MICROBIO 4.7 - HELMINTHS/WORMS (GI NEMATODES & CESTODES)

GI Nematodes and Cestodes

• Gastrointestinal nematodes and cestodes are the focus

Why Should You Care?

• Endemic nematodes exist worldwide

Class Objectives

• Compare and contrast nematodes and cestodes infecting the GI tract
• Describe the basic life cycle of the nematodes & cestodes
• Identify the route of transmission of GI helminths
• Classify parasites, the diseases they cause, and their presentation
• Identify when to prescribe anti-parasitic (protozoan and helminth) drugs

The Helminths

• Helminths include Nematodes, Trematodes, and Cestodes

Gastrointestinal Helminths Overview

• Pathogens include nematodes (roundworms) and cestodes (tapeworms).
• Encounter occurs via poor handling of human waste, leading to fecal helminth eggs contaminating soil, foodstuffs, and animal feeds.
• Nematodes enter via ingestion of eggs, such as Ascaris, Trichuris, or Enterobius or through direct skin penetration by soil larvae, such as Strongyloides or hookworm.
• Cestodes enter via ingestion of encysted worm larvae in undercooked beef, pork, or fish.
• Most adult intestinal helminths remain in the intestinal tract and do not replicate within the host; worm burden increases only with re-exposure to infectious eggs or larvae.
• Sustained autoinfection in strongyloidiasis may develop into Strongyloides hyperinfection, leading to diarrhea, pneumonitis, rash, and eosinophilia.
• Damage caused is usually asymptomatic, but a large worm burden can lead to intestinal obstruction from Ascaris, rectal prolapse from Trichuris, anal itching from Enterobius, and iron deficiency anemia from hookworm.
• Fish tapeworms steal B vitamins causing anemia
• Diagnosis involves identifying the eggs, larvae, or adult worms (or segments) in feces.
• Treatment includes albendazole, ivermectin, pyrantel pamoate, and praziquantel.
• Prevention involves sanitary disposal of human waste for nematodes and avoiding undercooked meat and fish for tapeworms.

Nematodes vs Cestodes

• Helminths are worms, multicellular and can be large in human hosts, potentially causing mechanical damage and obstruction.
• Most adult helminths are anaerobic.
• Reproduction in helminths may be hermaphroditic or involve separate male and female sexes.
• A muscular body enables motility.

Nemathelminthes - Nematodes

• Nematodes are "roundworms" (nemo = "thread").
• Nematodes are round and coiled with a definitive mouth and anus.
• The outer covering of nematodes is a cuticle.
• Trichinella spiralis is an example of a nematode.

Platyhelminthes - Cestodes

• Cestodes are "flatworms" or "tapeworms".
• Cestodes are segmented and covered in a tough tegument.
• They have two suckers on one end and are hermaphroditic.
• Echinococcus is an example of a cestode.

Summary of the GI Nematodes

• Healthy individuals often experience no or mild symptoms when the parasite load is low.
• Species are categorized by whether they remain in the GI tract or leave during their larval stage and how they enter the human host, sometimes there is overlap.

Intestinal Nematodes - Remain in GI Tract

• Trichuris trichiura: "whipworm"
• Enterobius vermicularis: "pinworm"

Intestinal Nematodes - Exits GI Tract

• Parasites migrate to the lungs, grow within alveoli, and cause peritonitis and eosinophilia.
• Hosts cough up and swallow parasites, which then travel to the GI tract to lay eggs.
  • Ascaris lumbricoides: "ringworm"
  • Strongyloides stercoralis: "threadworm"
  • Necator americanus: "New World hookworm"
  • Ancylostoma duodenale: "Old World hookworm"

Nematode Pathway to Infection

• Nematodes are introduced by ingesting eggs from contaminated food, water, or hands, or larvae may penetrate the skin.
• Nematodes reach the stomach, and adult forms lay eggs.
• Eggs are excreted in feces, contaminating the soil and aiding spread to new hosts.

Remain in GI Tract - Whipworm

• Trichuris trichiura also known as "Whipworm" (Trick-er-us trick-er-uh).
• Transmission occurs when hosts ingest eggs.
• Clinical presentation includes high worm burden with pain, abdomen distention, bloody diarrhea, malnutrition, anemia, and rectal prolapse, especially in children.
• Treatment involves benzimidazoles and ivermectin.

Remain in GI Tract - Pinworm

• Enterobius vermicularis is known as "Pinworm" (En-terr-o-bee-us ver-mih-q-lair-ihs).
• Transmission: The host ingests the eggs.
• Adult females travel to the perianal region and lay eggs in the skin folds, usually at night.
• When individuals scratch the area, they pick up eggs on their hands and transfer them to others via direct contact or fomites.
• Autoinfection: eggs hatch; new larvae travel to the rectum.
• Clinical presentation includes perianal itching (pruritus).
• Treatment: Benzimidazoles, pyrantel pamoate, and ivermectin.

Exits GI Tract - Ringworm

• Ascaris lumbricoides is also known as “Ringworm” (ass-cuh-rihs lum-brih-coy-dees).
• Common helminth infection, the largest roundworm parasitizing humans, measuring 15-30 cm long.
• Transmission: Hosts ingest eggs.
• Clinical presentation: malnutrition and stunted growth in children from obstruction of the gastrointestinal tract by adult worms; complications from invasion of the bile duct, liver, gallbladder, or appendix.
• Treatment: Benzimidazoles, pyrantel pamoate, and ivermectin.

Exits GI Tract - Threadworm

• Strongyloides stercoralis (Stron-jih-loy-dees stir-core-ah-liss).
• Transmission: Skin penetration.
• Eggs hatch within the host's GI tract.
• Larvae, not eggs, are excreted in feces (unique).
• Clinical presentation mimics peptic ulcers with inflammation, epigastric pain, vomiting, and diarrhea.
• Treatment: Benzimidizoles and ivermectin.

Exits GI Tract - Old & New World Hookworms

• Necator americanus is “New World” (nih-cate-or amer-ih-can-us).
• Transmitted through skin penetration.
• Adult worms have hook-like mouth parts that latch onto the intestinal mucosa and suck blood.
• Clinical presentation: anemia, GI problems, developmental impairment.
• Secondary bacterial infections due to hook wounds.
• Treatment: Benzimidizoles and pyrantel pamoate.
• Ancylostoma is "Old World" (ang-kill-ah-stow-ma).
• Transmitted by penetration of the skin, or ingestion.
• Clinical presentation: cutaneous larva migrans - "creeping eruptions" under the skin where the worms migrate subQ.
• Treatment: Benzimidizoles.

Cestodes

• Cestodes, also known as tapeworms, are hermaphroditic, containing both male and female reproductive organs.
• The body is called the strobila, comprising multiple segments known as proglottids.
• Gravid proglottids at the distal end house uteri with eggs.
• They have no digestive systems; they absorb nutrients through their body walls.
• The scolex has suckers and a rostellum with hooklets for attaching to the host.
• Cestode infections are often asymptomatic when infected with adult forms.
• Some experience abdominal pain, diarrhea, and other GI symptoms.
• Diagnosis is confirmed by finding eggs or proglottids in the stool.
• When humans host larval stages of some cestodes forms, it results in more serious infection that includes damaging cysts.

Cestode Life Cycle

• Cestodes can have complex life cycles with multiple intermediate hosts.
• Larvae exit the eggs and are ingested by intermediate hosts (insects/fish).
• Larvae undergo transformations that produce infective forms.
• Humans ingest infective larvae, usually by eating an intermediate host.
• They mature to adult form to produce gravid proglottids in the small intestine.
• Proglottids release eggs, which are passed to the environment in feces.

Cestodes - Tapeworms

• Taenia solium (pork tapeworm) and Taenia saginata (beef tapeworm) (tee-knee-uh).
• Transmission: Pigs or cows ingest vegetation contaminated with the parasites' eggs; humans eat the infected meat.
• Clinical presentation:
  • Taeniasis is mild or asymptomatic; high worm burden leads to GI problems.
  • Cysticercosis occurs when humans ingest eggs in food or water contaminated by human feces; eggs become larvae that form cysts.
  • Neurocysticercosis involves larvae forming cysts and calcified lesions in the CNS
• Significant cause of epilepsy worldwide
• Neurological manifestations depend on location and size of lesions.
• Treatment: Benzimidazole and Praziquantel, or Niclosamide.

Cestodes - Broad Tapeworm

• Diphyllobothrium latum is a broad or fish tapeworm.
• The largest human tapeworm, up to 10 meters long.
• Transmission is by eating undercooked infected fish.
• The life cycle includes two intermediate hosts: early larval forms eaten by microcrustaceans, which are then eaten by fish.
• Clinical presentation: serious infection can cause Vitamin B12 deficiency, pernicious anemia, and intestinal obstruction.
• Treatment: Praziquantel, alternative Niclosamide

Cestodes - Hydatid Tapeworm

• Echinococcus granulosus is a hydatid tapeworm.
• Transmission: Humans are accidental hosts when they ingest the eggs passed from canine feces.
• Clinical presentation: large, complex cysts form in human tissues. Cysts can rupture and cause anaphylactic shock and disseminate the parasites.
• Treatment: Benzimidazoles and surgical removal of cysts, or Niclosamide.