Questions: Vector-Borne Disease Ecology and Control

Question 1

West Nile virus cycles between birds and Culex mosquitoes. Humans become infected when bitten by infected mosquitoes, but infected humans have viral titers too low to infect feeding mosquitoes. What follows for control strategy?

• A. Treating infected humans with antivirals is the primary control approach, since it reduces the infectious reservoir
• B. Control must focus on reducing mosquito populations or the bird-mosquito transmission cycle, since treating human cases does not interrupt transmission
• C. Vaccination of humans is the most efficient control strategy since humans are the primary amplifying host
• D. Eliminating the Culex mosquito species entirely is both feasible and necessary for control

Answer: (B)

Because humans are dead-end hosts — infected humans cannot transmit to feeding mosquitoes — treating sick people does nothing to interrupt the transmission cycle. The virus amplifies between birds (reservoir hosts) and Culex mosquitoes; humans are accidental infections outside this cycle. This contrasts sharply with malaria, where humans ARE the reservoir and treating infections reduces the infectious pool available to mosquitoes. For West Nile, control must target the enzootic cycle: reduce Culex populations through larval source reduction, insecticides, or biological control. Option D is wrong because eliminating a vector species is rarely ecologically feasible or desirable.

Question 2

Why does climate warming cause malaria and dengue transmission to expand to higher elevations and latitudes, even in regions where the mosquito vectors are already present?

• A. Warmer temperatures increase human outdoor activity, raising exposure to mosquito bites
• B. Warmer temperatures shorten the extrinsic incubation period, allowing the pathogen to complete development inside the vector before the mosquito dies
• C. Warmer temperatures increase mosquito biting rates, increasing the probability of transmission per mosquito-human contact
• D. Warmer temperatures reduce the effectiveness of insecticides, allowing mosquito populations to grow unchecked

Answer: (B)

The extrinsic incubation period (EIP) is the critical bottleneck. The EIP is the time from when a vector takes an infectious blood meal until it can transmit in a subsequent bite. For malaria at 25°C, EIP is 10–12 days; in cooler temperatures it extends dramatically. Since mosquitoes live only 2–4 weeks, a long EIP means most vectors die before becoming infectious — transmission cannot be sustained. As temperatures rise in previously cool regions, the EIP shortens enough that vectors survive long enough to transmit. This is the precise mechanism linking climate change to range expansion: not just 'more mosquitoes' but 'mosquitoes that live long enough for the pathogen to complete its development.'

Question 3

Integrated vector management aims to eliminate vector species largely, using combined chemical, biological, and environmental strategies to achieve eradication.

• T. True
• F. False

Answer: False

IVM's goal is to reduce vector density below transmission thresholds, not to eliminate the species. Complete elimination of a vector species is rarely ecologically feasible (vectors often have broad geographic ranges and rapid reproductive rates), and elimination of a widespread arthropod species would have unpredictable ecological consequences. The practical target is reducing population density to levels where the basic reproduction number R₀ falls below 1 — at which point transmission cannot be sustained. IVM achieves this through combining larval source reduction, insecticides, biological control, and personal protection, chosen to minimize resistance development and ecological disruption.

Question 4

Applying the same insecticide uniformly and continuously across a region will eventually drive the mosquito population to zero as the most susceptible individuals are killed.

• T. True
• F. False

Answer: False

Uniform, continuous insecticide application creates strong directional selection for resistance, following the same evolutionary logic as antibiotic resistance. Mosquitoes with any heritable resistance survive and reproduce; over generations, resistance alleles spread through the population. Rather than driving the population to zero, prolonged uniform exposure drives the population toward resistance, ultimately making the insecticide ineffective. IVM addresses this by rotating insecticide classes (to prevent selection for resistance to any one mechanism), combining with non-chemical methods, and monitoring resistance levels — analogous to the combination therapy and stewardship strategies used for antibiotics.

Question 5

Explain why treating infected humans effectively controls malaria but not West Nile virus. What property of the transmission cycle determines whether human treatment reduces overall disease spread?

Model answer: The determining factor is whether humans are amplifying hosts (reservoir) or dead-end hosts. In malaria, humans are the primary reservoir: parasites replicate to high densities in human blood, and infected humans are the source of parasites for feeding mosquitoes. Treating infected humans reduces the infectious reservoir, directly interrupting transmission. In West Nile virus, the parasite amplifies between birds and Culex mosquitoes; human infections are incidental, and viral titers in human blood are too low to infect feeding mosquitoes. Treating human cases removes them from a dead-end branch of the transmission network that doesn't feed back into the cycle.

This distinction — reservoir host vs. dead-end host — is fundamental to designing effective control strategies. It explains why malaria control combines bed nets (reducing human-mosquito contact) with case treatment (reducing the infectious reservoir), while West Nile control focuses entirely on the bird-mosquito cycle. Misidentifying the reservoir leads to misdirected interventions: spending resources treating dead-end human cases for West Nile would have minimal impact on transmission, while neglecting the enzootic cycle would allow virus to continue amplifying. Understanding reservoir ecology before designing interventions is essential to public health practice.