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Parasitology is the study of parasites , their hosts , and the relationship between them
Definitions
What are Parasites
An organism that is entirely dependent on another organism referred to as the host for all or part of its life cycle and metabolic requirements
What is Parasitology
This is the area of biology concerned with the phenomena of dependence of one living organism on another. It is an aspect of symbiology which deals with the study of Parasites Medical parasitology One of the largest fields in parasitology, medical parasitology is the study of those parasites which infect humans Veterinary parasitology The study of parasites which cause economic losses in agriculture or aquaculture operations, or which infect companion animals
Tropical parasitology: study of parasites that affect man and animals in the Tropics
Types of Parasites
Site: Ectoparasites external parasites. Live temporarily or permanently on the surfaces of the host (skin, feathers, gillsetc). E.g. ticks, lice,etc. the occurrence of ectoparasites especially arthropods is called infestations.
Endoparasites internal parasites. Live inside or within the host tissues and organs. Endoparasites that live within the cells of the host tissues are called Intracellular parasites e.g. malarial parasites (plasmodium species). Those that live in tissues but not within the cells are called Extracellular parasite. Those living in the host lumen such as the gut, circulatory blood and lymph vessels, the respiratory and urino genital tracts are called coelozoic parasites, while those living in the tissues of the host are called histozoic.
Size: Microparasites small, unicellular and multiply rapidly within the vertebrate host often inside cells e.g. protozoans.
Macroparasites-large, multicellular and do not have direct reproduction within its vertebrate host. E.g helminthes
Others
Obligatory/obligate parasite-are absolutely dependent on the host and are incapable of an alternative or independent existence.
Facultative-are opportunistic parasites which can live a non parasitic existence and can survive for a limited period within another organism. They are capable of both free and parasitic existence E.g. S.stercoralis. Host
A host is the non parasitic member in a parasitic association. It harbours the parasite and provides the nourishment and shelter to the parasite.
Types: Definitive Host: Harbours the adult parasite, the most highly developed form of a parasite or where the parasite replicates sexually
Intermediate host: This is the host which alternates with the definitive host and harbours the larval or asexual stages of a parasite. Some parasites require two intermediate hosts for the completion of their life cycle.
Reservior: It is a host that harbours the parasite and serves as an important source of infection to other susceptible hosts.
Dead end Host: is the host in which the parasite cannot be further transmitted
Vector: A vector is an agent, usually an insect that transmits an infection from one human host to another. They transmit the infective stage of parasites to the other.
PHYLUM NEMATODA
In Tropical Africa, nematodes are of particular importance because they cause great morbidity, mortality and economic loss. Many of them are free living in soil, freshwater and marine habitat, while many others have adapted the parasitic mode of life; parasitizing plants and animals.
Morphology and Anatomy
There are 3 major features which are of profound impact on the form, function and mode of life of nematodes a. Body wall,pseudocoelom. and. hydrostatic skeleton. b. The reproductive system.c. The digestive system
Body wall, pseudocoelom and hydrostatic skeleton a.Body wall: This is made up of a cuticle , hypodermis and longitudinal muscle layer .
Cuticle :This is secreted by the epidermis. It can be smooth or covered with structures such as spines, bristles, warts, papillae, striations etc. The arrangements and positions of these structures are of taxonomic importance. The cuticle covers the entire external surface and also lines the buccal cavity, pharynx, rectum, cloaca, vagina and excretory pore. The cuticle is responsible in part for allowing the invasion of hostile environments such as dry terrestrial soils and the digestive tracts of hosts. This is because it helps to reduce the permeability of the body wall. The terrestrial or parasitic nematodes usually have dense fibrous inner layer of the cuticle while most of the free living marine and freshwater lack this inner layer. As the nematode grows, it sheds its cuticle and grows a new one through a series of 4 moults during its lifetime.
.The Hypodermis This is a layer lying beneath the cuticle which it secretes (or the basal lamina). It has 4 projections which make up the hypodermal cords. The cords project into the pseudocoel. The cords run longitudinally dividing the body wall musculature into 4 quadrats the dorsal and ventral cords carry the longitudinal nerve cords while the 2 lateral cords contain the excretory canals.
Longitudinal muscle layers Next to the hypodermis is a relatively thick muscular layer.The muscle layer is made up of single cells whose cytoplasm is differentiated into a contractile portion with myofibrils and a cytoplasmic or non contractile portion. These cells are arranged in rows and the number of rows per quadrant and the shape and disposition of the contractile portion are of value in separating and recognizing types of nematodes.
The Pseudocoelom
The body cavity is not a true coelom It is a Pseudocoelom containing a fluid under great hydrostatic pressure/ The fluid is the haemolymph which functions in the transport of solutes from one tissue to the other.
Hydrostatic skeleton
With no skeletal structures, nematodes rely on the body fluid in the pseudocoelom which being under great internal pressure serves as a Hydrostatic skeleton against which the muscles of the body wall act to bring about movement.
Other specialized structures of the Body surface
Lips: The mouth is usually surrounded by lips and also array of sensory bristles or papillae. In the primitive, free living nematodes, the number is six but in parasitic species the number varies from 0 to three.
Amphids: These are chemoreceptors found as a pair on each side of the cephalic end. These are mostly found in the free living species but in reduced form on many parasites.
Phasmids: These are sensory organs. Also found in pairs at the terminal end behind the anus.
The reproductive System. The great success of nematodes as parasites is derived mainly from their ability to maintain reproductive efficiency with little morphological specialization.
Male system
The male system is usually single (single testis). It is made up of a long thread like testis whose coils surround the intestine and fills up most of the body cavity not occupied by the intestine. Two types of testes are recognized among nematodes the telogonic type in which the proliferation of germ cells occurs only at the blind end of the elongate testis and the hologonic in which germ cell proliferation occurs along the entire length of the testis. Most species have spicules whose function is to keep the vulva open during copulation so that sperm can be injected into the female vulva against hydrostatic pressure. The sizes and shapes of the spicules vary among species and so are useful taxonomic features.
Female system Most nematodes have 2 ovaries(didelphic). The ovary is thread like and forms many coils around the intestine and reproductive system, filling most of the body cavity. Different terms used to describe the number and arrangement of reproductive system:
1. Monodelphic: where only one ovary and uterus are present. 2. Didelphic-two ovaries and two uteri are present 3. Polydelphic-more than two ovaries and uteri.
Classification
Nematodes parasites can be classified :
1. Based on location of the adult worm in the body 2. Based on mode of infection 3. Systemic (Zoological) classification.
Location of nematode Conjunctiva Mesentery Lungs Subcutaneous Small Large Lymphatic tissue intestine intestine system Loa loa - Strongyloides Loa loa Ascaris Trichuris Wucheraria Dracunculus stercoralis lumbricoides trichiura bancrofti Medinensis Onchocerca volvulus Ancylostoma Entorobius Mansonella duodenale vermicularis ozzardi Necator americanus
Trichinella spiralis
Strongyloides stercoralis
Mode of infection By ingestion By cutaneous By blood sucking penetration insects A.lumbricoides N. americanus Filaria. T. trichiura A. duodenale E. vermicularis S. stercoralis D. medinensis T. spiralis
Systemic (Zoological) classification.
A lot of controversy about classification of nematode into higher taxa. Division into two classes Aphasmidae and Phasmidae based on presence or absence of the caudal sense organs the phasmid is widely accepted. Recently, the phylum has been divided into two classes, Secernentea and Adenophorea depending on the nature of the excretory system. According to Kampfer et al (1998), the Phylum is divided into two classes. Class Adenophorea (=Aphasmida) and Class Secernentea (Phasmida) Class Adenophorea Have cephalic chemoreceptors called amphids Lack caudal chemoreceptors called Phasmids. Excretory system is simple or poorly developed. At times absent. OrderTrichocephalida family Trichinellidae.e.g Trichinella spiralis Family Trichuridae E.g. Trichuris trichiura Class Secernentea Ha ve cephalic chemoreceptors called Amphids, though poorly developed Have caudal chemoreceptors called Phasmids Excretory system complex in some with cuticulized duct and well developed collecting tubules Order Rhabditada . Family Strongyloididae.e.g. Strongyloides stercoralis Order Strongylida family Ancylostomidae.e.g. Ancylostoma duodenale Order Ascaridida family Ascarididae.e.g. Ascaris lumbricoides Order Oxyurida family Oxyuridae.eg. enterobius vermicularis Order Spirurida SuperFamily Filarioidea family Onchocercidae.e.g. Wucheraria bancrofti, Brugia malayi, Onchocerca volvulus, Loa loa, etc
Characteristics, lifecycle, morphology of these nematodes Order Trichocephalida Family: Trichinellidae Species: Trichinella spiralis
Morphology 1. lack of spicules in males, single gonad 2. size it is the smallest human nematode 3. the whiplike characteristic is not marked instead the body merely tapers towards the anterior end 4. females are ovoviviparous
Life cycle Th e life cycle is passed in one host only. However the larvae and adults develop in diff organs of one animal. The adult live in the intestine of man and other suitable host (rat, pig urban cycle, hyenas, leopards, jackals sylvatic cycle in tropics). After copulation. The females burrow into the mucosa and submucosa where being ovoviviparous, produce live larvae (about 1500). The larvae penetrate into the blood stream which distributes them to various organs of the body. They tend to settle in skeletal muscles (eg muscle of diaphragm, jaws, tongues etc). Here they penetrate individual muscle fibres where they grow up to 10 times their original size. During this period, they begin to coil forming the characteristic spiral. Host tissue reactions form a lemon shaped cyst around the larva which is now known as encapsulated larva . Within the cyst, the larva can survive for several months or years and still be viable. However, after about 6 to 18 months, calcification of the cyst and later of the larva themselves set in. Infection occurs when infected muscle is consumed by a suitable host. The larvae are released in the duodenum and develop rapidly, reaching maturity in only 40 hrs. In man, this is the dead end of the infection. T.spiralis however is maintained in the nature by any of these cycles a. pig to pig b.rat to rat c. feral cycle involving wild rodents and various carnivores.
Diagram
Diagnosis Trichinella infection is asymptomatic. Eggs or larvae are not detected in faecal samples. But there are some diagnostic techniques for detecting encapsulated larvae in the muscles: 1. X ray examination: may show the presence of calcified cysts in the muscles 2. Muscle biopsy digesting the biopsy specimen in enzymes and recover the larvae by seb=dimentation in a tube 3. Xenodiagnosis muscle biopsies are fed to lab rats and the presence of infection is confirmed if larvae developed in the muscles of the rat.
Order: Rhabditidae Family: Strongyloididae Species: Strongyloides stercoralis The order has species in which free living and parasitic generations alternate in the life cycle. Morphology 1. The parasitic females are parthenogenetic produce offsprings without being fertilized by the male. The parasitic females are slender and have oesophagus which are almost half the length without an end bulb. 2. The free living females are stout and have a rhabditiform oesophagus with an end bulb 3. Free living males are smaller than females with rhabditiform oesophagus and two spicules
Eggs Oval, transparent, thin shelled Life cycle The life cycle is very complex. 3 diff cycles can be recognized. Man is the only host. The infection is acquired when man walks barefooted on the soil contaminated with faeces. 1. The Homogonic development The parthenogenic females produce eggs which hatch in the mucosa into rhabditiform larvae. These larvae are passed out with the faeces. They develop into filariform larvae in the soil. These are the infective stage gaining entry into man by penetrating the skin and embarking on larval migration (bld stream heart lungs alveoli trachea mouth cavity and swallowed into the stomach). When they get to the intestine, they develop into adulthood. 2. The Heterogonic cycle Here, the rhabditiform larvae in the soil develop into free living sexually mature males and females which mate and produce eggs (in the soil). These eggs hatch into rhabditiform larvae which moult into filariform larva (still in the soil), these can either infect man by penetration (as in the direct cycle above ) or develop into free living adults again. Successive generations of free living adults can be produced in this way, thus increasing the number of larvae available which are capable of infecting man . 3. Auto infection Here, the rhabditiform larva moult into the filariform larva within the intestine and this without passing out of the body, re infects man by penetrating the intestinal mucosa or the perianal area, proceeding on larval migration before coming back to the intestine to grow to maturity.
Diff btw Rhabditiform and filariform larvae Filariform larvae are the skin penetrating infective forms of the parasite. They are longer and more slender than the Rhabditiform larvae. Rhabditiform larvae are developed directly from the gravid females. Rhabditiform larvae have short mouth and double bulb oesophagus while the filariform larvae have short mouth and cylindrical oesophagus. Diagram
Diagnosis Stool examination : diagnosis is by demonstrating Rhabditiform larvae in fresh stool.
Hookworms About 5 families of medical importance. One of the families is family Ancyclostomatidae. Species include Necator americanus( 1st identified in USA thus referred to as America murderer, necator means murderer and Ancyclostoma duodenale w hich are very prevalent. Morphology 1. Hookworms derived their names from the fact that the anterior end of the body is curved dorsally, giving the worms a hook like appearance 2. Males are distinquished from females based on prominent post copulatory bursa diagram
3. Adult worms are small, grayish white or brown, cylindrical worms. N.americanus is generally smaller than A duodenale.
Diffs btw N americanus and A duodenale A. duodenale N. americanus
size Larger and thicker Smaller and thinner Anterior end Bends in the same Bends in the opposite direction as the body direction of the body curvature curvature Copulatory bursa Dorsal ray is single. Total Dorsal ray is split from number of rays is 13 the base. Total number of rays is 14 Vulva opening situated behind the middle Situated in front of the of the body middle of the body
Buccal capsule six teeth, four hook like on Four cutting plates, ventral surface and two two each on ventral knob like on dorsal surface and dorsal surfaces
Diagram
Life cycle Life cycle is direct. Eggs are passed out in faeces. If the eggs are deposited on favourable soil, hatch into the 1 st stage rhabditiform larvae (btw 24 48 hrs). By the 3rd day, the larvae moult to the second stage still rhabditiform larvae which continues to feed on bacteria and organic debris.By the 5 th day, the larvae moult again to produce the 3 rd stage larvae. This is the filariform larva infective stage which does not feed but relies on food reserves in the intestine. They are found on the top 15mm of moist soil.Since they are sensitive to dessication and freezing, they undertake vertical migration up and down in the soil depending on the moisture content, temperature of the soil. Can survive for weeks under ideal condition. Human infection is usually through the skin of the feet, hands, buttocks etc areas in frequent contact with the soil. By mechanical and lytic action, the larvae migrate through the epidermis into the dermis and subcutaneous tissue. From the deeper tissue embark on larval migration into venules and lymphatic vessels where they are carried to the right side of the heart and then to the capillary , lungs, alveoli to the pharynx swallowed down the digestive tract to the small intestine. In the small intestine, the larvae undergo a 3 rd moult to become 4 th stage larvae. These attach to the villi and undergo the final moult to become adults.Male fertilizes females and eggs are produced in about 6 weeks and the cycle is repeated. Diagram
Laboratory diagnosis Demonstration of eggs in faeces.Demonstration of adult worm in stool. Indirect method by blood examination to reveal microcytic, hypochromic anaemia and eosinophilia.
ORDER ASCARIDIDA Family:ASCARIDIDAE Species: Ascaris lumbricoides This is the most widespread and commonest nematode parasite of man in tropical Africa. Ascaris lumbricoides has been recognized in man and Ascaris suum in pigs Morphology : Are large stout nematodes.Females are long, males are smaller with tightly curled posterior end. Diagram
Genital openings vulva of female is located in the anterior third of the body. Male has a cloacal opening at the posterior end, bearing a pair of simple spicules of equal length. Males also have papillae before and after the anus(pre and post) while females have post anal papillae. Mouth surrounded by 3 lips. . Diagram Egg : Egg is oval or rounded,. Fertilized and unfertilized eggs have double shell. Life cycle : Life cycle is direct. Man is the only known definitive host of A.l umbricoides. Unembryonated eggs are laid(not infective). Embryonation takes place outside the body under suitable conditions of temp, .moisture content and oxygen tension of the soil to produce the 1 st stage larva coiled within the egg shell in 9 13 days after passage of the egg. The 1st stage larva moults (1 st moulting) within the egg shell into the 2 nd stage larva (rhabditiform infective larva).3 weeks after the passage of the egg ,Man becomes infected by swallowing an egg containing the 2 nd stage larva (in food, drink or raw vegetables). Hatching takes place in the duodenum where the egg shell is softened by digestive juice in the duodenum. Larval migration This is the most remarkable feature of the life cycle. The larvae penetrate the gut wall and enter the blood circulation and the embark on a migration through the liver to the heart and to the lungs through the pulmonary circulation. They break from the capillaries into the alveoli, then to the trachea, to the pharynx and are swallowed back into the stomach and then to the intestine where they develop into adulthood
Diagnosis :Worm when passed in faeces after treatment Eggs fertilized eggs round or oval in shape, floats in saturated salt solution, unfertilized elliptical, does not float in saturated salt solution
ORDER: OXYURIDA FAMILY:OXYURIDAE SPECIES: Enterobius vermicularis (pin worm) Morphology: 1. Females 8 13 mm, males 2 5 mm in length 2. Mouth is surrounded by 3 lips 3. Muscular end bulb pharynx 4. males curled tail and single spicules 5. Adults live in caecum and large intestine of man.
Life cycle Gravid females migrate down the large intestine through the rectum and lay their eggs in the peri anal region at night. After laying about 10,000 eggs the female dies. The sticky and embryonated eggs become infective within 6 hrs. There are 3 means of transmission 1. When infected children scratch their peri anal region due to itching, the eggs get lodged in the fingers. Infection occurs when the fingers convey the eggs to the mouth. 2. At times, some eggs hatch in the peri anal region and the larvae enter through the anus and migrate up the caecum where they reach maturity. This is known as retroinfection. 3. The eggs which are light can become airborne and disperse. These can be inhaled and subsequently swallowed or can hatch inside the nostril. Also during nose picking, they could be transferred to fingers and later passed into the mouth and swallowed. Diagnosis Eggs cannot be detected in faeces. Best diagnosis is by cellophane tape swab(attachment of a piece of cellophane to peri anal region over night). Then examined for eggs under the microscope.
Order: SPIRURIDA FAMILY:DRACUNCULOIDIDAE SPECIES: Dracunculus medinensis Disease: Dracunculiasis or guinea worm Morphology: lips are absent. Males rarely seen,12 29x0.4mm,4 pairs of pre anal and 6 pairs of post anal papillae, a pair of spicules. Females one of the longest nematodes, 50 100cm with thickness of about 1.0mm. Lifecycle Lifecycle is completed in 2 hosts. A.Definitive hoshostt is man. The worm resides in the sub cutaneous tissue. B. The intermediate host are CyclCyclopsops in which the embryosembryos undergo developmental changes before they become infective to man. Life cycle: When gravid, the female(up to 50 to 100cm long)migrlong)migratesates to the superficialsuperficial subcutaneous tissues, usually in the legs and feet. Here it induces the formatiformationon of a blister which causes intense itching. This blister eventualeventuallyly bursts forming an ulcer. When immersed in cold water the anterior end of the worm is extruded through a hole in the ulcer, releasing thousands of larvae in a milky discharge from its ruptured uterusuterus into the water. When removed from the water, the ruptured part of tthehe worm dries up, sealingsealing off the remaining larvae in the uterus. Subsequent reimmersreimmersionion triggers off a further discharge of larvae, a process which is completed in 2 to 4 weekweeks,s, by which time the empty shell of the worm is ejected. The 1 st stage larvae swim freely in the water and have a leleasease of life of 4to7 days.days. Subsequent development depends on in gestion by intermediate hosts which are various species of freshwater copepods cyclops.. Once inside the stomach of the Cyclops, thethe larva penetrates into the haemocoel where it moults twice to become the 3 rd stage larva which is the infective stage. Man b ecomes infected by drinking water containing CyclopCyclopss harbouring 3rd stage infective larvae. Although the Cyclops are kikilled,lled, the larvae survsurviveive and are activated by the dilute hydrochloric acid in the ststomachomach to escape fromfrom the dead Cyclops and eventually penetrate through the intestinal wall.The worms embembarkark on larval migration from the intestine to the sub cutaneous tissues via the lymphatic systsystem.em. Adult worms emerge after about 4 months, copulation takes place after which the adult mmaleale dies and become encysted and the female migrate towards the extremiextremities(whichties(which are lilikelykely to come in contact with water).They start discharging larvae 1100 to 14 months afteafterr infection Diagnosis -intense itching and burning pain at the site of the blister which is a characteristic of the disease presence of actively moving larvae when blister burstsbursts
ORDER: SPIRURIDA FAMILY:FILARIOIDAE DISEASE:FILARIASIS
WUCHERARIA BANCROFTI Disease Elephantiasis Morphology Adult is long, thread like, creamy white worm. Female Posterior end is finely tuberculated, Male tail end is curled ventrally, contains 2 spicules of unequal length. Males and females remain coiled together. Lifecycle The adult live in the lymph glands and ducts where they form tight coils. The females which are ovoviviparous produce thousands of pre larval stages called microfilariae which are ensheathed in the egg membrane. These microfilariae pass into the blood stream through the thoracic duct. The microfilariae also exhibit nocturnal periodicity in the peripheral blood. They accumulate in the peripheral circulation around midnight (btw 11pm and 2am), but remain in the deep circulation particularly vessels of the lungs during the daytime. The intermediate hosts (vectors) are various species of mosquitoes(Culex, Aedes, Anopheles and Mansonia) female.When a mosquito bites man, it sucks blood containing microfilariae which lose their sheath in the stomach and migrate to the thoracic muscles within few hrs. They undergo the 1 st moult and become the 2 nd stage larvae which is the characteristic sausage shape Diagram
About 2 wks after, the 2 nd moult occurs producing the 3 rd stage (filariform) larva which is the infective stage. The larvae migrate to the proboscis sheath and when the mosquito bites man, the larvae escape into the skin and gain entrance into the body through the bite wound. They migrate through the lymphatic system, finally settling in their definitive site where they become sexually mature in about 9 months. Diagnosis Demonstration of microfilariae in stained thick blood film and unstained film Demonstration of adult worms in biopsy sample
Loa loa (Eye worm) Disease: Loiasis Found in the subcutaneous tissues of man, periodically crossing the eyeball under the conjunctiva and cornea (hence the name eye worm). The migration induces allergic responses resulting in the formation of cutaneous swellings known as “calabar swellings”. Morphology Presence of a small wart like bosses covering the cuticle is characteristics. Males 20 34mm long by 350 430 m in diameter Female 20 70mm long and 425 m in diameter. Microfilariae are sheathed measuring 250 300 m in length. Lifecycle Similar to that of W.bancrofti. The microfilariae also show periodicity, but this time diurnal, accumulating in the peripheral blood in the day and concentrating in the lungs at night. This appears to be an adaptation to the biting habits of the insect vectors which are certain species of the mangrove or deer fly Chrysops C.silacea, C.climidiata which are daytime biters. Mansonia africana however has been suggested to be important vector iinn Nigeria (Ogunba, 1977) Development of larvae in the vectors follow the same pattern as in W.bancrofti , but the 3 rd stage (filariform) larvae develop in the fat body ooff the fly before migratingmmigrigratingating to the mouth parts. Man becomes infected when the fly transmits larvae during feeding (bites man fforor blood meal). Life span in man is appr ox 15yrs. Diagnosis Clinical presence of calabar swellings presence of an adult worm crossing the conjunctiva Demonstration of microfilariae in the blood
ONCHOCERCA VOLVULUS Disease: Onchocerciasis