Presentation on theme: "Nematodes zAre surrounded by a secreted multilayered cuticula, have only longitudinal muscles which are organized in quadrants and a ‘pressurized’ pseudocoel."— Presentation transcript:
Nematodes zAre surrounded by a secreted multilayered cuticula, have only longitudinal muscles which are organized in quadrants and a ‘pressurized’ pseudocoel zA simple linear intestine made up of columnar cells with a muscular pharynx that pumps in food against the internal pressure zA linear reproductive tract along which germ line cells go through meiosis and mature into gametes (amoeboid sperm, eggs surrounded by an impermeable shell).
Nematodes zBelieve in order and fate. They develop to a defined number of cells and follow a highly deterministic developmental program. zFour larval stages and adult, molt at each developmental step. The L3 stage can developmentally arrest and ‘hang in’ waiting for better times or the next host. zDifferent life cycle stages can differ dramatically in their metabolism (aerobic larvae, anaerobic adults)
Nematodes or round worms 2 GI Nematodes: Trichuris, Ascaris & Hookworms Worm expulsion and the Th2 immune response The hygiene hypothesis of allergy Trichinella & the first modern public health campaign
The importance of gastro-intestinal (GI) nematode infections zRoughly halve of all people are currently infected with GI- nematodes zInfection is generally associated with low mortality, but there can be considerable morbidity
The importance of GI nematode infectioins ParasitePrevalence (millions) Morbidity (millions) Mortality (thousands) Ascaris Hookworms Trichuris Children are especially at risk to suffer from GI nematode related morbidity.
The WHO helminth control program for school aged children zChildren are continuously exposed to infection zChildren grow rapidly and are especially susceptible to infection and the associate morbidity zChronic helminth infection seems to have a negative impact on cognitive development z10 cents annually per child covers drug cost zThe program is heavily based on drug treatment
Trichuris trichuris (human whip worm) zAdults live in cecum where the female worm produces 20,000 eggs per day which are shed with the feces zEmbryonation occurs within 3 weeks in soil zInfection is through ingestion of eggs, larvae hatch and penetrate the mucosa and tunnel it zLarvae grow and molt 4 times until mature
Trichuris zAdults remain embedded with their thin front end in the mucosa zInfections with less than 10 worms are asymptomatic zModerate to heavy infections cause abdominal pain, diarrhea, weight loss zHeavily infected children may have profuse bloody diarrhea, cramps, tenesmus and urgency which may result in rectal prolapse zAdults can live for years so worm burden can build up Trichuris vulpes (dog whip worm in the intestine of a heavily infected dog)
Trichuris zDiagnosis by demonstration of characteristic eggs (“champagne corks”) zIn moderate infections eggs might be hard to find and concentration techniques might be required zBy procotoscopy, mucosa might be edematous and adult worms might be noted zTreatment with mebendazole zSanitary disposal of human feces prevents disease
Ascaris lumbricoides -- the human round worm zLarge intestinal nematodes (Ascaris lumbricoides infects humans, A. suum infects pigs, the species are almost indistinguishable, but have clear host specificity) zMales are cm and have incurved posterior end, females are cm zThe anterior end of both sexes shows three lips zOne billion people infected
Ascaris zFully embryonated eggs are swallowed and L2 hatches in the stomach and penetrate stomach or duodenal mucosa zLarvae enter blood stream and leave through alveoli into lung zLarvae molt several times in the lungs L3/L4 move up and get swallowed z2-3 months after infection the adult worms start laying eggs (200,000 daily) zEggs are shed with the feces and embryonate within 2-3 weeks
Ascaris zInfection depends on fecal contamination of food, water or soil zEggs are sensitive to sun light but otherwise extraordinarily resistant (ascarosides - special glycolipids secreted by the embryo) zFertilized eggs are shorter and rounder than unfertilized
Ascaris zOccasional pulmonary symptoms zIntestinal phase mostly asymptomatic, but worms can lead to a developmental slow down in children zDangerous complications are mostly observed in children under 10 zVolvulus, a mass of knotted worms obstructing the intestine zPenetration of the bile duct and liver by adult worms zPenetration of the intestinal wall, followed by peritonitis zWandering and obstruction can be an unwanted side effect linked to certain medications
Acylostoma duodenale & Necator americanus -- human hookworms zSmall nematodes (1-1.5 cm) zHead is slightly bend (hook) and the ‘mouth’ carries characteristic teeth (Ancylostoma) or plates (Necator, note that these are not real teeth but cuticular formations of the ‘buccal capsule’) zThe posterior end of the male worm is elaborated into a copulatory bursa
Hookworms zAncylostoma is found in Europe around the Mediterranean, on the West coast of South America and in parts of China and India zNecator is found over much of the western hemisphere, Africa and South East Asia zMore than a billion people infected
Hookworms zAdult worms live in the small intestine and female lay eggs a day over 5 years zEggs are passed with the feces zLarvae develop outside the body and molt twice zThe filariform or L3 larvae move to the surface in search for a host zIf they come into contact with the host they penetrate the skin, enter blood vessels and leave the circulatory system into the alveoli zThe lavae move up the trachea into the esophagus, are swallowed and finally reach the intestine, where they molt twice more before they reach maturity Infectious L3
Hookworms zSkin penetration and associated secondary bacterial infection can result in “ground itch” zPulmonary phase is usually asymptomatic zIntestinal phase: worms attach to the mucosa and feed on blood. Worms continuously move to new places exacerbating bleeding
Hookworms zThe main concern with hook worm disease is blood loss z 0.03 ml (N.a.) to 0.26 ml (A.d) per worm, up to 200 ml per day in heavy infections zChronic heavy infection results in anemia and iron deficiency zTogether with malnutrition infection can severely stunt growth and development in children zAnemia leads to weakness and fatigue in adults zAntihelminthic treatment with mebendazole
Creeping eruption or cutaneous larva migrans zHumans occasionally get infected by L3 larvae of dog and cat hookworms (e.g. Ancylostoma caninum or brazilienzis again by skin penetration) zThe larvae can not establish a productive infection in humans, but wander about in the subcoutaneous tissue, causing significant inflammation and painful swelling zResponds well to treatment
Hookworms zAcylostoma was commonly found in coal mines throughout Europe zHeavy infection rates due to ‘perfect’ conditions for transmission in mines could result in severe anemia and pathology zAt the turn of the 19th century, severe hookworm disease was an officially recognized occupational hazard in German coal mines and miners with the disease were eligible for workman’s compensation
Hookworms zHookworms were wide spread in the Southern USA (read Jimmy Carter’s boyhood biography for detail on hookworm and other infectious diseases in rural Georgia) zHookworm control programs were a big part of the public health campaign in the South during and right after World War II z % of school children from rural costal Georgia tested positive for hookworm infection Nurse brings hookworm medicine to a rural Alabama family in 1939 (National Library of Congress photo LC-USF D)
The immune response and GI nematodes zNematode infections are usually chronic and have high rates of reinfection zThis suggests that the worms have developed effective ways to a) avoid getting killed by the host and b) they are not inducing overly strong inflammatory responses, which could kill the host zHowever there are data from human epidemiological studies which indicate that a certain degree of protective immunity might develop (at least in some diseases) zThe many variables in the study populations make it hard to develop sound models.
Mouse models to study GI nematode immunity zThe use of a mouse model allows control over many factors variable in human studies zThe mouse immune system is very well studied and many reagents are available zA huge collection of so called knock-out mice with defined defects in specific aspects of their immune system can be used
Lymphocytes are required for worm expulsion zMice are infected with Trichuris muris eggs zAfter 11 days many worms can be detected, but after 33 days all adult worms are cleared zSCID mice lack T and B cells and can not expel worms zReconstitution of SCID mice with naïve lymphocytes from a normal mouse restores ability to control infections zThis approach of ‘taking it apart and then putting it back together’ has been very powerful to dissect the immune system and its responses to various pathogens
A quick review of Dr. Tarleton’s immunology intro Worm expulsion requires T-cells, more specifically a certain flavor of T helper cells
T-helper cells can develop into two flavors (TH1 or 2) zThe immune system and its cells make important choices zThe cells of the immune system talk to each other through messenger proteins called cytokines zT-cells both make and respond to many cytokines zStimulation with different cytokines leads to the development of two types of T-helper cells specialized for orchestrating two very different immune responses zTh1 and Th2 strongly down-regulate each other zThis polarization has important consequences for the downstream response and can spell life or death
TH1 cells promote cytotoxicity and destruction of intracellular pathogens zTH1 cells orchestrate a response directed to inhibit intracellular pathogens like viruses, certain bacteria (e.g. TB), or certain protozoan parasites (Leishmania - see later lecture) zThey secret Interferon gamma which activates intracellular killing mechanisms zThey activate macrophages and cytotoxic T-cells
TH2 cells promote neutralizing antibodies and mast cell activity zTH2 cells suppress the activation of macrophages and activate eosinophiles and mastcells zTH2 cells promote a strong antibody response based on neutralizing IgGs and IgEs zA TH2 response is most effective to combat extracellular pathogens
The key to expulsion is mounting the “right” response zCertain inbred mouse strains mount a protective response to Trichuris muris resulting in expulsion (resistant), others develop chronic infections with worm persisting in the gut (susceptible) zSusceptible mice develop a strong immune response, but it’s a Th1 response zIf however these susceptible mice are treated to artificially shift to a Th2 response they clear the infections zThe immune system is not hard wired but makes important choices which can control the outcome of an infection
Physiological changes induced by anti-worm immunity zRecent work show strong effects of the anti- worm immune response on the gut physiology zInstead of directly killing the worms these physiologial changes appear to render the intestine inhospitable for the parasite zIncrease in number of goblet cells (these are the cells in the intestinal epithelium which produce mucus - strong red stain in the histology image) zIncrease in mucin secretion zIncrease in intestinal mobility zIncrease in water influx into the intestinal lumen (some of these seem to be directly induced by cytokines and immune cells like mast cells)
Worm expulsion zIt is associated with a type 2 (TH2) response zThe effector mechanisms include immunological and physiological processes zMany worms have found ways to suppress or modulate the immune system zThis allows them to dodge an effective immune response and/or prevent the potential demise of their mammalian host due to massive inflammation
Immune modulation by worms can be detrimental or beneficial zNumerous recent epidemiological studies show that certain vaccines are less effective in children that are infected with worms than those that have been cured using drugs (these are mostly vaccines that require a robust TH1 response) zThis is backed up by many studies in mice that use vaccination or co-infection zOn the other hand worm infection can dampen autoimmune diseases and allergies (diseases due to an ‘over-active’ immune system)
The Hygiene Hypothesis zThere has been a considerable increase in the diagnosis of autoimmune diseases and allergies over the second half of the 20th century zPrevalence of allergies in urban areas appears higher than in rural environments zEnvironmental factors like pollution, nutrition etc. can be important for specific allergies but have shown little consistent overall association with allergies and autoimmunity zChildhood infections though show strong negative correlation with both autoimmune disease and allergies
Inverse correlation of type I diabetes and chronic infectious diseases Red delineates areas which harbour six or more of the low mortality neglected diseases (filariasis, leprosy, onchocerciasis, schistosomiasis, soil- transmitted helminths, and trachoma). Yellow delineates areas where there are relatively high incidences of T1D (> 8 per /year). Non coloured areas delineate where T1D < 8 per /year and where the ‘neglected diseases’ are not endemic.From: Zaccone et al. Parasite Immunol :515–523.
Inverse correlation of allergies with infections zAmong kids in various studies in different areas of the world around 30% have antibodies against dust mite allergen (suggesting they all are exposed) zBut whereas asthma is found in 12% of kids from Europe and Australia, only 3% have asthma in Gambia and Nigeria
The Hygiene Hypothesis zHigh pathogen burden stimulates the immune system to develop a robust regulatory network that keeps inflammation in check zWorms set up long lasting chronic infections, they induce strong TH2 responses and promote regulation of this process zUnderstanding this process in detail could point to new allergy interventions
The Hygiene Hypothesis: Treatment by worm infection? zSeveral clinical trials using Trichuris suis have been conducted to treat human Crohn’s disease or ulcerative colitis (autoimmune inflammation of the intestine) zT. suis does not productively infect humans yet modulates the immune response zMost studies show some clinical improvement for a fraction of the patients zFurther studies are needed to fully evaluate potential and mechanisms
The Hygiene Hypothesis: Treatment by worm infection? zOne example (double blind study with 54 patients suffering from acute ulcerative colitis): The patients received either a placebo or 2500 T. suis ova every 2 weeks for 12 weeks. z43.3% of the patients given T. suis improved compared with those given placebo (16.7%). zThe study also included a 12 week crossover limb where patients originally on placebo where switched to T. suis and those on T. suis were switched to placebo. In the crossover limb, 56.3% of the patients given T. suis improved compared with 13.3% of patients given placebo (see Elliot et al. (2007) International Journal for Parasitology 37: for detail) zDon’t try this at home!
Trichinella spiralis zTrichinosis is caused by infection with Trichinella spiralis z(additional morphologically indistinguishable species have now been described for sylvatic cycles) zInfection of all hosts occurs through larvae encysted in muscle tissue (carnivorism, salvage and or cannibalism is needed to maintain transmission)
Trichinella zThe larva is freed from its nurse cell, and enters the mucosa of the small intestine zLarvae grow, molt 4 time and young adults copulate within 32 hours of infection zThe females give birth to live L1 larvae while tunneling the epithelium zFemales die after 4-16 weeks
Trichinella zThe larvae enter blood vessels and are carried throughout the body zWhen they pass skeletal muscle (they prefer highly active muscles) they leave the vessels and penetrate the muscle and the muscle cells
Trichinella zThe larvae enters a muscle cell and lives as an intracellular parasite developing in the cytoplasm of the host cell zThe larvae manipulates the host cell to its needs (probably by secretion of suitable effector proteins, the molecular mechanism is not well understood) zThe end product is a nurse cell zA fine net of blood vessels forms around the nurse cell (angiogenesis)
Trichinella zThe host cell looses its myofilaments and several additional subcellular changes occur zBoth host cell and worm are enclosed by a collagen capsule (collagen mRNA has been detected in nurse cell, but some authors suggest the capsule is secreted by surrounding fibroblasts
Trichinella zStrong dose dependence of symptoms (>1200 larvae can be lethal) zActivity of the females can cause inflammatory reaction in the mucosa, leading to diarrhea and abdominal pain zParasitized muscle exhibits intense inflammation, cyst start to calcify in humans after 6 months
Trichinella zMuscle invasion stage begins in the second week after infection zFever and perorbital edema are followed by myalgia (muscle pain) and weakness zCharacteristic splinter hemorrhages can be found under finger nails zFever and chills can persist for weeks zHeadache is common and dizziness may develop zMuscle swelling, aching and tenderness occurs often zDeaths are rare and due to myocarditis (inflammation of the heart muscle), encephalitis and pneumonia (larvae in the diaphragm)
Trichinella control was one of the first effective modern public health measures zTrichinella was discovered in 1835 and linked to human disease 1860 zSeveral large outbreaks in Germany resulted in the formation of an expert commission in Berlin which included Rudolf Virchow one of the co-discoverers of the Trichinella life cycle zVirchow suggested mandatory inspection of all meat at the slaughterhouses a practice that was introduced in Prussia in 1877 zThis has greatly reduced trichinellosis as human health problem
Trichinella zDomestic infection through pigs has become rare in the US and Europe zMeat control, rat control, prompt removal of dead animals, limit access of wildlife to pigs zAlmost all case now are sylvatic transmission from game zYou want your bear burger well done!
Aniskiasis zHumans are accidental host of several nematodes which reach maturity in marine mammals zInfection through consumption of uncooked fish zLarvae often penetrate gastric and intestinal mucosa causing pronounced inflammation which can mimic Crohn disease zRapid onset of symptoms often within hours zDiagnosis by endoscopy, surgical or endoscopic removal of larvae in cases with severe symptoms