Medically Important Parasites: Helminths

Medically Important Parasites

• Parasites of medical importance can be classified as helminths (worms), protozoa (unicellular eukaryotes), or arthropods (invertebrates with articulated legs).

General Characteristics of Helminths

• Helminth means worm.
• Helminths are classified into two phyla: Platyhelminthes and Nemathelminthes.
• Platyhelminthes are flattened worms categorized into two classes: Trematoda (non-segmented flukes) and Cestoda (segmented tapeworms).
• Nemathelminthes are cylindrical worms.

Helminth Characteristics Compared

Trematoda Characteristics

• Trematoda are flattened and unsegmented, except for female Schistosomes, which are cylindrical.
• They lack a body cavity, and their organs are embedded in connective tissue.
• The digestive system is simple without an anus; nutrients are absorbed from the surroundings, and food remnants are vomited through the mouth.
• Trematodes possess organs of fixation in the form of oral and/or ventral suckers; a third genital sucker may also be present.
• They are hermaphroditic (monoecious), except for Schistosomes, which are unisexual (diecious).
• The life cycle of all trematodes requires a snail as an intermediate host, and a part of their life cycle occurs in water.

Digestive System of Trematodes

• The digestive system begins with a mouth opening surrounded by an oral sucker, leading to an oesophagus that is either completely or partially surrounded by a pharynx.
• From the oesophagus, the intestine bifurcates into two caeca that end blindly at the posterior end.
• Lacking an anus, trematodes absorb nutrients from their surroundings, and food remnants are expelled through the mouth.

Reproductive System of Trematodes

• The female reproductive system starts with an ovary that varies in shape and arrangement among different trematodes and it leads to an oviduct, ootype then uterus.
• The common genital pore, situated near the ventral sucker, is where the uterus opens.
• The ootype receives the vitelline duct and is surrounded by the shell (or Mehlis') gland, which secretes the eggshell.
• The male reproductive organs feature two testes, which vary in shape and arrangement and leads to two vas efferens.
• The two vasa join to form the vas deferens.
• The vas deferens leads to a seminal vesicle that may end in a muscular cirrus and opens into the common genital atrium anterior to the female opening.
• Testes and ovaries can be either ovate or branched and located beside each other or in front of each other.

Excretory and Nervous Systems of Trematodes

• Flame cells are distributed throughout the body.
• Excretory ducts merge, forming two main collecting tubules that join to form a bladder, opening posteriorly via an excretory pore.
• Nerve ganglia surround the pharynx, sending fibers to all organs.

Life Cycle of Trematodes

• Trematodes are heteroxenous parasites, needing at least two hosts to complete their lifecycle (indirect life cycle).
• A snail serves as the first intermediate host, and water is required for part of the life cycle.

Developmental Stages of Trematodes

• All trematode eggs are operculated, except for those of Schistosomes.

Egg Stage

• Eggs may be laid as immature (with a fertilized ovum) or mature (with a fully formed miracidium).
• Mature eggs hatch immediately to release the miracidium, while immature eggs need 2–3 weeks for maturation.
• Schistosome eggs are spinated.

Miracidium Stage

• A ciliated, conical organism emerges from the hatched egg that swims for a few hours (up to 24 hours) in water.
• It has a primitive gut, some penetration glands, two pairs of flame cells, light-sensitive eye spots, and some germ cells.
• In heterophyids, the miracidium hatches from the egg inside the snail intermediate host.

Sporocyst Stage

• Inside the snail intermediate host, the miracidium travels to the liver, loses its cilia, and transforms into a sporocyst.
• This sac-like structure contains germ cells that gradually develop into daughter sporocysts or rediae.

Redia Stage

• The redia is a sac-like structure with a mouth, pharynx, primitive gut, and birth pore.
• Germ cells inside the rediae proliferate and develop into daughter rediae or cercariae.
• Schistosomes do not have a redia stage in their life cycle.

Cercaria Stage

• The cercaria consists of a body and a tail.
• The body resembles the adult worm with oral and ventral suckers, a rudimentary gut, and an excretory system.
• Cercariae possess glands adapted for penetration or encystation, depending on the cercaria type.
• Cercariae find their way to outside water where they may encyst on vegetation for ingestion by the definitive host, penetrate a second intermediate host (fish and crabs), or penetrate the skin of the definitive host directly.
• Types of Cercariae includes:
  • Leptocercus cercaria: with a simple tail
  • Lophocercus cercaria: tail is surrounded by a membrane
  • Furcocercus cercaria: with forked tail
  • Microcercus cercaria: tail is a knob-like structure

Encysted Metacercaria Stage

• Once cercariae leave the snail and enter water, they may encyst on vegetation for ingestion by the definitive host or penetrate a second intermediate host (fish and crabs) to encyst in muscles.
• Metacercaria is the infective stage of most trematodes.