Medical Entomology Exam 2018/2019 --- SECTION A (Answer ALL Questions) Question 1 (6 marks) Differentiate the following terminologies as used in medical entomol
Medical Entomology Exam 2018/2019 --- SECTION A (Answer ALL Questions) Question 1 (6 marks) Differentiate the following terminologies as used in medical entomology: i) Propagative ii) Cyclodevelopmental iii) Cyclopropagative Answer: i) Propagative Transmission (2 marks): - The pathogen multiplies within the vector but undergoes no developmental changes - The organism increases in number but remains essentially the same morphologically - Example: Yersinia pestis in fleas during plague transmission - Vector acts as a biological incubator for pathogen reproduction ii) Cyclodevelopmental Transmission (2 marks): - The pathogen undergoes developmental changes within the vector but does not multiply - Essential morphological and physiological changes occur in the vector - Example: Malaria parasites ( Plasmodium spp.) in Anopheles mosquitoes - Sporozoites develop in mosquito salivary glands iii) Cyclopropagative Transmission (2 marks): - Combination of both propagative and cyclodevelopmental transmission - The pathogen both multiplies AND undergoes developmental changes within the vector - Example: Trypanosoma cruzi in triatomine bugs (Chagas disease) - Organism transforms through different morphological forms while multiplying --- Question 2 (6 marks) Describe six methods of controlling molluscs in the environment. Answer: 1. Chemical Control (1 mark): - Use of molluscicides such as metaldehyde and methiocarb - Copper sulfate and niclosamide for aquatic snails 2. Biological Control (1 mark): - Introduction of predators such as chickens, ducks, and fish - Use of competitor snail species 3. Physical Barriers (1 mark): - Use of barrier plants around water sources - Physical screening of water intake points 4. Environmental Modification (1 mark): - Drainage of stagnant water bodies - Removal of aquatic vegetation that harbors snails 5. Chemical Repellents (1 mark): - Use of repellents such as salt spreading around areas - Lime application to alter pH 6. Sanitation and Hygiene (1 mark): - Proper sewage disposal to prevent contamination - Community education on avoiding contact with infested water --- Question 3 (6 marks) With examples, give three classes of insecticides for controlling mosquitoes. Answer: 1. Organochlorines (2 marks): - Mechanism: Bind to GABA receptors, causing formation of GABA-A-chloride ionophore complex - Examples: DDT (dichlorodiphenyltrichloroethane), endosulfate - Effect: Disruption of nervous system causing paralysis 2. Organophosphates (2 marks): - Mechanism: Inhibit acetylcholinesterase, making acetylcholine unavailable - Examples: Malathion, temephos, fenitrothion - Effect: Continuous nerve stimulation leading to death 3. Pyrethroids (2 marks): - Mechanism: Target sodium channels in nerve membranes - Examples: Permethrin, deltamethrin, cypermethrin - Effect: Disruption of nerve impulse transmission causing knockdown and death --- Question 4 (6 marks) Fill in the following blank spaces (only one answer): i) Culex species are major vectors of: Filariasis ii) Onchocerca volvulus is transmitted by: Black flies/Simulium species iii) Leishmania tropica is transmitted by: Phlebotomus species/Sandflies iv) Fresh water snails are major vectors for: Schistosoma haematobium causing Schistosomiasis Answer Explanations: - Culex mosquitoes transmit Wuchereria bancrofti causing lymphatic filariasis - Simulium species transmit Onchocerca volvulus causing river blindness - Phlebotomus sandflies transmit Leishmania species causing leishmaniasis - Freshwater snails ( Bulinus spp.) are intermediate hosts for Schistosoma haematobium --- Question 5 (6 marks) Outline the development of filarial nematodes in their vectors. Answer: Stage 1 - Ingestion (1 mark): - Microfilariae are ingested by vector during blood meal from infected host - Microfilariae present in peripheral blood of infected individual Stage 2 - Migration to Flight Muscles (2 marks): - Microfilariae penetrate gut wall and migrate to thoracic flight muscles - Development occurs in the muscle tissue of the vector Stage 3 - First Molt (L1 to L2) (1 mark): - First larval stage (L1) molts to second larval stage (L2) - Occurs approximately 7-14 days post-infection Stage 4 - Second Molt (L2 to L3) (1 mark): - Second larval stage molts to third larval stage (L3) - L3 is the infective stage for humans Stage 5 - Migration to Proboscis (1 mark): - Infective L3 larvae migrate from flight muscles to proboscis/mouthparts - Ready for transmission during next blood meal (14-21 days total development) --- Question 6 (6 marks) Outline the control of leishmaniasis in the environment. Answer: 1. Vector Control (2 marks): - Spraying insecticides in sandfly breeding sites - Use of residual spraying in houses and animal shelters 2. Environmental Modification (1 mark): - Cleaning termite mounds and organic debris where sandflies breed - Removal of vegetation close to human habitations 3. Personal Protection (1 mark): - Use of repellents and protective clothing - Sleeping under fine-mesh bed nets 4. Reservoir Control (1 mark): - Treatment and control of infected domestic animals (dogs) - Reduction of rodent populations near human settlements 5. Community Education (1 mark): - Health education about risk factors and prevention - Early case detection and treatment to reduce reservoir --- Question 7 (6 marks) With relevant examples, describe mechanical transmission of pathogens. Answer: Definition (1 mark): Mechanical transmission occurs when arthropods transfer pathogens externally without biological development of the pathogen within the vector. Mechanism (2 marks): - Pathogens adhere to external body parts (legs, mouthparts, body hairs) - Contamination occurs through contact with infected materials - Immediate or short-term transfer to food, wounds, or mucous membranes Examples (3 marks): House Fly ( Musca domestica ): - Transmits Vibrio cholerae (cholera), Salmonella typhi (typhoid) - Picks up pathogens from feces and transfers to food Cockroaches ( Blatta species): - Transmit various enteric bacteria causing gastroenteritis - Contaminate food and surfaces while foraging Blow Flies: - Transmit wound pathogens and cause secondary infections - Transfer bacteria from decaying organic matter to open wounds --- Question 8 (6 marks) Describe the epidemiology and significance of triatomine bugs. Answer: Geographic Distribution (1 mark): - Found primarily in Central and South America - Also present in southern United States and some Caribbean islands Vector Role (2 marks): - Vectors for Trypanosoma cruzi causing Chagas disease - Transmission occurs through infected feces deposited during feeding - Also known as "kissing bugs" due to facial feeding preference Epidemiological Significance (2 marks): - Major public health problem affecting millions in Latin America - Primarily affects rural populations living in poor housing conditions - Disease has chronic manifestations including cardiomyopathy and megacolon Clinical Importance (1 mark): - Acute phase may present with fever, malaise, and localized swelling - Chronic phase can cause serious cardiac and digestive complications - Often goes undiagnosed for years, leading to significant morbidity --- SECTION B: Answer TWO Questions (22 marks each) Question 9 (22 marks) Using an illustration, outline the life cycle of a three host tick. Answer: Overview (2 marks): Three-host ticks use different hosts for larval, nymphal, and adult stages, dropping off between each feeding to molt in the environment. Stage 1: Egg Stage (3 marks): - Adult female drops from host after blood meal - Lays thousands of eggs in soil or vegetation - Eggs incubate for weeks to months depending on environmental conditions - Six-legged larvae hatch from eggs Stage 2: Larval Stage (First Host) (4 marks): - Larvae seek small mammals (rodents, birds) as first host - Attach and feed for 3-7 days - Drop off host when engorged - Molt to eight-legged nymphs in environment - Can survive months without feeding Stag