---start parasit.lec.09.03.97----- #10, #11 Dr Johnstone 9/3/97 ("lecture 2, parasit 4001") INTRODUCTION TO THE NEMATODES URL nematodes include strogyles of horses, heartworm of dogs, etc. if you look at a classification tree, you see that all the parasitic worms are in the animal kingdom. the worms of veterinary importance are either platyhelminthes (flatworms) or the nematoda (roundworms) classification of nematodes: phylum - highest level we'll discuss order class superfamily family subfamily genus species - lowest level we'll discuss eg strongylus vulgaris, toxocara canis, etc. strongylus is a genus name, and vulgaris is a species name. this is found in horses. species==interbreeding population of animals that doesn't interbreed with other population sharing the same habitat. strongylus equinus and strongylus edentatus are also found in cecum and colon of equidae, but do not interbreed. all are similar in size and appearance and are hard to distinguish with the naked eye. they share the same host, same habitat, but do not interbreed - so they are different species. but their similarities are such that they lead us to the conclusion that they all had a common ancestor, and this common ancestry is recognized by our classifying them in the same genus. all are in same subfamilly strongylinae, family strongylidae, etc. classification allows us to organize information about parasites in a structured and helpful way. eg, don't memorize all details about each species - know the info about the family, and the superfamily, and so forth. only a few features will differ for the distinct species. nematodes are often called roundworms eg toxocara canis, the roundworm of dogs. they are cylindrical worms. the head end and tail end tapers. they have an outer skin called a "cuticle" which is often modified. basic nematode structure (don't have to know all this information): we will be more preoccupied with life cycles in this course. you will likely decide that parasitologists are obsessed with life cycles. it is essential to understand them. anyway. STRUCTURE of nematodes: covered with CUTICLE (outer skin) which may be modified oesophagostomum radiatum - parasite of lg intestine of ruminants and pigs. the cuticle has several areas where it is inflated, or rounded out. the esophageal area of the worm has ALAE around it - winglike expansion - in the cervical area. VESICLES are inflations of the cuticle around the mouth - cephalic vesicles - and ones by the esophagus are cervical vesicles another roundworm he shows us has no vesicles, no alae. but it has PAPILLAE on the sides, which are spinelike projections found in the esophageal region, believed to be sensory. at the tail end of many nematodes we see the caudal alae, and caudal papillae. CAUDAL ALAE- cuticle is expanded to form a bursa at the end, and the bursa is suported by rays, which are papillae, all of which are expansions of the cuticle. this slide shows the hookworm of cattle. this bursa is the COPULATORY BURSA. it's used to grasp the female during copulation. not all nematodes have this bursa. females don't have it either. female strongyle with two males attached to it are shown on this slide - the bursae of the males fans out, grasps the female over the vulva, spicules are extruded, and sperm is passed into the vagina of the female through the spicule. this configuration is used in classifying nematodes into BURSATE and NON BURSATE nematodes. BURSATE nematodes - order strongylida NON BURSATE: orders ascaridida, oxyurida, spirurida, rhabditida, enoplida under the cuticle is HYPODERMIS which is a syncitium. on the inner aspect is four longitudinal thickenings called CORDS. lateral cords are excretory system, dorsal and ventral are nervous system. inside that are somatic muscles, found only between the longitudinal cords. the body cavity is lined with a membrane, is fluid filled, and contains internal organs. the intestine is a simple tube with a single layer of columnar cells on basal membrane lining it. there are microvilli projecting from the inner surface of those cells. goign back to GI tract - it consists of three parts. a mouth opening, leading to a buccal cavity; an esophagus, and a simple intestine. mouth opening varies by species from simple opening to large opening with thick cuticular walls and teeth. esophagus is thick walled, lined with cuticle, contains muscle, nerve, and glands in its walls. the esophageal muscles serve to dilate the lumen of the esophagus in a pumping fashion, to suck in liquid food and pump it to intestine. in some worms, the sucking action can reach 120 pulsations/minute. that's why these worms are so pathogenic - voracious feeders. intestine opens into rectum in females or cloaca in males, which is lined in cuticle and terminates in anus..opens as excretory duct or pore. the nervous system consists of nerve ring, ring of nerve ganglia around the esophagus, from which nerve trunks extend and go through dorsal and ventral regions of hypodermis. sexes are separate in most nematodes, males usually smaller than females. male has one or sometimes two tubular testes, stores sperm in seminal vesicle, carries it to cloaca in vas deferens. dorsal to cloaca are the spicules. they are in a pouch that opens into the cloaca, so that during copulation the bursa spreads, the spicules are extruded into the vulva/vagina of the female. females have two tubular ovaries, each has an oviduct which leads to uterus, vagina, vulva - sometimes there is a vulva flap to protect it. trichostrongylus sp: horse parasite. we see the small mouth opening,excretory pore. simple structure strongylus vulgaris: huge mouth opening, more complex, teeth present. adults of strongylus vulgaris are pathogenic because they take big bites out of the cecum/colon of the horse. they cause bleeding ulcers. cuperia- small intestinal parasite of ruminants- we see the paired spicules. the shape of the spicules allows you to speciate the nematodes, btw. we won't really have to do that in this course :) each structure has a function, and the shape and configuration is usually species specific. nematode LIFE CYCLE: nematodes are worms, and seem simple on the surface. but there is a complexity. they are highly differentiated. sexes are separate and sexual reproduction is the norm. basic life cycle: nematodes are rich in diversity of their life cycles. although most have a life cycle that relates to this basic form, the complications in each family/genus/species gives to this group of nematodes an extraordinary diversification. despite this, you can identify this basic life cycle: assume an animal is infected, parasitized, with males and females. the worms have sex and the females lay eggs. the eggs pass to the outside of the host. larval stage L1 develops and hatches. molts to L2. grows. molts to L3. L3 is infective stage for the host. parasitized host == "definitive" or "final" host. L3 enters host, molts to L4. grows, molts to immature adult. grows. becomes mature. has sex. lather, rinse, repeat. the part outside the host is called "preparasitic phase" and part inside the host is parasitic phase. nematodes have a cuticle, fairly rigid, which constrains growth. in order to grow, since all stages have a cuticle, they have to grow a new, larger cuticle, and cast off the old one. this is called MOLTING. in the complete life cycle there are four molts. L1, L2, L3, L4, immature adult (L5), mature adult. this is an important basic concept to grasp. everything else is built on this. we know that immediate transfer of infection from host to host is rare. that's because of this life cycle. the preparasitic phase has to occur before the parasitic phase can occur. in many nematodes the preparasitic stages are free living, in others, they parasitize a secondary host - eg, dirofilaria immitis develops inside the mosquito, which ingests L1 from the host (microfilaria), and then L2 and L3 must develop inside the mosquito - they can't develop outside the mosquito. another dog is infected when a mosquito harboring L3 takes a blood meal and injects the L3 into the dog. then you have development of L4 and adult stages which end up in the heart of the dog (predilection site) again: eggs are passed. eggs hatch into L1 L1 grows molts L2 grows molts L3 infects host molts L4 grows molts becomes adult, reaches predilection site. often molt from L2 to L3 is incomplete, and doesn't become complete until L3 gets into new host. Eg, it will have both the L3 and the L2 cuticles on it - the L2 cuticle is called a "sheath". so you have "ensheathed 3rd stage larvae" which enter the host. L1 and L2 are "active feeding stages" but L3 is ensheathed, so it can't feed, because its mouth is covered, so it lives on stored nutrients, until it enters the host. sheath is protective. loss of sheath is required for infection to occur. so you have sheathed L3, say a sheep eats it. it will exsheath in the rumen, where conditions exist to induce exsheathment. this is important. host factors provide stimulus for exsheathment. these induce larvae to produce exsheathing fluid, rich in leucine aminopeptidase which loosens the sheath in a ring near the gland which makes the fluid. this weakens the sheat there, near the anterior end of the worm, and the cap pops off, and larva wiggles out. so the exsheathed L3 then begins to parasitize the host. then it can grow to L4. if it doesn't lose the sheath, it can't do that. exsheathment is one of the determinants of host specificity. if infective L3 doesn't get appropriate stimuli characteristic of its host, it will not exsheath and feed on the host. consider horse and sheep grazing together. consider a nematode of sheep called hemonchus contortus that causes blood loss in sheep. imagine sheep are grazing the pasture, so it is contaminated with L3 infectious larvae. if another sheep comes along and eats those larvae, the L3 with exsheath in the rumen, L3 will enter the abomasum and molt to L4. but what if a horse comes along and eats the infective L3? say it eats them. well, they will not exsheath, because a horse has no rumen. the L3 do not exsheath and do not infect the horse. this is an important feature that we can use in parasite control. again - the infective L3 has a sheath. it enters the host, exsheaths, and then becomes a parasitic L3 inside the host. types of nematode life cycles: DIRECT and INDIRECT direct: preparasitic stages (L1, L2, L3) are freeliving in the environment two types of direct life cycles: 1. eggs hatch - ostertagia ostertagi. if eggs hatch, than the L1, L2, L3 are free living, and infective stage is ensheathed L3, and host is infected by eating ensheathed L3 or perhaps by skin penetration in hookworm. 2. eggs do not hatch - ascaris suum. preparasitic larvae develop inside the egg. so infective stage for host animal will be the egg containing L2. route of infection usually ingestion of the egg containing L2 or L3 depending on species. in ascaris suum L2 is the infective stage. so remember the basic life cycle is just a guideline. do not panic yet. we will come back to specifics later. ostertagia ostertagi life cycle: adults live in abomasum of infected host. female lays eggs, eggs get out of host, eggs hatch, L1, L2, and L3 develop in feces, active L3 migrate onto pasture, host eats L3, it exsheaths, becomes parasitic, grows, becomes adult in host. ascaris suum lives in small intestine of pigs. lays eggs. eggs are passed out of host in feces. in egg, L1 develops but egg doesn't hatch. develops into L2 inside egg. infective stage is an egg holding an L2. pig eats egg. pig stomach is where egg hatches into L2. L2 migrates through pig, etc. INDIRECT life cycles an intermediate host is used/required for development of preparasitic larvae. two types: -definitive host ingests intermediate host (deer brainworm) -intermediate host serves as vector (eg dirofilaria immitis) deer brainworm - predilection site is meningeal blood vessel of deer brain. L1 is carried to heart, then to lungs, are coughed up, swallowed, go out through feces - so deer passes L1 in feces. slugs and snails that feed on deer feces take up the L1 which penetrate the feet of the snails. inside the snails, the L1 develops into L2, then into L3. L3 is the infective stage. then another deer eats the slug or snail infected with L3, snail is digested, L3 migrates back to brain of deer heartworm we should probably all know about. ---break--- re: textbook - just get the veterinary parasitology reference manual, the green spiral bound book. TYPE OF NEMATODE HOSTS * definitive: host in which we find the mature, egg laying adult. eg, dog is definitive host of dirofilaria immitis * intermediate: host in which preparasitic development takes place. mollusks, arthropods, mammals, etc. dirofilaria uses the mosquito as its intermediate host. * paratenic or transport: a host in which preparasitic infective stage larvae are found, but no development is taking place. the parasite uses the transport host to take it to the definitive host. the paratenic host is usually eaten as prey by the definitive host. ascaris suum of pigs - the infective stage is the egg containing the 2nd stage larva, found in soil. earthworm will ingest the egg, which will hatch. L2 will sit in the earthworm but won't grow. along comes a pig, which likes to eat earthworms. it eats the infective L2 when it eats the earthworm, and then L2 begins the parasitic phase of the life cycle in the pig. so ascaris suum uses the worm as a way to get into the pig. the worm is the paratenic host of the parasite. ARRESTED DEVELOPMENT: HYPOBIOSIS == inhibition of larval development look at the basic life cycle again. normally, a sheep grazing a pasture ingesting L3 of hemonchus contortus...L3 will become parasitic in the sheep. the time it takes from the sheep eating the larvae until eggs are passed in the feces is the "prepatent period" and this varies between 2-3 weeks with the species of hemonchus ingested. think of sheep being raised in PA with seasons that change - cold winter, mild spring, etc. there are two things driving the life cycle. temperature and moisture. warm & wet weather encourages rapid growth of preparasitic stages into L3. the colder it is the slower the growth. below 10 degrees C, no development occurs. optimal temp is 22-26 deg C. In winter, then, when it gets cold, if the females keep making eggs, they are not likely to survive. the parasite needs to survive, so it has to avoid winter conditions. so this parasite has developed something called arrested development. go back to fall. the L3 are on the pasture. temperatures are dropping. this signals the L3 on the pasture - once temp drops below critical point - any L3 ingested by sheep do NOT develop into adults. They exsheath, they molt to L4, and then they undergo hypobiosis. they stay as arrested L4 throughout the winter. it warms up in the spring, and L4s that have overwintered in the host begin to resume their development, molting to adults and laying eggs and so forth. this allows survival over the winter or in tropical areas where there is a dry season (dryness is the signal there instaed of coldness). this is important for three reasons. one, it ensures survival of nematodes during periods of environmental adversity. two, when large numbers of the larvae undergo arrested development and then resume, the resumption may result in outbreaks of disease (eg in spring), and if no outbreak of disease, certainly there will be seasonal contamination of environment as eggs are laid agian in the spring, right when animals start grazing again. third, arrested larvae are depressed metabolically, and are not as susceptible to anthelmintics. someone asked how larvae know when to resume growing. probably, they are preprogrammed for a period of arrest which will only last so long, but really we do not know. so the larvae will resume in waves, not all at once, probably depending on how they are genetically programmed. (end of intro to nematodes) (begin specifics) THE TRICHOSTRONGYLOIDEA "lectures 3 and 4" handouts. these lectures are also on the web. TRICHOSTRONGYLOIDEA are a superfamily belonging to the order STRONGYLIDA the STRONGYLIDA are the order these are the bursate nematodes. all males have a copulatory bursa. all have a buccal capsule, varying in size. they have both direct and indirect life cycles. this order contains four superfamilies, the trichostrongyloidea, the strongyloidea, the ancylostomoatoidea, and the metastrongyloidea (note, ancylostomoatoidea may be considered part of strongyloidea) the strongyle type eggs are thin shelled, smooth surfaced, and ellipsoidal in shape. we cannot speciate based on the egg. you have to culture the eggs to L3 and speciate L3, but since most of the drugs that kill one of these nematodes will kill the rest of them, it usually isn't important to speciate. also can dx species based on geography and clinical signs. trichostrongyloidea are found all over the world, and parasitize horses, swine, etc. there are two families in this superfamily: TRICHOSTRONGYLIDAE and DICTOCAULIDAE the TRICHOSTRONGYLIDAE are all found in the stomach of the host, and are small hairlike worms measuring from .5 to 3 cm. males have pronounced copulatory bursa. life cycle is direct. infection is by ingestion of ensheathed L3. small buccal capsules. 6 genera of this family are listed in the handout. the prepatent period is about 2-3 wks depending on species of nematode. are found worldwide in various climates, so arrested development is important in their life cycle. see handout for details of life cycle species: trichostrongylus axei, colubriformis, and vitrinus taxei is seen in ruminants, horses, and pigs. predilection site abomasum or stomach. prepatent period 2-3 weeks in ruminants, a bit longer in horses. causes hyperemic gastritis in horses the species seen in intestine cause enteritis and diarrhea but this is usually a secondary pathogen, not a primary pathogen Haemonchus contortus- abomasum of sheep and goats haemonchus placei- abomasum of cattle otherwise both haemonchus spp act the same way, but the prepatent period of placei is a bit longer than for contortus. otherise they're the same. haemonchus is the biggest abomasal nematode and is easily seen in situ. 1-3 cm long. females bigger than males, more easily seen. they are bloodsuckers, have small lancet in buccal capsule. they extrude lancet, slit host capillary and lap up blood it causes blood loss, anemia, hypoproteinemia. the females need oxyhemoglobin. the female worms have white ovaries and bloodfilled intestines. they look rather striking, like a barberpole of red intestine winding around white ovary - "barber pole worm". life cycle typical of the family. clinical signs are all related to blood loss. these worms are fairly voracious blood feeders. even in light infections we see anemia, pale mms, etc. because so much blood is lost, a lot of serum protein is lost, albumin is lost, will see hypoalbuminemia, edema, pale conjunctiva and mms. slide: submandibular edema on sheep with h.contortus infxn. slide: submandibular edema on cow with h.placei...also called "bottlejaw" this is a common sign in these animals. pale cows/sheep with bottlejaw usually have haemonchus. epidemiology of haemonchus contortus...how is it transmitted? important to know this so you know how to control it. at what point is it best to intervene to prevent an outbreak? if you see a flock of sheep with pale mms and bottlejaw, you have failed to prevent the problem. key to prevention is to undersand the life cycle and know what drives the life cycle to completion in the definitive host. epidemiology- those features associated with interrelation of parasite, host and external environment. factors influencing transmission of parasite between infected hosts, through the external environment. * BIOTIC POTENTIAL - ability of female to produce eggs. hemonchus females produce LOTS of eggs. small infections still produce high fecal egg counts. so the environment of the host can get heavily contaminated very rapidly. the parasite has a high biotic potential because of the seasons. not every egg will develop an L3 that gets back into the host. many are lost along the way. so the nematode pushes out a huge number of eggs, to increase the odds. * temperature and moisture dependency of egg-->L1-->L2-->L3 seen mostly in warm places. high humidity is essential for development of preparasitic larvae, and optimal growth is at 72-76 degrees. * arrested development is key. will arrest as L4 in abomasum then spring comes, L4 grows into adult, females lay eggs. 60 yrs ago it was found that every spring, grazing sheep had an increase in fecal egg counts. this rise was more pronounced in pregnant/lactating animals * PPR (post partum rise) and self-cure - pregnant sheep had high egg counts, but after weaning their lambs, fecal egg count dropped off. assume pregnant female flock. they lamb out in late winter and then in spring the fecal egg count rises (6-8 wks after lambing). we wean the lambs in may. then the egg counts of the lactating females drop, because the infection is being cleared. take a subgroup of these sheep and treat them with a drug like ivermectin that is effective against arrested larvae, we can prevent this PPR from occuring. why does PPR occur? go back to previous season. sheep graze all summer. temp drops in fall. L3 on pasture get the signal to arrest development after getting into host. so they become halted L4s in the sheep and stay in there all winter. now, sheep have Ab against this worm, but why do they wait til lambs are weaned to clear the parasite? well, there is an induced immunosuppression. the female sheep tolerates the infection. the immune response is suppressed, we're not sure exactly why, but the primary means seems to be due to the hormones of lactation, mainly prolactin. so when the lambs are weaned, and prolactin levels drop off, the infection is cleared. it is still somewhat murky, but this occurs routinely, annually, in sheep infected with various nematodes. to recap: the lactating ewe is immunosuppressed, and we see a rise in the number of eggs passed during lactation. this is important. if lg numbers of arrested L4 resume at the same time, the lactating sheep with sucking lambs will get very sick. so we need to prevent this! we can do this by using ivermectin - treat each ewe at lambing time. this is important a) to prevent sickness in lactating ewes, and b) to prevent contamination of the pasture. lambs grazing that pasture could get very sick and die. so it is important to treat to prevent lamb mortality! also any other sheep grazing the field could get heavy infections. this is an almost perfect adaptation of the parasite to synchronize with the life cycle of the host. * immune status of definitive host. only in sheep over 6 mos. we see a periodic natural expulsion of the nematode. DICTOCAULIDAE are found in lungs. ---end parasit.lec.09.03.97---