches the pattern described in the question: a disease once confined to foothills becoming common at higher altitudes as the vector’s climatic niche expands.​
  ➡ Conclusion: This option precisely addresses the ecological mechanism and is the best and correct answer.

Key concept: Climate change and vector altitudinal shift

Rising global and regional temperatures in Africa are enabling mosquito vectors to move to higher latitudes and elevations, expanding disease risk into highland regions that were historically too cool for sustained transmission. In African highlands, even small increases in temperature can significantly increase mosquito abundance and lengthen transmission seasons, which explains the newly observed prevalence of mosquito-borne disease at higher altitudes.​