The Dynamics of Vector-Borne disease (1) Vector competence: the ability of a vector to transmit a pathogen Two important definitions: (2) Vectorial capacity: rate at which future inoculations arise from a currently infective case For example, Can the pathogen grow inside the vector? Can the vector be transmitted by the vector? 1"Vectorial capacity = C m = number of vectors per human a = number of human blood meals per mosquito per day V = vector competence P = daily survival rate n = incubation period in vector (extrinsic incubation period) C= The average number of new infections arising daily from a single infection 2"4 Host Searching Host searching consists of three phases: 1. Appetitive searching 2. Activation and orientation 3. Attraction 3"b)#Vision#e.g., Black flies: """""Savannah and open country species are drawn to silhouette traps Forest dwelling relatives -> no attraction to silhouette traps #• #Most widely used by diurnal insects in open habitat #• #This typically works on shorter distances than olfaction Mosquitoes -> 5 to 20 meters but, Tsetse -> can detect a moving target (1.3m x 1.0m) from 50m #4"3. Attraction Arthropods makes a decision to land and start feeding 1. Vision 2. Smell 3. Heat Often mixture of these Several cues are used to make the decision: 4"The role of heat in attraction Bedbug# Rhodnius#prolixus#But also#some#role#in""#• #Mosquitoes • Fleas • Blackflies • Lice #Heat plays an important role in 5"Vertebrate haemostasis Bleeding at site of tissue damage is stopped by series of interrelated mechanisms: 1. Vasoconstriction 2. Platelet aggregation 3. Coagulation Pain: Tissue damage leads to inflammation: Pain, redness and swelling 5 Anti-haemostatic Components in Saliva 6"The Pharmacology of Blood Feeding Insect Saliva #No digestive enzymes! Almost all species studied produce: 1. Vasodilator 2. Anti-platelet compound 3. anti-coagulant#Saliva compounds often also reduce pain, but almost none have anesthetics, instead they have 4. anti-inflammatory agents 7"8"6 Managing the blood meal Temporary ectoparasites take large bloodmeals Hosts may be hard to find Feeding is risky abdomen is very extendible Permanent ectoparasites take small blood meals e.g. Anopluran lice take only 20-30% of body weight in blood meal Ticks also take small blood meals at a time 9"Managing the Blood Meal cont’d #80% of blood meal is water #• Anopheles gambiae gets rid of water while feeding #• #Rhodnius prolixus (kissing bug) can get rid of 40% of weight of meal in 30 minutes#10"7 Types of feeding strategies: 1. Only larval forms feed on blood e.g., Congo Floor Maggot 2. Insects that feed exclusively on blood throughout their entire life (e.g. kissing bugs; bedbugs; lice) 3. Insects in which only adults feed on blood (i) both adults are obligatory blood feeders (e.g., fleas) (ii) both adults are optional blood feeders and both adults take other meals (e.g., stable flies) (iii) only female adult feeds on blood, male never (e.g., mosquitoes, black flies, sand flies) 11"Autogeny Autogeny: ability to lay eggs before blood feeding for first time E.g. some mosquitoes, blackflies, sandflies, tabanids, Facultative; This allows the insect to decide between blood feeding or reproduction, depending on the quality of larval feeding and availability of host Obligatory; If hosts are rare 12"An Introduction to Mosquitoes “No animal on earth has so profoundly touched so directly and profoundly the lives of some many human beings. For all of history and all over the globe she has been nuisance, a pain and an angel of death. Mosquitoes have felled great leaders, decimated armies, and decided the fate of nations. All this, and she is roughly the size and weight of a grape seed” Andrew Spielman 13"Males vs. Females Male 14"Mosquitoes Classification 1.Toxorynchitinae Order: Diptera Family: Culicidae 2. Anophelinae 3. Culicinae #• Around 3,400 species described #• 42 genera #• 3 subfamilies 15"1. Toxorhynchitinae #• 94 species #• Genus Toxorynchites # • Mainly tropical, Some • species in US 16"Larvae are predaceous on other mosquito larvae Larvae live mostly in container habitats Adult males and females feed on plant nectar May kill/eat as many of 400 larvae Sometimes reared as biological control -> with limited succes: - hard to rear large enough numbers - very susceptible to insecticides Toxorhynchites Do not feed on blood and do not transmit disease 17"How to recognize Toxorhynchites mosquitoes have a characteristic bend in the proboscis 18"2. Anophelinae Human Malaria (transmitted mostly by 30-40 species) Lymphatic Filariasis Arboviruses (Arthropod-borne virusses) Not considered major nuisance species: - They do not occur in the very large numbers that some other mosquito species do #• #3 genera: Anopheles, Chagasi, Bironella • 473 species described • All females blood feed • Only Anopheles species transmits disease#19"Anopheles How to recognize? Adults rest at 45O angle, with abdomen pointing upwards Most adults have dark and pale scales on wings Eggs Larval position Horizontal Floating device 20"Anopheles Adults are crespescular/nocturnal Blood feeding and oviposition occurs from dusk till dawn Females lay 50-200 eggs Eggs cannot resist dessication Eggs hatch in 2-3 days (Tropics) 21"Anopheles Clean, unpolluted water Various habitats; Temporary pools open sunlit pools Rice Fields Edges of stream salt water marshes (e.g. An. melas, An. merus) Grassy ditches Larval Ecology Larvae are filter feeders on micro-organisms in the water; 22"3. Culicinae Yellow fever Dengue Fever Other Arboviruses Fileriasis #• 38 genera • 2,750 species • Important genera: Aedes Culex Mansonia Haemagogus Sabethes 23"Angle of the body Eggs do not have floats: Length and shape of the palps Siphon 3. Culicinae Abdomen points downward Body at angle to watersurface Anophelinae Culicinae 24"Culex Egg raft • Some species with very wide distribution • Eggs are laid in wide variety of habitats • Pools, puddles, ditches, borrow pits, rice fields #e.g., Culex quinquefasciatus: • Breeds in water with