- Nematodes are slender thread-like worms.
- Transmitted by the bite of blood-sucking insects.
- The female worms are longer than males.
- Adult worm has a length of 80-100mm and has a breadth of 0.25-0.30mm
- Female worms are viviparous and give birth to larvae known as microfilariae
- Sheathed microfilariae: Those microfilariae which remain egg membranes which envelop them as sheath.
- Unsheathed microfilariae: Those in which the egg membranes are ruptured are known as unsheathed microfilariae.
- Periodicity: It is the largest number of microfilariae occuring in blood.
- Nocturnal periodicity: when the largest number of microfilariae occur in blood
Eg: wuchereria bancrofti
- Diurnal periodicity: when the largest number of microfilariae occur in blood
Eg: Loa Loa
- Nonperiodic: when the microfilariae circulate at constant levels during the day and night
Eg: Onchocerca volvulus
- Subperiodic or nocturnally subperiodic: when the microfilariae can be detected in the blood throughout the day but are detected in higher numbers during late afternoon or night.
- Microfilariae are found in capillaries and blood vessels of lungs during the period when they are not present in peripheral blood.
- Life cycle of filarial nematodes is passed in two hosts
- Definitive host is man
- Intermediate host are blood-sucking arthropods
- Infection with any of the filarial worms may be called filariasis.
- Lymphatic filariasis is caused by
- Wuchereria bancrofti
- Brugia malayi
- Brugia timori
- History and distribution:
- Elephantiasis: painful, disfiguring swelling of the legs and genital organs-is a classical sign of late stage disease.
- Wuchereria bancrofti is distributed widely in the tropics and subtropics .
- The largest number of cases of filariasis occurs in India.
- The adult worms reside in the lymphatic system of man.
- The microfilariae are found in blood.
- Adults are whitish, translucent, thread-like worms with smooth cuticle and tapering ends.
- Female larger than male.
- Posterior end of the female worm is straight ,while that of male is curved vertically and contains two spicules of unequal length.
- Female is viviparous and directly releases sheathed microfilariae into lymph
- It has a colourless, translucent body with a blunt head, and pointed tail.
- It can move forward and backward within the sheath which is much longer than embryo.
- It is covered by hyaline sheath.
- Along the central axis of the microfilariae, a coloum of granules can be seen, which are called somatic cells or nuclei.
- These granules are absent at specific locations -a feature which helps in the identification of the species.
- These locations are
- Cephalic space
- Nerve ring
- Anterior V-spot(rudimentary excretory system)
- Posterior V-spot
- Microfilariae do not multiply or undergo any further development in the human body.
- If they are not taken up by a female vector mosquito ,they die.
- Lifespan is 2-3 months.
- They show nocturnal periodicity in peripheral circulation.
- Wuchereria bancrofti passes it’s life cycle in two hosts
- Man (definite host) and female mosquito (intermediate host)
- Infective form: actively motile third stage filariform larva is infective to man.
- Mode of transmission:by bite of mosquito carrying filariform larva
Development in mosquito:
- Within 2-6hrs after ingestion,exsheating occurs.
- They penetrate the stomach wall and migrate to thoracic muscles after 4-17 hours
- During the next 2 days ,they metamorphose into first stage larva(sausage shaped)
- Within a week , increases in size and becomes second stage larva.
- It develops internal structures and formes third stage filariform larva which is infective to man.
- There is no multiplication of the microfilariae develops into one infective larva only.
- The time taken from the entry of the microfilariae into the mosquito till the development of the infective third stage larva located in its proboscis sheath which constitutes extrinsic incubation period.
- It’s duration is 10-20days.
Development in man:
- After penetration it is usually carried to abdominal or inguinal lymph nodes , where they develop into adult forms.
- There is no multiplication at this stage and only one adult develops from one larva, male or female.
- They become sexually mature in about 6 months and mate.
- They releases 50,000 microfilariae .
- The microfilariae are ingested with the blood meal by mosquito and the cycle is repeated.
- Prepatent period: The period from the entry of infective third-stage larvae into the human host till the first appearance of microfilariae in circulation is called the biological incubation period .
- This is usually 8-12months.
- Clinical incubation period: the period from the entry of the infective larvae ,till the development of the earliest clinical manifestation is called clinical incubation period.
- Usually 8-16 months.
- Infection caused by wuchereria bancrofti is termed as wuchereriasis or bancroftian filariasis.
- The disease can present as
- It occurs due to blockage of lymph vessels and lymph nodes by adult worms.
- Classical manifestations:
- Most patients appear clinically asymptomatic but subclinical disease include hematuria or proteinuria.
- Acute adenolymphangitis is characterized by high fever, lymphatic inflammation and transient local edema.
- It occurs as a result of hypersensitivity reaction to microfilarial antigens, not directly due to lymphatic involvement.
- Clinical manifestations:
- Massive eosinophila
- Pulmonary symptoms like dry nocturnal cough,dyspnea and asthmatic wheezing.
- Classical features of lymphatic filariasis are absent.
- Meyers kouwenaar syndrome is a synonym for occult filariasis.
- Direct evidence:
- Detection of microfilariae
- By examination of thick and thin blood smear , stained with giemsa stain
- By examination of unstained mount of blood under microscope
- Detection of adult worm
- Lymph node biopsy
- On x-ray (if worms are classified)
- High frequency ultrasound and Doppler within the scrotum.
- Eosinophila in blood
- Elevated serum IgE levels
- Antigen detection
- ICT(immunochromatographic test)
- Both the tests have sensitivity and specificity
- Antibody detection
- CFT(complement fixation test)
- IHA(indirect hemagglutination)
- IFA(indirect fluorescent antibody)
- These tests have low sensitivity and specificity.
- Done by PCR
- The test is positive only when microfilariae are present in peripheral blood.
- Negative in chronic filariasis.
- Diethylcarbamazine is the drug of choice
- Given orally 6mg/kg body weight for a period of 12 days
- It has both macro and microfilaricidal properties.
- Following treatment with diethylcarbamazine severe allergic reaction may occur due to death of microfilariae.
- It kills both microfilaria and adult worm.
- Antihistamines or corticosteroids may require to control the allergic phenomenon.
- Eradication of vector mosquito.
- Detection and treatment of carriers.