Medical helminthology

Introduction to

MEDICAL HELMINTHOLOGY

Medical helminthology is a branch of Parasitology that applies for the study of a group of a vastly numerous and different worms that are parasitic to humans and animals. They are grouped in two main phyla which are divided as usual to classes, subclasses, families, genera and species.

1- Phylum Platyhelminths, which includes two medically important classes; Trematoda and Cestoda.

2- Phylum Nemathelminths, which includes the medically important class Nematoda.

INTRODUCTION TO PHYLUM PLATYHELMINTHS

General characters;

They are bilaterally symmetrical.

They have a definite ‘head end’, with associated sensory and motor elements.

All are dorsoventrally flattened and have no body cavity.

They are leaf shaped or oval, but some are elongate such as tapeworms.

Their size varies from nearly microscopic to over 5 meters in length.

The body covering (tegument) varies in structure among classes.

Locomotion occurs by muscle fibers which are arranged in one or two layers near the body surface.

The nervous system is simple, of ‘‘ladder type’’ and there are also tactile cells, chemoreceptors and eyespots.

The digestive system is started with the mouth opening and ended by the intestinal coeca.

 The osmoregulatory and excretory system is based on the protonephridium  (flame cell). It is a single cell with a tuft of cilia that extend to a delicate tubule. This is joined to other tubules that form excretory channels opening in one or more excretory pores.

The reproductive system follows a main pattern but has vast variation amongst different groups. Almost all are hermaphroditic. Self-fertilization is common though cross fertilization can happen also.

Platyhelminths are divided into 4 classes, 2 of them have medical importance which are:

Class Trematoda, these are mainly parasites of the digestive tract of all vertebrates. They need a mollusc intermediate host, and maybe a second vertebrate intermediate host.

Class Cestoidea, they are parasites of almost all classes of vertebrates. They always need an intermediate host.

CLASS TREMATODA

GENERAL CHARACTERS

Parasites of all classes of vertebrates.

In many cases they need more than one intermediate host to complete their life cycle.

Have special different developmental stages in the intermediate host where they reproduce asexually.

Body is dorsoventrally flattened and oval in shape, some are filiform.

There is a powerful oral sucker surrounding the oral opening, most have another ventral sucker (acetabulum) and some have a genital sucker as well.

The digestive system started with the mouth opening which is surrounded by the oral suckers,then the oesophygus which is surrounded by the muscular pharynx. The oesophygus is devided just above the ventral sucker into two simple intestinal coeca.

Muscular and nervous system:

Musculature is mainly a subtegumental or suprficial muscle layers. The nervous system consists of two anterior ganglia connected by a commissure.

 The excretory system is based on the flame cell .Their ductules collect in larger collecting ducts. These on each side end into excretory bladder in the posterior end of the trematode.

Reproductive system is composed of both male and female genital systems in the same worm (hermaphrodite).

 1-The male genital system is formed of two testes; each gives out a vas eferens. The two ducts join to form one vas deferens that dilates distally to form a seminal vesicle. The seminal vesicle is surrounded by prostatic glandular cells, and opens in the cirrus, which is the male copulatory organ. The cirrus opens through the male genital orifice that lies besides the female genital orifice in the genital atrium.

 2-The female genital system is formed of a single ovary which is usually round or oval, but may be lobated or even branched. The ovary gives out a short oviduct has a seminal   receptacle. At the base of seminal receptacle there is slender tube called Laurer’s canal which ends either blindly in the parenchyma or opens through the tegument and is considered vestigial vagina. The oviduct is joined by the vitelline duct which provides the ovum with yolk cells coming from the vitelline glands. Vitelline glands are numerous, and give out vitelline ductules which collect to form one duct on each side, these join to form a central common duct which joins the oviduct. After junction with the vitelline duct the oviduct expands to form the ootype which is surrounded by numerous monocellular Mehli’s glands that produce the shell material of the egg in the ootype. The ootype opens into the uterus where the eggs are completely formed in its proximal part and are sent distally to be expelled through the female genital pore in the genital atrium.

Development:

At least two hosts share in the life cycle of trematodes. One is a vertebrate where sexual reproduction takes place, and the other is a mollusc (snail) where asexual reproduction occurs.

Egg: has an operculum at one end (except schistosomes) through which the first larval stage escapes when it reaches water.

The miracidium: (1st. larval stage) is a tiny, ciliated pyriform organism. It has nerve endings and penetration glands which secrete histeolytic enzymes. There is a variety of sensory organs and endings in the miracidium like chemoreceptors, photoreceptors (eye spots), thermo- and statoreceptors. Miracidia survive only for few hours so they rapidly seek their snail host and penetrate its soft body attracted by its mucus secretions.

Sporocyst; is the 2nd. larval stage that forms inside the snail. The embryonic cells inside the sporocyst may develop into another sporocyst generation (daughter sporocyst) or into the next stages – redia or cercaria.

The redia: (3rd. larval stage) feed on snail tissue and grow in size. Their embryonic cells develop into either daughter rediae or cercariae .

Cercariae: represent the juvenile stage of the vertebrate-inhabiting adult.

Some have a stylet in the anterior margin of the oral sucker (xiphidocercaria).

 Some cercariae have cystogenous glands which form a cyst wall around the cercaria which loses its tail and becomes an encysted metacrcaria.

 According to the tail shape cercaria are either:

Leptocercous, with simple tail.

Lophocercous (cercaria ornatatae), with membranous tail.

Furcocercous, with forked tail.

Microcercous with small knob-like tail.

Cercariaium without a tail.

Metacercaria: It is a quiescent stage between cercaria and adult.

Juvenile stage: This begins when the definitive host is reached either by penetration (schistosomes) or after being swallowed then excystation occurs.

CLASS CESTOIDEA

SUBCLASS CESTODA

These are called the tapeworms (cestos = tape). Sexually mature forms live in the intestine of all classes of vertebrates.

General characters:

• As platyhelmitnths, they are dorsoventrally flattened, and have no body cavity.
• They are ribbon-like, and of variable lengths, ranging from few mms. up to several meters.
• The body is formed of three main parts:

1. The scolex (head) is the anterior end of the worm and usually it is equipped with fixation organs to maintain its position in the gut. The organs of fixation are in the form of suckers (acetabula), grooves or bothria, and protrusible leaf-like organs (bothridia). Scolices having acetabula may carry hooks for anchoring them to the gut. Hooks are arranged in one or more circles anterior to suckers on a muscular protrusible dome-shaped area at the apex of scolex called rostellum. Scolex has the chief neural ganglia of the worm, and it bears numerous sensory endings.
2.  The neck: it lies behind the scolex, usually thinner in size. It is an undifferentiated zone containing the stem or germinal cells that give rise to the following part of the body.
3. The strobila: it is a unique structure for cestodes. It is a linear series of sets of reproductive organs; each set is called proglottis or proglottid. Immature recently formed proglottids are near the neck and they are more mature when we go posteriorly. Mature proglottids carry reproductive organs of both sexes. When the uterus becomes full of eggs, which cannot pass outside, it compresses other organs causing their disappearance and the gravid proglottid is formed. There are constrictions between proglottids, but there is no true segmentation because the tegument, muscle, and some organs are continuous between them.

• Alimentary canal is absent. Cestodes get the nutrients through active transport of readily digested food.
• The tissues of most cestodes contain cells that form calcareous corpuscles. These are highly refractile bodies of 12-32 μm. in diameter
•      and are formed of inorganic elements.
• The muscular system in the cestodes is composed of contractile longitudinal and circular bundles in the tegument. If any part of the strobila is cut, it can move spontaneously and independently, so groups of gravid proglottids can pass independently from the anus.
• The nervous system is centered in the scolex.
• The excretory or osmoregulatory system is composed of main canals that run longitudinally from the scolex down to the posterior end, usually in two lateral pairs one ventral and anotherdorsal.  
•  

• The reproductive system of cestodes is represented in each proglottid. The male system is composed of many testes distributed dorsally and give out vasa efferentia collecting in semineferous tubule, seminal vesicle and cirrus ending in the genital pore with the vagina. The female system is formed of an ovary, tubular uterus connected to a vagina opening in the genital pore, vitelline glands differ in shape according to the species, and open with the shell glands into the ootype between the ovary and uterus.
• The development of cestodes differs according to the orders. In common, they are heterozygous, having a definitive host in which they live in the intestine, and an intermediate host that harbors the larval stages in its tissues. Table showing the comparison between the two orders:
•  
• There are two orders of medical importance that infect man and domestic animals; these are Pseudophyllidea and Cyclophyllidea.

PHYLUM NEMATODA

General Characters:

Bilaterally symmetrical.

Unsegmented body.

Elongate, tapered at both ends.

Cylindrical, round in cross sections (roundworms).

Vary markedly in their size from less than 1mm up to more than one meter.

Include parasitic as well as free living worms.

Usually have separate sexes. Males are smaller than females and commonly have ventrally curved posterior end.

Have a body cavity (pseudocoel), containing fluid within which lie the digestive and reproductive systems.

The digestive system: a simple tube extending from the mouth to the anus.

The mouth opens at the anterior end and may have lips, papillae, plates, a buccal capsule, or provided with teeth.

The oesophagus follows the mouth. It is triradiate in cross section, lined with cuticle and is provided with three oesophageal glands.

It may acquire the different shapes:

1- Club-shaped.

2- Double bulbed: an anterior club-shaped part and a posterior spherical part).

3- Rhabditiform: an anterior cylinderical and a posterior pyriform parts separated by a short constricted isthmus).

4- Cylindrical (filariform oesophagus): a muscular anterior part and a glandular posterior part.

5- Cellular: a row of cells (stichocytes) attached to the oesophageal tube.

The intestine follows and ends by the anus.

The anus opens on the ventral surface close to the posterior end in the female and joins the genital duct to open in a cloaca in the male.

Parasitic nematodes feed on blood, tissue cells and fluids, intestinal contents, or some combination of these.

The excretory system consists of lateral canals, ventral glands or both, which open near the anterior end through a ventral excretory pore.

The genital system:

The male system is composed of:

A long convoluted tube consisting of the testis, vas deferens, seminal vesicle and the ejaculatory duct, which opens in the cloaca.

The accessory copulatory organs as: spicules, gubernaculum, sement glands, papillae and bursa.

The female system consists of one, or two convoluted tubes, each differentiated into the ovary, oviduct, seminal receptacle, uterus and ovijector that join the other one (in case of doubled systems) forming a vagina which opens into the vulva.

Classifications of phylum Nematoda:

According to location in the definitive host:

Nematodes infecting human beings may be divided into intestinal and tissue nematodes:

   * Intestinal nematodes: They include:

  [1] Nematodes that have a direct life cycle (mouth to intestine):

                      1- Enterobius vermicularis.

           2- Trichiuris trichiura

           3- Capillaria spp.

           4- Trichinella spiralis

[2] Nematodes that have a migratory cycle through the lung, they include: -

1. a) Nematodes that pass through the mouth to the lungs then to the intestine:

           5- Ascaris lumbricoides.

1. b) Nematodes that pass through the skin to the lungs then to the intestine:

          6- Ancylostoma duodenale and Necator americanus.

          7- Strongyloides stercoralis

* Tissue nematodes:

       They include:

                    8- Larva migrans.

                    9- Dracunculus medinensis.

                  10- Filariae.

II-  Based on Whether they Lay Eggs or Larvae

1. Oviparous -Laying eggs:

Ascaris. Trichuris.

2. Viviparous—Producing larvae:

Trichinella, Wuchereria, Brugia, Dracunculus.

3. Ovoviviparous-Laying eggs containing fully formed larvae which hatch out immediately: Strongyloides.

IV-According to the presence of special caudal chemical receptors called phasmids into:

Phasmidial : all have phasmids except:

Aphasmidial nematodes Family Trichiuridae as: Trichiuris trichiura , Capillaria spp. and Trichinella spiralis)