Vector-borne disease transmission depends critically on vector competence—the mosquito, tick, or other arthropod's biological and behavioral ability to carry and transmit a pathogen. Vector competence is determined by vector species, parasite's extrinsic incubation period, temperature, humidity, and urban ecology. Climate change and urbanization directly affect vector distribution and transmission dynamics, explaining the emergence and geographic expansion of diseases like dengue, Zika, and Lyme disease into new regions.
Map the geographic range of a vector-borne disease and relate changes over time to temperature and precipitation patterns.
Assuming all mosquito species transmit the same pathogens—competence varies widely by species and population genetics.
From infectious disease epidemiology, you know that disease transmission requires a susceptible host, a pathogen, and a route of transmission. For vector-borne diseases, that route runs through a living arthropod — a mosquito, tick, sandfly, or other invertebrate that doesn't just mechanically carry the pathogen but allows it to replicate and mature before transmitting it to a new host. Vector competence is the biological capacity of a specific vector species (or population) to take up, maintain, and transmit a pathogen. It is not a binary on/off property — it is a spectrum shaped by the vector's genetic makeup, the pathogen's compatibility with that species' cells, and the environmental conditions governing the interaction.
The concept of the extrinsic incubation period (EIP) is central to understanding why temperature matters so much. After a mosquito takes a blood meal from an infected host, the pathogen must replicate within the mosquito's gut, disseminate to the salivary glands, and reach high enough concentrations to be transmitted in a subsequent bite. This takes time — 8–12 days for dengue in *Aedes aegypti* at 28°C, but considerably longer at lower temperatures. At temperatures below a critical threshold, the pathogen simply cannot complete its development cycle before the mosquito dies of old age (mosquito lifespans are also temperature-dependent). This is why vector-borne diseases are geographically constrained by climate: not because the mosquitoes can't survive in colder regions, but because the EIP would exceed the mosquito's lifespan there, making transmission biologically impossible. As average temperatures rise even 1–2°C, the EIP shortens and the geographic zone where transmission is possible expands toward higher latitudes and elevations.
Vector competence interacts with vectorial capacity — a broader mathematical concept capturing the full transmission potential of a vector population. Vectorial capacity incorporates vector density, human-biting rate, daily survival probability, and the EIP. Small changes in any component can have non-linear effects on transmission: if daily mosquito survival increases from 0.85 to 0.90 (a modest improvement in vector lifespan under warmer, wetter conditions), the probability of surviving long enough to complete the EIP roughly doubles. This is why climate change projections for vector-borne diseases are not incremental — they predict expansion into entirely new geographic zones where transmission was previously impossible. Dengue, endemic in the tropics for decades, has now transmitted locally in Florida, Texas, and southern Europe. Lyme disease vectors (*Ixodes scapularis*) have colonized Canada. These are not accidents but predictable consequences of warming temperatures extending the thermal envelope for transmission.
Urban ecology adds a second driver. Aedes aegypti — the primary dengue, Zika, and chikungunya vector — is an intensely urban mosquito that breeds in small, clean water containers: flower pots, discarded tires, bottle caps. Urban expansion creates ideal habitat. Planned cities with good sanitation infrastructure and piped water reduce container breeding; dense informal settlements with intermittent water supply (causing household water storage) create the ideal epidemiological environment. This means that climate change and urbanization are not independent drivers — they interact, and the areas experiencing the most rapid informal urbanization in tropical regions face the compound risk of expanding vector range meeting expanding vector habitat. Effective vector control requires integrating both ecological drivers into surveillance and intervention design.
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