Question

Filariasis is caused by.
A. Dead adult filariae
B. Presence of bacteria in filarial wall
C. Microfilariae
D. Biting of filarial worm

Answer 1

Hint: The worm is present in the peripheral blood which is taken up by the mosquito and then spread to a healthy person.

Complete step by step answer:
Filariasis is a parasitic disease caused by infection with round forms that belong to the family of Filariodiae. The worms which cause this disease are either Wuchereria bancrofti or Brugia malayi. The disease is spread by blood-feeding insect vectors. Due to the repeated and long-duration exposure to these vectors, filariasis may cause symptoms in the lymphatics, skin, and eyes.
Pathogenesis- the vector bites a person and releases the larva into the skin which then travels into the lymph vessels. In the lymphatics, the larva grows into the adult worm which can persist for several years. The adult female worm then releases many microfilariae into the blood. During the daytime, microfilariae are present in the deep vascular bed, however, it migrates into the peripheral blood at night. The mosquito, when it takes up this blood at night and bites a healthy person, the same cycle is repeated in that person too.
People are mostly asymptomatic to this infection or can develop acute and chronic conditions. The parasite damages the lymph system, may cause elephantiasis, secondary bacterial inflammation, etc. The adult worm and the microfilariae reside in the lymphatics of the upper limb, lower limb, and genitalia.
Diagnosis can be done by collecting blood samples at night to coincide with the appearance of microfilariae in the blood. Other methods include serological examination, PCR-based assays, etc. Prevention can be done by mass treatment and personal protection like good hygiene, periodic examination of blood for infection in endemic areas, etc.
In humans, the disease is transmitted by the larval form of the parasite.

Hence, the correct option is option C, Microfilariae.

Note:
The last stage of the larva i.e. the third larval stage in the mosquito is the infective stage. At this stage, the larva initiates infection in humans.