---start---- parasit 10.02.97 Dr Lok parasit lecture 19: INTRODUCTION TO ENTOMOLOGY/important Diptera And now for something completely different... we're leaving the helminths behind. Now we're moving into arthropods: vectors and ectoparasites. Arthropods and their impact on animal health. No one here has ever had a course in entomology - because the field is disappearing. we're going to talk about arthropods in a general way. BACKGROUND: what are arthropods and why do we care? arthropods are a large group of organisms - a phylum equal in size to the nematodes or the chordates, or the platyhelminthes. if we just took one family - say the beetles - there are over 300,000 species of beetles alone, so it is av ery large and diverse group of organisms. arthropods are more species than all others combined. why are they so successful? High reproductive potential, for one. One female tick can give rise to hundreds or thousands of progeny. Many times this high repro potential is coupled to a short generation time. the life cycle may only be a few months, or even only a couple of weeks. this combination is called "genetic plasticity" ability to respond to environmental pressure very quickly. this is shared with bacteria and other microbial pathogens we've discussed. a good example of this genetic plasticity is the rise of insecticide resistance. many populations of mosquitoes are wholly resistant to DDT. so arthropods are successful competitors with humans and animals. they do compete with us for resources - food, fiber, etc. they cause major economic losses of food, crops, etc. in some places in the world where people have subsistence agrarian communities, these are major concerns. But they are not our big problem in this course. our concern is the impact arthropods have on human and animal health. there are two ways they do this: direct way: arthropods can be the causative agent of disease - eg, annoyance and blood loss disease :) is caused by mosquitoes. Lots of mosquitoes could cause severe blood loss. Dermatosis is also caused frequently by arthropod ectoparasites. "sheep scab" is a type of mange caused by a mite that is very small. Myiasis - stomach bots from a horse - larval form of highly specialized fly. these immature stages must live in the stomach of an equid to continue to grow. We'll be talking about this later. Arthropods can also cause ALLERGY or envenomization. insects like wasps will envenomize their prey and can cause dz that way, or can elicit a very allergic response. More people die from anaphylactic reactions against hymenopterous venom every year than from snake bite or other venoms. Some spiders are also very venomous - the brown recluse is very small but it secretes a stable proteolytic venom which causes severe local reactions that persist and spread and can cause loss of limbs or death. the black widow also has a potent and stable venom. so envenomization can be a direct cause of disease. indirect way: as a vector or intermediate host of a disease agent. example: mosquitoes and other biting flies can act as vectors of different types of protozoan and metazoan parasites. there are various kinds of mechanisms and nuances to this kind of interaction. arthropods can be mechanical carriers of disease agents, moving them from host to host by simple contamination. or, they can support some cyclical change in the agent or parasite involved. note: VECTOR seeks out its host. INTERMEDIATE HOST is sought out by definitive host. A mosquito acting as vector of heartworm seeks out the dog. A mouse acting as intermediate host is sought out by the cat that eats it. what are the characteristics of the phylum arthropoda? slide: snipe fly. illustrates basic characteristics. - bilateral symmetry - elongate multisegmented body - paired, jointed appendages - exoskeleton of secreted non-cellular material (in insects/arachnids, chitin) (in crustaceans, calcium carbonate) - ventral nerve cord (instead of vertebrate dorsal cord) - open circulatory system with dorsal "heart". blood and cells percolating through body cavity assisted by a pulsatile venous sinus dorsally. what are some familiar kinds of animals in this phylum? Classes such as the clas Crustacea, of little medical significance although are intermediate hosts for some flatworm parasites. Chilopoda - centipedes - some of these inject irritating substances with hollow fangs, but are also of little significance. don't serve as vectors. Diplopida - millipedes - as above. can cause some skin problems with secreted defensive irritants, but really of peripheral importance. medically important arthropod classes: Insecta aka hexapoda - 6 legged arthropods. includes the orders diptera (true flies), phthiraptera (lice), siphonaptera (fleas), and more (hemiptera, true bugs eg "kissing bug". note that these bugs can be vectors for some trypanosomes. also the bed bug belongs to this order and it feeds on people who sit on infested furniture.) arachnida: include animals like scorpions (scorpionida) which envenomize, spiders which are another order (araneida), and the order acarina (mites and ticks) How are arthropods constructed and how do they work? generalized morphology: some structural landmarks constitute taxonomic ID characteristics. You have to know what the parts are to know if it is there or not. see "exploded view" on p 2 of handout. INSECT BODY head, thorax, and abdomen are three main segments. head contains primary sensory structures - compound eyes, tympanae, feeding apparatus thorax has wings and legs if present. is divided into prothorax, mesothorax, and metathorax, each of which has a pair of legs in the insecta. the meta and mesothoraces also each have a pair of wings in some groups. abdomen contains repro and gi tracts. it is clearly segmented. there are "spiracles" lining it which are openings to the respiratory tract. ARACHNID BODY two regions: head and thorax are fused into a "cephalothorax" and it contains legs, eyes, etc. the abdomen has repro and digestive tracts. never any wings, but may be several pairs of legs. exoskeleton is multilayered. endocuticle, exocuticle, epicuticle. the epicuticle is a waxy layer that conserves water (seals it in). exocuticle contains chitin which is crosslinked to other things to provide dark color and hardness. don't memorize this. but it is all noncellular, and the whole cuticle is secreted by epithelial layer. GENERALIZED INTERNAL ARTHROPOD: note that throughout the handout some structures are marked with asterisks. these areas can be sites of pathogen growth in an arthropod. GI tract: paired salivary glands and ducts. these are definitely the sites of pathogen proliferation and development, they are sources of infection. the gut is divided into three segments - foregut +/- crop, proventriculus; midgut analogous to mammal stomach. somewhat expansile. hindgut roughly analogous to a colon - water absorbed from here. behind this are malpighian tubules which are kind of analogous to the kidney, and hwich is the site of development of dirofilaria immitis respiratory tract - oxygen isn't carried by blood cells. it comes in directly by a finely divided tube system. individual cells are penetrated by "tracheoles" which bring oxygen in from the outside through the spiracles. in some aquatic insects respiration can occur across the cuticle as well. sometimes contraction of abdomen causes "inspiration" spiracles vary widely and are good taxonomic cues for speciation of arthropods. excretory system: malpighian tubules, continuous with gut, remove nitrogenous waste from body and shunt it into hindgut. repro: simple. either paired ovaries or testes joined by ducts. also can be site for pathogen development. there are many arthropod associated microbial diseases that can be passed mother to offspring because of this. also many female insects have a small sac off the common oviduct called the spermitheca which stores sperm. female insects may therefore be totally monogamous. they mate one time and store the sperm and never associate with males ever again. this is used strategically in reproductive control of arthropod populations. fat body: site of intermediary metabolism. kindof a liver. loose aggregation of fat cells. "CNS": loose use of the term. sure there's a supraesophageal ganglion near the head, but there are many ganglia, and the NS is highly DEcentralized. sensory organs have different shapes/forms: simple and compound eyes for recieving light stimuli, other receptors for heat/chemical/mechanical stimuli endocrine: arthropods do have endocrine systems. development and reproduction is under strict hormonal control. this is important b/c many modern growth regulating types of insecticides mimic some of these hormones. DEVELOPMENTAL BIOLOGY: insects insects use two different kinds of strategies. both are types of metamorphoses - sequential changes in body forms. the first type of metamorphosis is "simple" or heterometabolous metamorphosis. a nymph stage emerges from egg and molts a few times through various stages until it reaches adult stage. the nymphs look like small adults. not only do they look like parents,but they inhabit the same ecological habitat,feed on the same substrate, and so forth. adult female lays egg that hatches into nymph. complete/holometabolous metamorphosis. complete series ofradical changes in body form. immature stages look nothing like adult. larval stage hatches from egg. larvae undergo one or more instars or forms each punctuated by a molt. all these larval stages are active feeding forms which store up nutrient reserves and they finally molt into a quiescent stage called a pupa which is nonfeeding. it doesn't do anything but sit there, and in the case of the pupa the adult forms, and escapes by some method after it's all ready to go. not only are larval stages physically different from adults, but their ecological niche and the substrates they feed on are different as well. ACARINA: confusing terminology (ticks/mites) hatching from eggs we have 6 legged larval stage, followed by 8 legged nymphal stage which feeds on blood followed by adult. We've talked about role of arthropods as vectors/int hosts. What is it that gives "vector competence" to a group of arthropods? Why are some of them vectors and others not? Why aren't grasshoppers good vectors for vertebrate pathogens, while mosquitoes are? what's the evolutionary explanation? the first major trend that has given rise to vector competence is the trend in evolution of mouthpart morphology. primitive insects eg grasshoppers have chewing or mandibular type mouthparts. they just crush plant or other material and ingest it. in more evolved arthropods the mouthparts have evolved into peircing stylet or sucking tube or something that can pierce the animal and suck blood or transfer fluid in some way. the other trend is change in food preference. many arthropods are obligate plant feeders. some have evolved toward hematophagy - blood feeding - and these arthropods are more likely to be good vectors. so something has to have the right equipment and the right food preference. assuming you have that, there are still other things... determining factors include the extrinsic factors of temperature, which must be appropriate, and temporal and spatial association - a mosquito will not be a good vector of dog heartworm if it isn't interested in feeding on dogs. some mosquitoes feed only on bird blood, and they are not good d.immitis vectors. there are also intrinsic factors - vector susceptibility to infection may be under control of genetic factors or nutritional factors. you can take lab populations of mosquitoes and manipulate them so they are highly susceptible to d.immitis, or totally refractory to d.immitis, due to genetic variation. ---end---