Helminths

• Nematodes


  • Many are free living, some are parasites of humans, animals and plants
  • Exhibit a high degree of preadaptation which contributes to their success
  • Biochemistry/physiology is highly adapted to survive wide range of conditions, utilize wide range of food sources, and withstand environmental insults
  • An extremely successful group of animals
• Nematodes do NOT undergo asexual reproduction (parthenogenesis has been reported – i.e. development of gametes without fertilisation – but this is rare in parasitic nematodes of animals)
• Nematodes have a life cycle with four larval stages and four moults
• World Health Organisation definition of health: 'Health is the state of complete physical, mental and social well-being and not merely the absence of disease or infirmity'
• Estimated global prevalence and associated morbidity of soil-transmitted helminths
• Disability-adjusted life year (DALY)

  The sum of the years of life lost due to premature mortality
• 20% of world's population harbour at least one species of gastrointestinal (GI) nematode
• Polyparasitism is also common i.e. many people harbour many different species of parasites (GI nematodes and other parasites as well)
• Population dynamics of nematode parasites

  • Most intestinal nematodes do not multiply within an individual host
  • Number of adult worms in a host is due to number of infection events (L3 larvae) - degree of exposure
  • Intensity of infection important – not prevalence
  • In endemic populations number of worms in an individual is OVERDISPERSED
• >70% of worms found in <15% of hosts
• Most heavily infected hosts are at greatest risk of morbidity and are the major source of infective stages
• Overdispersed distribution


  A small number of hosts have a large number of worms, while a large number of hosts have a small number of worms
• Ascaris lumbricoides (direct life cycle)

  1. Egg

  2. L1 larvae

  3. L2 larvae

  4. L3 larvae

  5. L4 larvae

  6. Adult
• Wuchereria bancrofti (vector transmitted)

  1. Egg

  2. L1 larvae

  3. L2 larvae

  4. L3 larvae

  5. L4 larvae

  6. Adult
• Reasons for high prevalence of soil-transmitted helminth infections

  • Widespread distribution of nematodes
  • Resilience of eggs to harsh environmental conditions
  • High number of eggs produced per parasite
  • Poor socioeconomic conditions
  • Lack of education
• Ascariasis


  A disease caused by infection with Ascaris lumbricoides
• Ascaris lumbricoides life cycle

  1. Ingestion of infective eggs

  2. Larvae hatch and invade intestinal mucosa

  3. Larvae carried via portal and systemic circulation to lungs

  4. Larvae mature in lungs and ascend bronchial tree to throat

  5. Larvae swallowed and develop into adult worms in small intestine
• Ascaris lumbricoides has a worldwide distribution, with highest prevalence in tropical and subtropical regions and areas with inadequate sanitation
• Pathology of ascariasis


  • Associated with ingestion and migration of larvae
  • Associated with adult parasites in the intestine
  • Due to "wandering" adults outside of the intestine
• Pathology associated with ingestion and migration of Ascaris larvae


  • Severe haemorrhagic pneumonia can lead to life-threatening respiratory failure
  • More commonly, smaller haemorrhages may lead to breathing difficulties, pneumonia and/or fever
  • Many parasite proteins are highly allergenic, causing eosinophilia and allergic hypersensitivity reactions such as asthma
• Pathology associated with adult Ascaris worms in the intestine


  • Generalised digestive disorders - abdominal discomfort, nausea
  • Contribute to malnutrition in the host, especially if host is undernourished
  • Mass of worms can physically block the intestine, requiring surgical removal
• Pathology associated with "wandering" adult Ascaris worms


  • Can enter bile duct, causing blockage and interference in fat metabolism
  • Can break through appendix or intestinal wall, causing peritonitis
  • Can travel up the intestinal tract and be vomited up or emerge through the nose
  • If they enter the trachea, they may cause suffocation
• Diagnostic methods for ascariasis

  • Coprological - detection of eggs in faeces
  • Serological - detection of antibodies or antigens
  • Molecular - PCR detection of parasite DNA from eggs
  • Image-based diagnostics
• Diagnosis in non-endemic regions depends on thorough investigation of travel history
• Intestinal obstruction


  Mass of worms physically block intestine and need to be surgically removed
• 85% of obstructions occur in children aged 1-5 years
• Adult worms in the intestine

  Can leave small intestine and enter other organs
• Wandering adults outside of the intestine

  • Can block bile duct causing jaundice and fat metabolism issues
  • Can break through appendix or intestinal wall causing peritonitis
  • Can be vomited up or emerge through the nose
  • Can enter trachea causing suffocation
• Filariasis


  Disease caused by infection with nematodes of the family Filaroididea
• Types of filariasis


  • Lymphatic filariasis - worms occupy lymphatic system
  • Subcutaneous filariasis - worms occupy subcutaneous layer of skin
  • Serous cavity filariasis - worms occupy peritoneal, pleural or pericardial cavity
• Filariasis is transmitted by mosquitoes and black flies
• 856 million people in 52 countries live in areas that require preventive chemotherapy to stop the spread of lymphatic filariasis
• Wuchereria bancrofti


  Filarial worm transmitted by mosquito bite that causes a type of filariasis called Elephantiasis
• Elephantiasis


  • Although the parasite damages the lymph system, most infected people are asymptomatic
  • Lymphoedema - improper functioning of lymph system results in swelling
  • Swelling and decreased lymph system function make it difficult to fight infection - bacterial infections cause hardening and thickening of skin
  • Also causes tropical pulmonary eosinophilia syndrome with symptoms of cough, shortness of breath and wheezing
• Consequences of filariasis include social stigma, sub-optimal mental health, loss of income-earning opportunities, increased medical expenses, and immense socioeconomic burdens of isolation and poverty
• Morbidity management and disability prevention


  • Vital for improving public health
  • Surgery can alleviate most cases of hydrocele
  • Clinical severity and disease progression can be reduced with simple hygiene measures, skin care, exercises and elevation of affected limbs
  • People with lymphoedema must have access to continuing care throughout their lives
• Prevention & Control of lymphatic filariasis

  1. Avoid mosquito bites by sleeping under a net, wearing long clothing, and using repellent

  2. Annual mass drug treatment of entire communities and control of mosquitoes to reduce microfilariae in blood and diminish transmission
• Mebendazole


  Anti-nematode drug that binds to β-tubulin and inhibits microtubule assembly, impairing glucose uptake and reducing energy production, causing paralysis and death of the parasite
• Preventing infection with soil-transmitted helminths

  1. Periodic administration of anti-helminthic drugs to eliminate infecting worms

  2. Health and hygiene education to prevent re-infection

  3. Improved sanitation to reduce soil contamination with infective eggs
• WHO global target - eliminate morbidity due to soil transmitted helminthiases in children by 2030