Filariasis: Introduction, Classification, Life Cycle, Clinical Features, Lab Diagnosis, Treatment and Control

Filariasis Life cycle and lab diagnosis

Introduction of Filariasis

Filariasis is an infectious disease caused by nematodes of the Filarioidea type e.g. Wuchereria bancrofti, Brugia malayi,  Oncocerca volvulus, and Loa loa. These are spread by blood-feeding insects such as black flies and mosquitoes. The filarial worms reside in the subcutaneous tissues, lymphatic system, or body cavities of humans,

Classification of filarial worms  based on location in the body 

  1. lymphatic filariasis – causative agents: Wuchereria bancrofti, Brugia malayi, Brugia timorii, 
  2. Subcutaneous filariasis : Causative agents are-Loa loa, Oncocerca volvulus, Mansonella streptocerca
  3. Serous cavity filariasis: Causative agents are-Mansonella perstans, Mansonella ozzardi (They are virtually non-pathogenic)


The females of parasites are viviparously giving birth to larvae so-called microfilaria. This parasite includes four genera and species are parasitic to humans. These filarial nematodes are as follows: 1. Wuchereria bancrofti 2. Brugia malayi 3. Oncocerca volvulus and 4. Loa loa but mainly two genera i.e. Wuchereria and Brugia are encountered in peripheral blood.

Wuchereria bancrofti

Geographical Distribution



South china

South America

Central and West Africa


Lymphatics and lymph nodes


It has two stages adult worm and larva (microfilaria).

Adult worm

The adults are whitish, translucent, thread-like worms with smooth cuticles and tapering ends. The female is larger (70–100 × 0.25 mm) than the male (25–40 × 0.1 mm). The posterior end of the female worm is straight, while that of the male is curved vertically and contains 2 spicules of unequal length. Males and females remain coiled together usually in the abdominal and inguinal lymphatics and in the testicular tissues. The female worm is viviparous and directly liberates sheathed microfilariae into the lymph.

Lifespan: 10 to 15 years


The microfilaria has a colorless, translucent body with a blunt head, and pointed tail It measures 250–300 µm in length and 6–10 µm in thickness
It is covered by a hyaline sheath, within which it can actively move forwards and backward Along the central axis of the microfilaria, a column of granules can be seen, which are called somatic cells or nuclei. The granules are absent at certain specific locations—a feature that helps in the identification of the species. Microfilariae do not multiply or undergo any further development in the human body. Their lifespan is believed to be about 2–3 months. It is estimated that a micro filarial density of at least 15 per drop of blood is necessary for infecting mosquitoes


The microfilariae circulate in the bloodstream. They show a nocturnal periodicity in peripheral circulation; being seen in large numbers in peripheral blood only at night (between 10 pm and 4 am). This correlates with the night-biting habit of the vector mosquito.

Life Cycle

Definitive host: Man

Intermediate host: Female mosquito, of different species, acts as vectors in different geographic areas. The vector in most other parts of Asia is Culex quinquefasciatus (C. fatigans).

Infective form: Actively motile third-stage filariform larva

Mode of transmission: Bite of a mosquito carrying filariform larva.

Microfilaria in peripheral capillaries ( at night )

Microfilaria ingested by mosquito reaching the stomach

Microfilaria, shed sheath, penetrate the gut wall, enter thoracic muscles

Short larvae ( first stage )

Second stage larvae

Third stage larva

Mosquito bites man and depositing of infective larva on skin

Larva skin penetration, lymphatics and becomes adult male and female

Adult worms male and female remain coiled together ( abdomen, inguinal lymphatics, testicular tissues) and again cycles starts

Clinical features of Filariasis ( due to Wuchereria bancrofti )

Clinical features of filariasis differ according to the stages of the worm and the nature of its location in our body.

Due to larvae entry into the blood





Low-grade fever



Fugitive swelling  ( skin or limb)

Occult Filariasis

Due to hypersensitivity to filarial antigen

Microfilaria is not detectable in blood but seen at affected sites

May present as tropical pulmonary eosinophilia


Loss of weight


Dry nocturnal cough


Asthmatic wheezing

blood eosinophil count 3000 to 5000 cu mm

May also show






Adult worm

Lymphangitis (testicles, epididymis, spermatic cord, etc)

Lymphadenitis (groin, axilla)




Laboratory Diagnosis  of Filariasis

  • Demonstration of Microfilaria: Microfilaria can be demonstrated in blood, chylous urine, exudate of lymph varix, and hydrocele fluid. Peripheral blood is the specimen of choice. The method has the advantage that the species of the infecting filaria can be identified from the morphology of the microfilaria seen.
  • Unstained Film: Examination under the low-power microscope shows the actively motile microfilariae lashing the blood cells around. The timing of blood collection is critical and should be based on the periodicity of the microfilariae.
  • Concentration Techniques: When the microfilaria density is low, concentration techniques are used: Knott’s concentration technique

Nucleopore filtration

DEC provocation test

  •  Biopsy: Adult filarial worms can be seen in sections of biopsied lymph nodes, but this is not employed in routine diagnosis
  • Skin Test: Intradermal injection of filarial antigens induces an immediate hypersensitivity reaction. But the diagnostic value of the skin test is very limited due to the high rate of false-positive and negative reactions.
  • Serodiagnosis: Complement fixation, indirect hemagglutination (IHA), indirect fluorescent antibody (IFA), immunodiffusion, and immune enzyme tests have been described. But the tests are not sufficiently sensitive or specific to be used either for individual diagnosis or surveys. Demonstration of Circulating Antigen Highly sensitive and specific test for detection of specific circulating filarial antigen (CFA)The Trop bio test is a semiquantitative ELISA for detection of CFA in serum or plasma specimen. Immunochromatographic filariasis card test (ICT) is a new and rapid filarial antigen test that detects soluble W. bancrofti antigens using a monoclonal antibody (ADIZ) in the serum of infected humans. Both assays have sensitivities of 93–100% and specificities approaching 100%.
  • Molecular Diagnostic Technique: Polymerase chain reaction (PCR) can detect filarial DNA from a patient’s blood, only when circulating microfilaria are present in peripheral blood but not in chronic carrier state usually the test provides sensitivities that are up to 10 fold greater than parasitic detection by direct examination and is 100% specific.
  • Other tests:

Indirect Evidence

Eosinophilia (5–15%) is a common finding in filariasis.

Elevated serum IgE levels can also be seen.

Treatment of Filariasis

Diethylcarbamazine (DEC): effective against adult and microfilariae. Ivermectin: destroy microfilariae but not adults.

Vector Control

Drainage and filling:

urban chemical and biological larvicides

Indoor residual insecticide


Outdoor residual insecticide spraying

Personal protection


impregnated materials: nets, curtains, clothing

House screening

House location



Further Readings

  1. Merkell and Voge’s medical parasitology
    9th edition.
  2. Parasitology: 12th edition
    By K. D. Chatterjee
  3. District laboratory practice in Tropical countries –Part-I.
    By Monica Chesbrough.
  4. Isenberg clinical microbiology procedures Handbook
    2nd edition. Vol. 2
  5. Atlas of Medical Helminthology and protozoology -4th edn  -P.L.  Chiodini, A.H. Moody, D.W. Manser
  6. Medical Parasitology by Abhay R. Satoskar, Gary L. Simon, Peter J. Hotez and Moriya Tsuji
  7. Atlas of Human Parasitology, Lawrence R Ash, Thomas C. Orihel, 3 rd ed, Publisher ASCP Press, Chicago.
  8. Molecular Medical Parasitology. Editors: J. Joseph Marr, Timothy W. Nilsen, and Richard W. Komuniecki, Publisher Academic Press, an imprint of Elsevier Science.
  9. Topley & Wilsons’ Principle of parasitology. Editors: M.T. Parker & L.H. Collier, 8 th ed 1990, Publisher Edward Arnold publication, London.
  10. Illustrated Medical Microbiology 2nd ed, Satish Gupte


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