By Kim Harbal and Marcus Williamson

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Strongyloides Stercoralis

• By Kim Harbal and Marcus Williamson

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Taxonomy

• CLASS SECERNENTEA
• SUBCLASS RHABDITIA
• ORDER RHABDITIDA
• SUBORDER RHABDITINA
• SUPERFAMILY RHABDITOIDEA
• FAMILY STRONGYLOIDIDAE

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Where is this Parasite Found?

• Tropical and Sub-tropcial regions
• Temperate regions
• Rural areas, institutional settings, and in lower
  socioeconomic groups
• Mostly found in warm moist areas that favor the
  survival of the juvenile stages

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Geographic Distribution

• Currently, an estimated 100-200 million people
  are infected worldwide in 70 countries
• In the US Appalachians, especially in eastern
  Tennessee, Kentucky, and West Virginia.
• Internationally Sub-Saharan Africa, South and
  Southeast Asia, Central America, and South
  America, and parts of Eastern Europe

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Question 1

• Name a state that has Strongyloides stercoralis?

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Geographic Distribution
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General Things to Know

• Commonly called a Threadworm
• The smallest nematode parasites of humans
• Most Strongyloides and their eggs are found in
  the soil
• Infections are initiated when exposed skin
  contacts contaminated soil
• Autoinfection commonly occurs allowing infection
  to persist for decades
• More common in people who suffer from alcholism,
  who are caucasion, males, or have an occupation
  that involves soil contact
• Not likely to be transmitted through water

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Hosts

• Definitive
• Humans
• Dogs
• Cats
• Other mammals

• Intermediate Hosts
• None

There are 38 species of threadworms found in
sheep, swine, goat, ox, deer, camel, rabbit,
primates, dogs, and cats
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Question 2

• How many species of Strongyloides are there?

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Egg stage

• Eggs are about 50 um to 68 um long and 30 um by
  34 um wide.
• Females produce several dozen eggs and release
  them in to the gut lumen or submucosa
• Not normally diagnostic because eggs hatch before
  exiting the human host

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Rhabditiform larvae stage (L1)

• Non infective stage
• Size ranges from 250-300 microns in length and 60
  microns in diameter
• Passed with feces

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Rhabditiform larvae stage (L1)

• Distinguishable characteristics
• short buccal cavity
• clear esophagus
• prominent genital primordium

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Rhabditiform Stage (contd)
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Filariform Stage (L3)

• Reside in the small intestine of the host
• They are about 650 microns long and 50 microns in
  length
• Have a long esophagus
• This is the infective stage of Strongyloidiasis

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Filariform Stage (contd)

• Have a long non bulbous esophagus of about 2/5
  of the length of the body
• Female filariform larvae are slender and faster
  moving than the rhabditiform stage
• Male filariform larvae are not parasitic, only
  the females are
• Filariform larvae penetrate the skin by releasing
  hydrolitic enzymes

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Question 3

• Name one of the three distinguishable
  characteristics of rhabditiform larvae from other
  species of nematodes?

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Question 4

• What larvae form is the infective stage?

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Free living Females

• Have a rhabditiform esophagus
• Females are about 1 mm in length by 50-75 um wide
• Contain embryonated eggs
• Females have a vulva that is about equatorial
• Females uteri contains more eggs than most
  parasitic females

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Free living Males

• Have a rhabditiform esophagus
• Males are up to 0.9 mm long and 40um to 50 um
  wide
• Have two simple spicules and a gubernaculum
• Their pointed tail is a curved ventrad

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Parthenogenetic females

• Parthenogenetic females reach a length of 2.0-3.0
  mm
• 1/3 of the worm is the esophagus
• Inhabit crypts of the small intestines
• Female is stout and has a vulva that is about
  equatorial
• Vulva is in the posterior third of the body
• The uteri carries only a few eggs at a time

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Question 5

• T/F The adult male is found in the definitive
  host.

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Life Cycle

• Adults lay eggs in the intestines within the
  definitive host
• Rhabditiform larvae hatch from the eggs in the
  intestines
• Three possibilities from here
• Free living adult worms (Heterogonic)
• Filariform larvae (Homogenic - Parasitic
  Generation)
• Autoinfection

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Free living adult worms (Heterogonic)

• Rhabditiform larvae will be excreted in the stool
• In this developmental option, the rhabditiform
  will molt four times and will either become a
  female or male adult worm
• The free-living adults then mate and produce eggs
• The rhabditiform larvae then hatch from the eggs
• The larvae then can either produce a new
  generation of adults or become filariform larvae

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Homogenic Life Cycle

• Rhaabditiform larvae will be excreted in the
  stool
• The rhabditiform larvae molt twice and develop
  into filariform larvae
• No adult stages outside of host with this life
  cycle

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Autoinfection

• In this, the rhabditiform larvae develop into
  filariform larvae before they leave the hosts
  intestines
• The filariform larvae then penetrate the
  intestinal mucosa or perianal skin
• They then follow the life cycle in the hostto be
  explained next

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Parasite in the host

• Now, the filariform larvae from each cycle enter
  the circulatory system and travel to the heart
• They come out of the pulmonary artery and enter
  into the lungs
• Penetrate the alveolar spaces and are carried to
  the trachea and pharynx, swallowed, and make
  there way to the small intestines
• In the small intestines they molt twice and
  become adult female worms
• By parthenogenesis they produce eggs, yielding
  rhabditiform larvae
• The prepatent period lasts about 1 month
• Possible cycles start over again

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Question 6

• Name 1 of the 3 cycles that occurs with
  Strongyloides stercoralis?

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Question 7

• What larvae stage is excreted in the stool?

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Transmission

• Through contaminated soil
• Ingestion
• Skin contact
• Autoinfection
• Transmammary infection (with dogs)
• Presumably it can occur in humans

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Symptoms

• Most Strongyloides infections are asymptomatic
  and can survive decades undiagnosed
• The longest documented asymptomatic infection was
  more than 65 years
• Symptomatic infections typically manifest in
  gastrointestinal, pulmonary, and dermatologic
  systems
• In immunocompromised persons, symptomatic
  infections can be devastating and carry a 60-85
  mortality rate
• Acute Infection
• Chronic infection
• Hyperinfection
• Disseminated Infection

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Types of Infections

• Acute Infection
• Symptoms occur after a few weeks whether it is a
  cough, abdominal pain, diarrhea, and/or anorexia.
  
• Most likely detection of hundreds of larvae
• Chronic infection
• manifested by epigastric pain, recurrent hives
  (urticaria) and transient red lines on the skin
  that appear and move rapidly (larva currens) and
  sporadic diarrhea
• Hyperinfection
• Most patients that experience this are
  immunocompromised
• Most likely from accelerated autoinfection
• Large number of parasites present
• Large numbers of larvae in stool and/or sputum
• Disseminated Infection
• Migration of larvae to organs beyond the range of
  the pulmonary autoinfective cycle

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Gastrointestinal Symptoms

• Bloating, distension
• Diffuse abdominal pain
• Diarrhea, typically nonbloody
• Anorexia, weight loss, nausea
• Malabsorption syndromes may occur in chronic
  infections

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Question 8

• T/F It can be transmitted by breast milk

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Dermatologic Symptoms

• Ground itch - Papulovesicular pruritic rash,
  usually on the feet
• Cutaneous infection Dermatitis is produced by
  migration of the infective juveniles through the
  skin
• Petechial/purpuric rash (with disseminated
  disease)
• Inflammation
• Slight hemorrhage and swelling
• Larva currens Skin lesions, hives, or rash
  often on the trunk and buttocks (allergic
  response due to migrating larvae

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Cardiopulmonary Symptoms

• Wheezing or coughing
• Mild to severe symptoms of pneumonia when worms
  are present in the lungs
• Hyperinfection syndrome
• massive proliferation of larval forms
• Pulpatations
• Atriofibrillations
• Choking sensation

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Diagnosis

• Examination of feces and duodenal contents
• Finding rhabdiform or filariform larvae in a
  fresh stool
• If not in a fresh stool, may be misdiagnosed as
  hookworm
• Finding larvae in the sputum
• Culture techniques
• ELISA
• Immunodiagnosis by indirect immunofluorescence
• Antigen whole Strongyloides ratti larvae

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Question 9

• Name a way the parasite can be diagnosed?

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Culture Technique

• Fairly new technique
• Movement of the larval forms can be seen
• Bacterial colonies of normal oral flora are
  formed along the moving trail
• Performed using a chocolate agar sputum culture

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Treatment and Prevention

• Thiamendazole
• Keeps nematode larvae from growing into adult
  forms
• Cambendazole
• Ivermection 100 effective for clearing
  Strongyloides infection with no side effects

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Control

• Proper hygiene
• Do not allow infected hosts to breast feed
• Do not walk barefoot in soil
• In Hospitals
• Make sure you wear clean masks, gloves, and
  aprons
• Wash hands frequently

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Question 10

• What drug is 100 effective against
  Strongyloidiasis?

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Question 11

• Are you more likely to get Strongyloidiasis by
  swimming in a lake or walking barefoot in the
  woods?

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Works cited

• Schmidt, G., Roberts, L. (2005). Foundations
  of Parasitology (7th ed.). New York, NY
  McGraw-Hill
• Schmidt, G.S., and L.S. Roberts. 1989. Chapter
  25. Order Rhabditata Pioneering Parasites. in
  Foundations of Parasitology. Times Mirror/Mosby
  College Publishing. St. Louis. 750 pages.
• http//www.dpd.cdc.gov/dpdx/HTML/Strongyloidiasis.
  htm
• http//www.emedicine.com/emerg/topic843.htm
• http//www.cdfound.to.it/HTML/str1.htmss1
• http//www.wormtreatment.com/worm/strongyloides/
• http//martin.parasitology.mcgill.ca/jimspage/biol
  /strongyl.htm
• http//cmr.asm.org/cgi/content/full/17/1/208?cknc
  kAcute20Strongyloidiasis