Vector Biology 101 General Insect Biology & Relevance to Disease and Parasite Transmission Taxonomy and Systematics Phylum Arthropoda: The phylum is probably monophyletic, but with 4 distinct groups. It includes lobsters, crabs, shrimp, centipedes, millipedes, daddy longlegs, insects, ticks, and mites, and spiders. There are over 1 million species of arthropods, making up the largest phylum in the animal kingdom. Phylum Arthropoda SubPhylum: Crustacea - lobsters, crabs, etc. SubPhylum: Chelicerata Class Arachnida- spiders, mites, ticks, scorpions, etc. SubPhylum: Myriapoda Class Diplopoda - millipedes Class Chilopoda - centipedes Subphylum: Hexapoda Class Insecta - beetles, flies, moths, mosquitoes, etc. Phylum Arthropoda Metameric - (body exhibits true segmentation - replication of muscles and nerves) Tagmosis - (segments of the body are modified and grouped together to form mouthparts and body regions such as the thorax) Chitinous exoskeleton – nitrogenous polysaccharide Hypothetical Insect Evolution “Worm-like” ancestry Metamerism (true segmentation) Tagmatosis (segments modified and grouped together to form larger body parts) Class Insecta (Hexapoda) Six legs Three distinct body segments Two pair of wings Eyes are often well developed Insect Characteristics THREE distinct body regions: Head (feeding, sensory, CNS) Thorax (locomotion, respiration) Abdomen (feeding, reproduction) General External Morphology of an Insect Cerci Organization - digestive tract Muscular pharynx works like a pump sucking food in Salivary glands and reservoirs store saliva (parasites exploit this niche to reach their host) Food can be stored in the crop Proventriculus is muscular - Proventriculus limits food passing to the midgut. Organization - digestive tract Midgut- principal site of digestion and absorption. Malphigian tubules (excretory system) - water and solute reabsorption Many insect secrete a peritrophic membrane - encloses the food mass Gastric caeca increase the absorptive area Insect development Morphogenesis Hemimetabolous Gradual change from one instar to the next Body proportions change Wings develop at later stages Holometabolous Striking change from last larval instar to adult Sexually mature adult emerges General Insect Head The Insect Head Apis (honeybee) mouthparts Housefly mouthparts Anopheles (mosquito) mouthparts Piercing Sucking Mouthparts Other Medically Important Arthropods (non-insects) Arthropods affect the health and well-being of humans and animals in several ways: Direct Causes of Disease or Distress Vectors or Hosts of Pathogenic Organisms Natural Enemies of other medically harmful insects Direct Causes of Disease: Ectoparasites - ticks, fleas, mites Endoparasites - chigoe flea, myiasis Envenomization - wasps, bees, spiders Allergic Reactions - dust mites Annoyance - mosquitoes, black flies Delusory parasitosis (DP) - psychosis Vector Biology Arthropods as vectors of infectious disease (parasites) Vectorbone Agents of Infectious Disease Parasites Bacteria Fungi Worms Viruses Modes of Transmission Mechanical Transmission Biological Transmission Propogative (multiplicative) Developmental (cyclo-developmental) Cyclopropagative Pathways of biological pathogen transmission: Vertical transmission: Transovarial transmission Horizontal transmission: Venereal transmission Trans-stadial transmission Modes of Transmission General Category Specific Route Parasite/vector example Transovarial Babesia bigemina/tick Vertical Transtadial Borrelia burgdorferi/tick Venereal LaCrosse virus/mosquito (male↔female) Co-feeding Borrelia burgdorferi/tick Salivation Plasmodium spp./mosquito Stercorarian Trypanosoma cruzi/triatomid Horizontal Regurgitation Yersinia pestis/flea Assisted escape/passive Borrelia recurrentis/louse transfer Active escape/active Onchocerca spp./black fly invasion Ingestion by host Diplidium caninum/flea The Mode of Disease transmission Arthropod-borne disease (ticks, mosquitoes, flies) Modes of Transmission General Category Specific Route Parasite/vector example Transovarial Babesia bigemina/tick Vertical Transtadial Borrelia burgdorferi/tick Venereal LaCrosse virus/mosquito (male↔female) Co-feeding Borrelia burgdorferi/tick Salivation Plasmodium spp./mosquito Stercorarian Trypanosoma cruzi/triatomid Horizontal Regurgitation Yersinia pestis/flea Assisted escape/passive Borrelia recurrentis/louse transfer Active escape/active Onchocerca spp./black fly invasion Ingestion by host Diplidium caninum/flea What makes a good vector? Transmission efficiency: Transmission frequency: Vector Efficiencies and Capabilities Competence Determined in the laboratory Physical ability of a vector to acquire infection and infect a naïve host A competent lab vector is not necessarily important in disease transmission in the field. Capacity Measured in the field Uses components such as: number of vectors per human, number of human bloodmeals per day per vector, daily survival rate, and the extrinsic incubation rate of pathogen; Vector efficiency is expressed in terms of low - high capacity Vector Biting Behaviors Activities Periods Nocturnal Diurnal Crepuscular Host specificity (blood meal source): Anthropophilic/anthropophagous Ornithophilic/ornithophagous Zoophilic/Zoophagous Feeding location: Exophilic Endophilic The Bug Walk Ticks and Mites - Ticks Generally macroscopic Haller’s organ - sensory, Ticks and mites are chelicerata - not olfaction and humidity insects Hypostome is toothed and exposed General body regions Transmit a number of human Gnathosoma (“head”)- carries the diseases chelicerata, hypostome Idiosoma - covered by a single Mites sclerotized plate and can be Generally microscopic further subdivided Lack Haller’s organ Hypostome is unarmed and hidden - small mouthparts Often possess hairs Family Ixodidae Hard ticks - hard cuticle Remain attached to the host for several days for feeding and become engorges (200-600x larger). Secrete a cement substance that helps securing them to the host No body segmentation Mouthparts visible from dorsal side 8 legs Questing in the open Family Argasidae Soft ticks Feed repeatedly, resting away from a host between meals Mouthparts visible from ventral side Questing in a protected environment (nest, etc.) Mostly ectoparasites of birds. Mites (small chelicerata 0.2-4 mm) Mite infestation caused by Sarcoptes scabiei Female mite burrows into skin, leaves tunnels, eggs, secretory and excrement behind. Causes sever itching, secondary infections Males remain on surface Order Siphonaptera - fleas Most are ectoparasites of mammals Bilaterally flattened Piercing/sucking mouthparts Feed exclusively on blood Highly evolved jumping mechanism - secondary loss of wings Important vector is disease transmission Holometabolous Medically important fleas Xenopsylla cheopis - rat flea - feeds readily on rodents AND humans Main vector of Yersinea pestis - the causitive agent of bubonic plague Medically important fleas Tunga penetrans - chigoe, jigger, (chigger) Endoparasitic flea - only a small posterior portion is exposed Anoplura Wingless Dorso-ventrally flattened Heterometabol Pediculus humanus occurs in two subspecies the head louse and the body louse The body louse lives mostly in the clothing and attaches its eggs to its fibers The head louse lives in the hair and attaches eggs (nits) to the hair Anoplura Lice are hemimetabolous, and lay specilized eggs (nits)" Anoplura !The crab louse is restricted to pubic hair !Lice transmit several forms of rickettsial typhus (febrile disease with high mortalities) !Both lice and typhus have flourished whenever people went to war. Often typhus killed more soldiers than combat Order Hemiptera - “half wing” !One of the largest orders of insects - 55,000 species !Still 2 sets of wings ! 1st set - leathery basal portion, membranous distal portion ! 2nd set - full membranous set !Dorsoventral flattened !Predacious insects !Family Reduviidae (reduviids) ! Assassin bugs - predators on other insects ! Kissing bugs ! Vector for Trypanosoma cruzi Order Hemiptera - “half wing” !Family Cimicidae !Ectoparasites !Cimex lectularis !Annoyance but not known to transmit nay disease Order Diptera !Flies !One pair of wings !Holometabolous !Huge group of insects with a number of medically important species. !120,000 species Family Culicidae !Virus and parasite transmission !West nile, encephalitis, Malaria, filiarial diseases !Sophisticated mouthparts !Long piercing proboscis !Length as long as thorax !Holometabolous with aquatic larvae !Distinguishing features among the medically important species Subfamily Phlebotominae !Sand flies !Very small - 1-3 mm !Only females feed on blood - usually at night or at dusk and dawn !Hairs on body and wings !Weak fliers - only capable of navigating short distances !Transmit Leishmania sp. Family Glossinidae !Tsetse flies !Fast flying insects !Day feeders (male and female feed) !Proboscis stick out in front like a tongue !Wings are folded like scissors when resting !Only in Africa !Transmit Trypanosoma brucei Polypneustic tubes: Family Glossinidae respiratory function ! Larviparous - give birth to larvae ready to pupate ! Deposit larvae in sand ! Larva dig in to sand ! ~ 15 min after “birth” pupation begins Family Simuliidae !Black flies - although most species are actually gray or tan. !Small stout body - thorax looks hunched !Active during the day !Painful bites !Transmits the disease Onchocerciasis Family Tabanidae !Horse and deer flies !Large colorful eyes !Large (6-25 mm) and powerful daytime feeders !Only females take a bloodmeal - required for egg development !We know them as serious pests - very painful bite !Transmit protozoan and helminth parasites Mullen, G. and L. Durden, Medical and Veterinary Entomology, 2002, Academic Press On Amazon.com for $97.42 (new) or $90.42 (used) .