Medical Entomology Questions

Dr. Ngala (Must Know) ## Vector Control Methods **1. Explain how Bacillus thuringiensis (Bt) works as a biological control agent and why it's preferred over chemical insecticides.** **Answer:** Bt is a bacterial biopesticide that blocks insect chitin biosynthesis pathways, killing mosquito larvae. It's preferred over chemical insecticides because: - Specific to target insects (narrow spectrum) - Doesn't persist in the environment - No bioaccumulation in fatty tissues - Minimal impact on non-target organisms --- **2. Compare and contrast organophosphates and carbamates in terms of mechanism of action and safety profile.** **Answer:** - **Mechanism:** Both block/bind to acetylcholinesterase, preventing acetylcholine breakdown - **Safety differences:**Carbamates have lower mammalian/human toxicity - Carbamates have better biodegradability - Carbamate inhibition is **reversible** - Chronic carbamate exposure less likely to cause illness than organophosphates --- **3. Why were chlorinated hydrocarbons like DDT banned despite their effectiveness? Discuss two specific concerns.** **Answer:** - **Long environmental persistence** - Relatively stable, don't break down easily - **Bioaccumulation** - Lipid/fat soluble, accumulate in fatty tissues through the food chain, reaching toxic levels in top predators --- **4. Describe the sterile male technique and explain why it's considered a genetic engineering approach to vector control.** **Answer:** Sterile males are produced and released to mate with wild females, resulting in no viable offspring and population reduction. It's considered genetic engineering because it modifies the reproductive capacity of the vector population, changing the target site/genetic makeup without using chemicals. --- **5. Explain the mechanism of action of pyrethroids and why they are commonly used in ITNs and LLINs.** **Answer:** Pyrethroids bind to voltage-gated sodium channels and delay their inactivation, increasing membrane permeability. This results in excitation, lack of coordination, and paralysis (knockdown effect). They're used in ITNs/LLINs because: - Relatively safe for humans - Rapid action (knockdown effect) - Effective at low doses - Example: Permethrin, Deltamethrin --- **6. What is IRS (Indoor Residual Spraying) and why is it effective against endophilic mosquitoes?** **Answer:** IRS is the application of insecticides on interior walls of houses and structures. It's effective against endophilic mosquitoes (those that rest indoors) because: - Mosquitoes rest on walls after blood feeding - Contact with treated surfaces kills them - Provides long-lasting protection (weeks to months) - Targets mosquitoes when they're most vulnerable --- **7. Describe how Toxorhynchites species function as biological control agents.** **Answer:** Toxorhynchites mosquito larvae (e.g., T. rutilus) are predators that feed on larvae of disease-transmitting mosquitoes including Anopheles, Culex, and Aedes. Adults feed only on sugar solutions (not blood), making them non-vectors. They provide natural population control without chemicals. --- **8. What are Insect Growth Regulators (IGRs) and how do they control vector populations?** **Answer:** IGRs are chemicals that block insect sugar/chitin biosynthesis pathways, disrupting normal development and molting. Examples include Pyriproxyfen and Diflubenzuron. They prevent larvae from developing into adults, reducing vector populations without killing adult mosquitoes directly. --- ## Mosquito Biology and Control **9. Explain why Anopheles mosquitoes are more efficient malaria vectors than Culex or Aedes species. Include at least two behavioral characteristics.** **Answer:** Anopheles behavioral characteristics that enhance malaria transmission: - **Anthropophilic** - Prefer human blood meals - **Endophagic** - Feed indoors where humans sleep - **Endophilic** - Rest indoors after feeding These behaviors maximize human-mosquito contact and facilitate Plasmodium parasite transmission. --- **10. How can you distinguish between Anopheles and Culex larvae in their aquatic habitat?** **Answer:** - **Anopheles larvae:** Lie **parallel** to water surface, breathe through spiracles on body side - **Culex larvae:** Hang at **45° angle** to surface, breathe via a **siphon** --- **11. Describe the bimodal biting pattern of Aedes mosquitoes and explain its significance for disease transmission.** **Answer:** Aedes mosquitoes have **two peak feeding times:** - Early morning - Late evening **Significance:** This pattern increases disease transmission because mosquitoes bite multiple hosts during peak human activity periods, facilitating arbovirus spread (dengue, Zika, chikungunya, yellow fever). --- **12. What is transovarial transmission? Provide one example of a disease transmitted this way.** **Answer:** Transovarial transmission is when a pathogen passes from infected female mosquito to her egg