Helminths: Nematodes - Blood and Tissue (Filarial) Nematodes

Filarial Nematodes

  • Transmitted by blood-sucking arthropods.
  • Larval stages reside in insect vectors.
  • Adult worm is pathogenic in humans.
  • Female worms produce larvae called microfilariae.

Morphologic Features of Microfilariae

  • Key features for identification:
    • Presence or absence of a sheath.
    • Arrangement and number of tail nuclei.
Sheath Present
  • Wuchereria bancrofti
    • Tail nuclei do not extend to the tip.
  • Loa loa
    • Continuous row of nuclei.
  • Brugia malayi
    • Not continuous, two nuclei at the tip.
Sheath Absent
  • M. ozzardi
    • Tail nuclei extend to the tip.
  • M. perstans
    • Tail nuclei extend to the tip.
  • M. streptocerca
    • Tail nuclei extend to the tip (Shepherd's hook).

Nuclei in the Tail

The following nematodes have nuclei in the tail:

  • Wuchereria bancrofti
  • Brugia malayi
  • Loa loa
  • Onchocerca volvulus
  • Mansonella perstans
  • Mansonella streptocerca
  • Mansonella ozzardi

Wuchereria bancrofti

  • Pronunciation: wooch-ur-eer’ee-uh/ban-krof’tye
  • Commonly known as Bancroft’s filaria.
  • Transmitted by a mosquito, usually in tropics.
  • Humans are the definitive hosts.
  • Microfilaria: no nucleus found in tail, sheath present.
  • Diseases:
    • Bancroftian filariasis
    • Hydrocele (scrotum)
    • Chyluria (urine)
    • Elephantiasis—Obstruction of lymphatic vessels in the extremities
    • Tropical pulmonary eosinophilia (TPE)
  • Diagnosis:
    • Identification of microfilariae in blood, fluids, or tissue.
    • Molecular diagnostics.
  • Treatment: Diethylcarbamazine; ivermectin.

Brugia spp.

  • Lymphatic filarial parasites:
    • Brugia malayi (Far East) (broog’ee-uh/may-lay eye)
    • Brugia timori (Indonesia)
  • Transmitted by mosquitoes (Anopheles and Mansonia).
  • Do not involve genital lymphatic vessels.
  • Diagnosis:
    • Adult worms are isolated in blood.
    • B. malayi has four to five subterminal and two terminal nuclei in the tail, with a sheath.
    • B. timori has five to eight subterminal nuclei and one terminal nucleus in the tail.

Loa Loa

  • Pronunciation: lo’uh/lo’uh
  • Commonly known as African eye worm.
  • Transmitted by the tabanid or deerfly.
  • Microfilariae circulate in the bloodstream and reside in the subconjunctiva.
  • Diagnosis:
    • Worm is identified in the eye, tissue, or peripheral blood.
    • Calabar swellings are where the worm is migrating in the skin (may migrate across the bridge of the nose).
    • Contains a sheath that does not stain with Giemsa.
    • Nuclei extend to the tail in an irregular fashion.

Onchocerca volvulus

  • Pronunciation: onk’o-sur’kuh/vol’vew-lus
  • Resides in tissue nodules of the host.
  • Transmitted by the female black fly (Simulium).
  • Found in Central America and Africa.
  • Diseases:
    • River blindness—Subcutaneous infection (2nd leading cause of infectious blindness worldwide)
    • Skin infections
    • Lymphadenopathy in the inguinal and femoral regions
  • Diagnosis:
    • Adult worm is identified in the tissue (nodule or skin snip).
    • Microfilariae may be visible in the cornea of the eye.
    • Microfilariae lack a sheath and have nonnucleated tapered tails.
  • Mnemonic: "On" is "no" spelled backwards, indicating no sheath and no nuclei in the tip of the tail.
  • Treatment: ivermectin

Mansonella spp.

  • Transmitted by biting midges (Culicoides).
  • Pathogenesis:
    • M. streptocerca is found in the skin and causes a pruritic rash.
    • M. perstans resides in pericardial, pleural, and peritoneal cavities.
    • M. ozzardi is found in the blood.
  • Diagnosis:
    • Do not possess sheaths.
    • M. streptocerca and M. perstans have nuclei that extend to the end of the tail; nuclei of M. ozzardi do not extend to the tip.
    • The tail of M. streptocerca is referred to as a “shepherd’s crook.”