Nematodes-Aphasmids

• Helminths


  Parasitic worms

  
• Topics covered


  • Introduction to Helminths
  • General Characteristics of Nematodes
  • Aphasmids vs. Phasmids
  • Trichinella spiralis
  • Trichuris trichiura
  • Capillaria philippinensis
  • Capillaria hepatica
  • Dioctophyme renale

  
• Nematodes


  • Phylum: Nemathelminthes
  • Adult worms have elongated, bilaterally symmetrical, cylindrical, smooth, unsegmented, flesh-colored bodies
  • Body is usually tapered to a pointed posterior end, and to a rounded anterior end
  • The body is covered by a noncellular, highly resistant coating "the cuticle"
  • They have a complete digestive system with mouth, oesophagus, midgut and anus
  • All are separate sexes (dioecious); the female is usually larger than the male
  • Classified according to their chemoreceptors: Amphids (cephalic) and Phasmids (caudal)
  • Classified on the presence or absence of larva in shell: Oviparous, Ovoviviparous, Viviparous
  • Classified into 2 main categories according to their primary location: Intestinal nematodes and Tissue nematodes (filariae)

  
• Aphasmids


  • Caudal papillae is reduced or absent
  • Excretory system without lateral canals
  • Phasmids – ABSENT
  • Esophagus is cylindrical – STICHOSOME/TROPHPOSOME
  • Unsegmented eggs with polar plugs or hatches in the uterus
  • 1st larval stage with stylet

  
• Examples of Aphasmids


  • Trichinella spiralis
  • Trichuris trichiura
  • Capillaria philippinensis
  • Capillaria hepatica
  • Dioctophyme renale
• Trichinella spiralis


  • The most important cause of trichinellosis in humans, and is the species that is most adapted to domestic and wild pigs
  • Common name: Trichina worm
  • Common associated disease/condition: Trichinosis, Trichinellosis (A zoonotic infection)

  
• Trichinella spiralis Encysted Larva


  • Average juvenile size: 75-120 µm long, 4-7 µm wide
  • Average mature size: Up to 1 mm in length
  • Encysted in: Nurse cells of striated muscle
  • Notable features: Inflammatory infiltrate present around nurse cell

  
• Trichinella spiralis Adult Worm

  • Adult Female: 4 by 0.05 mm, Blunt, posterior end; single ovary with vulva in anterior fifth of body
  • Adult Male: 2 by 0.04 mm, Curved posterior end with two rounded appendages
  • Notable features (Common to both): thin anterior end, Small mouth, longer digestive tract
• Trichinella spiralis Life Cycle/Biology

  1. Infective encysted larvae enter the host through ingestion of raw or insufficiently cooked meat

  2. After exposure to gastric acid and pepsin, the larvae are released from the cysts

  3. Larvae will invade the small bowel mucosa where they develop into adult worms (they will undergo four molts)

  4. Maturation takes about 2 days and adult worms begin to mate 5 to 7 days post infection. After 1 week, the females release larvae (viviparous)

  5. These larvae penetrate the mucosa pass through the lymphatic system, and to striated muscles where they encyst

  
• Trichinella spiralis Pathogenesis


  • Enteric Stage: Most of the light infections are asymptomatic; however, heavy infections can be life-threatening. Symptoms start appearing during the first week of infection. Invasion of the gut by large number of parasites can provoke watery diarrhea (most common feature); Abdominal pain, constipation, nausea or vomiting may also be seen
  • Stage of Larval Migration: Hypersensitivity reaction: The migrating Trichinella larvae provoke a marked local and systemic hypersensitivity reaction, with fever and hypereosinophilia. Periorbital and facial edema is common. Hemorrhages are seen in the sub-conjunctiva, retina and nail beds ("splinter" hemorrhages). Maculopapular rash. Migration to heart, CNS and lungs is common
  • Stage of Muscle Encystment: Common symptoms are myositis with myalgia, muscle edema, and weakness. Most commonly involved muscles: extraocular muscles followed by biceps; and muscles of the jaw, neck, lower back, and diaphragm

  
• Trichinella spiralis Laboratory Diagnosis


  • Definite Diagnosis: demonstration of the larvae in muscle biopsy. Sample: muscle biopsy near tendon insertions of deltoid (at least 1g) gives better yield. Direct slide technique: fresh muscle tissue compressed between glass slides and examined microscopically. Histopathologic study: can be done using H & E stain. Enzymatic digestion of muscle mass by trypsin and mounting the digested tissue may yield better result
  • Serology: Parasite specific antibody can be detected by ELISA or countercurrent immunoelectrophoresis (CIEP). Positive after 2 weeks of infection. It confirms the diagnosis but cannot differentiate past and present infection. Other serological method: Bentonite flocculation test
  • Bachman Intradermal Test: Cannot differentiate past infection from present infection. Bachman antigen: prepared from Trichinella larva from rabbit muscle. Intradermal injection of bachman antigen causes an immediate small induration surrounded by erythema of 5 cm diameter in 15-20 minute
  • Animal Inoculation in Rats (Beck's Test): Rats are fed with muscle tissue of suspected patients and after appropriate time, they are examined for T. spiralis larvae in the diaphragm
  • Other Tests: Blood eosinophilia, Increased Total WBC count, Elevated muscle enzyme (e.g. Creatine phosphokinase), X-ray (to detect calcified muscle cyst)
• Trichinella spiralis has worldwide distribution, most common in EUROPE and UNITED STATES. Found in animals including: pig, deer, bear, walrus, and rat
• Trichinella spiralis Prevention and Control

  • Health education
  • Meat cooked at min. 77C (170F)
  • Freezing meat
  • Storage at -15 C for 20 days or -30 C for 6 days
  • Regular animal monitoring
  • Proper disposal of suspected carcasses

  
• Trichinella spiralis Treatment


  • Mild Infection: Symptomatic treatment is required with bed rest, antipyretics, and analgesics
  • Moderate Infection: Mebendazole and albendazole are active against enteric stages of the parasite, but their efficacy against encysted larvae has not been conclusively demonstrated
  • Severe Infection: Glucocorticoid is added which is beneficial for severe myositis and myocarditis

  
• Trichuris trichiura


  • Common name: WHIPWHORM
  • Common associated disease/condition: Trichuriasis, whipworm infection
  • Habitat: Large intestine of man (cecum and appendix)
• Trichuris trichiura Egg


  • Size: 50-55 by 25 µm
  • Shape: Barrel, football; prominent hyaline polar plug at each end
  • Embryo: Unicellular; undeveloped
  • Shell: Smooth; yellow-brown color because of bile contact

  
• Trichuris trichiura Adults


  • Size: 2.5-5 cm long; males usually smaller than females; Anterior three fifth is thin, hair like, coiled (like rope of a whip) and posterior two fifth is short and thick
  • Shape: Colorless; resembles a whip handle; contains a slender esophagus
  • Posterior End: Pinkish-gray; resembles whip itself; contains digestive and reproductive systems; males possess prominent curled tail
  • Female is longer (35–50 mm) and its posterior part is either shaped like a comma or arc (resembles a handle of a whip)
  • Life-span: 1 year
• Trichuris trichiura Life Cycle/Biology

  1. Unembryonated eggs are passed with the stool

  2. In the soil, the eggs develop into a 2-cell stage

  3. Then forms an advanced cleavage stage

  4. The eggs then embryonate (infective stage; eggs become infective in 15-30 days)

  5. After ingestion (soil-contaminated hands or food), the eggs hatch in the small intestine, and release larvae

  6. Larvae mature and establish themselves as adults in the colon

  
• Trichuris trichiura Pathogenesis


  • In people with heavy infections: Adult female worm gets buried in the large intestinal mucosa that leads to:

  Mechanical distortion: Leading to inflamed, edematous, and friable mucosa

  Allergic response by the host: Increased numbers of macrophages infiltrates in the lamina propria

  
• Trichuris tail


  • Female is longer (35–50 mm) and its posterior part is either shaped like a comma or arc (resembles a handle of a whip)
  • Life-span: 1 year
• Trichuris parasite life cycle

  1. Unembryonated eggs are passed with the stool

  2. In the soil, the eggs develop into a 2-cell stage

  3. Then forms an advanced cleavage stage

  4. The eggs then embryonate (infective stage; eggs become infective in 15-30 days)

  5. After ingestion (soil-contaminated hands or food), the eggs hatch in the small intestine, and release larvae

  6. Larvae mature and establish themselves as adults in the colon

  
• Common Trichuris manifestations


  • Abdominal pain, anorexia, etc
  • Trichuris dysentery syndrome: bloody or mucoid diarrhea resembling inflammatory bowel disease
  • Iron deficiency anemia due to blood loss (0.005 ml bld loss/day/worm)
  • Recurrent rectal prolapse (due to heavy worm load in the rectum and malnutrition)
  • Growth retardation and impaired cognitive function (due to the release of anti-inflammatory cytokines induced by the secretory molecules of Trichuris species)

  
• Stool examination for Trichuris diagnosis

  • Microscopic examination of a single fecal smear is sufficient for diagnosis of symptomatic cases
  • The characteristic 50 × 22 µm barrel shaped Trichuris eggs (with mucus plugs at the ends) are readily detected on stool examination either by direct wet mount or following concentration of the stool
  • Formalin is preferred over polyvinyl alcohol to preserve the stool samples
  • Whip shaped adult worms of 3–5 cm long, are occasionally seen on proctoscopy
  • Methods for stool exam: DFS/ Direct Fecal smear, Kato-Thick smear (highly recommended to determine intensity of infection), Kato-Katz, Concentration Technique

  
• Serological diagnosis of Trichuris

  • Peripheral eosinophilia
  • Increased serum IgE
• Trichuriasis epidemiology


  • Occurs in both temperate and tropical countries but is more widely distributed in warm, moist areas of the world
  • 5 to 15 years of age are most frequently infected
  • Most prevalent in East Asia and Pacific Island regions
  • Least prevalent in Middle East and North African Regions

  
• Trichuriasis prevention and control

  • WHO recommends: biannual mass drug administration with mebendazole or albendazole
  • Provision of safe water
  • Environmental sanitation and hygiene education
  • Treatment of infected individuals
  • Sanitary disposal of feces
• Trichuriasis treatment


  • Mebendazole (500 mg once) or albendazole (400 mg daily for three doses) is safe and moderately effective for treatment, with cure rates of 70-90%
  • Ivermectin (200 mg/kg daily for three doses) is also safe but less effective
• Capillaria philippinensis


  • Common name: PUDOC WORM/ MYSTERY WORM
  • Common associated disease/condition: Human intestinal capillariasis
  • Habitat: Small intestine

  
• Capillaria philippinensis eggs/ova


  • Size: 35 to 45 um by 20-25 um; Smaller than Capillaria hepatica
  • Shape: Peanut-shaped or guitar shaped; straight bipolar mucus plugs
  • Embryo: Unembryonated when laid
  • Shell: Smaller and striated shell

  
• Capillaria philippinensis adults


  • Size: Resembles Trichuris except that it is smaller female is 2.5–4.3 mm long and males are 2.3–3.17 mm long
  • Male: Has a spicule, Long nonspiny sheath
  • Female: Vulva is located at the junction of anterior and middle third
• Capillaria philippinensis life cycle

  1. Unembryonated, thick-shelled eggs are passed in the human stool

  2. Eggs become embryonated in the external environment in 5-10 days (in water)

  3. Freshwater or brackish water fish gets infected by ingesting embroyonated ova; Eggs hatch in the intestine of the fish and grow into infective larvae penetrate the intestine, and migrate to the tissues

  4. Ingestion of raw or undercooked fish results in infection of the human host

  5. Adults of Capillaria philippinensis are very small and reside in the human small intestine, where they burrow in the mucosa

  6. Females may produce two types of eggs the unembryonated (feces) and the embryonated lacking shells (hatch inside definitive host) (oviparous)

  7. Autoinfection and hyperinfection may occur due to the larvae that re-invades small intestine mucosa

  
• Capillaria philippinensis diagnosis


  • STOOL EXAMINATION: Finding eggs/ adult worm in feces by direct smear or wet mount or by stool concentration methods
  • INTESTINAL BIOPSIES
  • No valid serological testing is available

  
• Capillaria philippinensis endemic areas

  • Endemic in the Philippines, Thailand; Sporadic in East and Southeast Asian countries
  • Northern Egypt has number of cases
  • Areas in the Philippines: Northern Luzon (epidemic), Southern Leyte, Zambales, Zamboanga del Norte and Sur, Agusan del Sur, Misamis Occidental
• Capillaria philippinensis prevention and control


  • Health education
  • Discouraging eating of raw fish/ Ensure properly cooked fish
• Capillaria philippinensis treatment


  • Prolonged treatment with albendazole (200 mg take daily for 10 days) is required
  • Severely ill patients require fluid replacement and supportive therapy
• Capillaria hepatica


  • Common name: Calodium hepaticum
  • Common associated disease/condition: Hepatic capillariasis
  • Habitat: Parenchyma of liver
• Capillaria hepatica eggs/ova


  • Size: 50 – 70 um by 30 – 35 um
  • Shape: Ellipsoid shape; Smooth poles / shallow polar prominences
  • Embryo: Unembryonated when laid
  • Shell: striated sheath

  
• Capillaria hepatica adults


  • Size: 20 mm in length; 70 to 100 um at anterior end
  • Shape: slender, delicate nematodes
  • Note: Rarely seen intact due to short life span
• Capillaria hepatica life cycle

  1. Adult worms are located deep within the liver parenchyma and lay hundreds of eggs in the surrounding parenchymal tissue

  2. Eggs trapped in the parenchyma can not be passed in the feces and remain in the liver until the animal dies, or more likely, is eaten by a predator or scavenger

  3. Eggs ingested by scavengers are unembryonated and are passed in through the digestive tract into and out in feces

  4. Eggs embryonated in the where they require air and damp soil to become infective

  5. The cycle continues when embryonated eggs are eaten by a suitable mammalian host

  6. Infective eggs hatch in the intestine, releasing first stage larvae (oviparous) . The larvae penetrate the intestinal wall and migrate via the portal vein to the liver parenchyma within 3-4 days. Larvae take about 3-4 weeks to mature into adults and mate. Humans are usually infected after ingesting embryonated eggs in fecally-contaminated food, water, or soil

  
• Capillaria hepatica diagnosis


  • Specific diagnosis: demonstrating adult worms and/or eggs in liver tissue at biopsy or necropsy
  • C. hepatica eggs in stool is a spurious finding results from the ingestion of liver of infected animal