Helminths and Platyhelminthes

Helminths

• These are parasitic worms that can infect humans and animals
• They are not always parasitic; some are facultative
• Three common types of helminths:
  • Nematodes (roundworms)
  • Cestodes (tapeworms)
  • Trematodes (flukes)

Platyhelminthes

• There are three body layers: ectoderm, mesoderm, and endoderm
• They are acoelomate, meaning they do not have a body cavity and occasionally observed
• The main body structure is acoelomate and dorso-ventrally flattened
• Exhibit Bilateral symmetry and do not have an anus
• The mesoderm develops into parenchyma
• The organs (reproductive, nerves, muscular, excretory, etc.) are embedded within the parenchyma
• Their organ system is incomplete when embedded
• They do not have specialized respiratory organs
• Gas exchange is by diffusion through outer body layer
• They have specialized excretory 'organs' known as flame cells
• These cells link into a tubular system to remove liquid nitrogenous waste
• Most members of the phylum are hermaphrodite

Cestodes

• Adults only inhabit one type of habitat, such as gut or derivatives like the bile or pancreatic duct of a vertebrae
• All adult forms are endoparasites that inhabit the gut or gut derivative of a vertebrae
• Lacking a ciliated epidermis and alimentary canal (gut)
• The adult form is divided into proglottids (segments)
• The anterior 'head' end is known as the scolex, with well-developed adhesive organs
• Mostly hermaphroditic
• They inhabit the gut and most are endoparasites

Trematodes

• These are commonly known as flukes and have two main orders:
  • Digenea
  • Monogenea
• They have integuments, which cover the area of nutrient absorption
• The body is undivided
• It has a mouth opening into a pharynx, esophagus, and the gut with a bi-lobed blind-ending diverticula (alimentary canal)
• Most species are hermaphrodites except for the Schistosomatidae species
• They have well-developed adhesive organs

Nemathelminthes

• Nematodes are commonly known as roundworms
• They have three body layers with a body cavity known as pseudocoelom
• The body cavity is fluid-filled, which forms a hydrostatic skeleton
• A cylindrical body shape with a non-ciliated outer layer known as cuticle
• Sexes are separate
• The gonads are tubular. Their ducts form a continuous structure
• Exhibits worldwide distribution and occupies terrestrial and aquatic habits

Nematodes

• Most numerous animals on earth
  • Can be 90,000 in 1 rotting apple
  • Can be 1074 in 6.7 mL of coastal mud
  • Estimated 9 billion per acre in good farmland
• Parasitic infections can be enormous
  • Example: T. spiralis is a blood/tissue nematode
• White worms are not segmented
• They are covered with a secreted cuticle from the hypodermis that grows as the worm grows
• Sexes are separate and find each other with pheromones
• Males lack flagellum
  • Move by pseudopodia
  • Pseudopodia is the mechanism of movement

Other Characteristics of Nematodes

• Males are usually smaller than females. They are equipped with copulatory organs that often show features specific to each species
• Development from the egg includes four larval stages and four molting before the adult stage is reached
• Certain species need an intermediate host to complete development
• Females are larger than males
  • Curved posterior end
• Equipped with chemoreceptors:
  • Cephalic or amphids (head end)
  • Caudal or phasmids (tail end)
    • Chemoreceptors are the gateway to enter the system
    • Found in the head
• Certain species are equipped with copulatory bursae, hooks, teeth, plates, and papillae
  • Necatur americanus uses teeth

Intestinal Nematodes

• They maintain their position in their habitats by:
  • Oral attachment (hookworm) -Also ingests blood cells
  • Anchorage with their attenuated ends (Trichuris)
  • Penetration to the tissues (Strongyloides)
  • Retention on the folds of the mucosa (Ascaris)
• In the small intestine:
  • Ascaris lumbricoides (large roundworm)
  • Hookworms
    • Ancylostoma duodenale
    • Necator americanus
  • Strongyloides stercoralis (dwarf threadworm)
    • More fatal
  • Capillaria philippinensis
• In the large intestine:
  • Trichuris trichiura (whipworm)
    • ¾ of the body is not burrowed
  • Enterobius vermicularis (pinworm)
    • Could easily contaminate

Methods of Obtaining Food as Nematodes

• Sucking and ingestion of blood (hookworm)
• Ingestion of lyzed tissues
• Feeding on intestinal content
• Ingestion of nourishment from body fluids

Ascariasis

• Depends on the number of worms
• Worms subsist on liquid content of small intestines
  • Do not suck blood or graze on mucosa
• Asymptomatic in Small to medium infections
  • May cause “sensitization phenomenon”
    • Allergic reaction to worm waste
    • Rashes, eye pain, asthma, insomnia, restlessness
• Wandering adults are dangerous
  • Overcrowding can lead to wandering
    • Upstream to pancreatic or bile ducts which may cause liver damage
    • If reaches stomach, vomiting of worm
    • Downstream to appendix or out anus
• Females like to crawl through small spaces and look for curly tail of male
  • She may crawl through the nose, an ear, or another opening; aspiration of worm can cause death
• Heavy infections can block intestines and are sometimes fatal
  • Can cause preformation
• Juvenile migrates through the lungs, causing Ascaris pneumonitis
  • Fatal in heavy infections
• Penetration of intestine or appendix may occur and is frequently fatal

Diagnosis and Treatment of Ascariasis

• Eggs in feces and juveniles in sputum can be detected
• The eggs are difficult to identify to species
• Dead adults may be found in feces
• Mebendazole will kill adults but not the migrating larvae
  • May need to repeat treatment
• Dead adults usually pass out through the anus

Ascorides

• The largest of the nematode parasites
  • Some are foot long
• They are stout, big worms, with a mouth surrounded by three lips
• Most are typically intestinal parasites and are usually heavy infections
  • Buccal cavity contains no teeth

Ascaris Lumbricoides

• Humans are the only definitive host
• The intermediate host is none
• Located in the small intestine
• Geographic distribution is cosmopolitan (25% of the world population is infected)
  • Known as a human parasite for over 2000 years

Transmission of Ascariasis

• Ingestion of eggs in contaminated food or water
  • Usage of night soil on crops increases transmission
• Needs to require shade and mild temperatures
• Eggs are very resistant and can embryonate in very strong chemicals
  • Some examples are 2 percent Formalin, potassium dichromate, and 50 percent hydrochloric, nitric, acetic, and sulfuric acid
• Very long life
  • At least 10 years
• Eggs can splash up onto vegetables and can be picked up and transported by cockroaches
• Wind borne can carry the eggs if trapped on mucus membranes, then swallowed
• Has been found on German bank notes

Prevention of Ascariasis

• Practicing proper sanitation and personal hygiene
• Following adequate health education
• Avoiding the usage of human feces as fertilizer

Trichuris Trichiura (Whipworm)

• Humans are normally the definitive host
  • Dogs are a possible definitive host
• There is no intermediate host
  • Pigs and chickens are transport hosts
• Flies will transport eggs on legs
• Location: large intestine from caecum and appendix to rectum
  • Burrows head into mucosa
• Geographical distribution is cosmopolitan
  • Warm climate
  • High rainfall
  • Has unsanitary conditions and high prevalence
• Geographical distribution is cosmopolitan
• Females may lay 3,000 to 20,000 eggs a day for many years
• Transmission is through ingestion of embryonated eggs in usually contaminated food
• Requires high humid, warm climate and shade to develop properly
• Needs soil for embryonization

Pathology and Symptoms of Trichuris Trichiura

• Low-level infections (<100 worms) are asymptomatic
• In large infections, can result in and diarrhea, bloody stool, abdominal pain and rectal prolapse
• Chronic infections in children can lead to growth retardation and finger and toe clubbing
• Often associated with an Ascaris lumbricoides infections
  • Mode of transmission is typically the same

Diagnosis and Treatment of Trichuris Trichiura

• Bipolar eggs in feces.
• Colonoscopy can uncover worm infections.
• Mebendazole or albendazole
• Rectal prolapse treated with surgery
  • Deworming is twice a year