THE NIGHT TRAVELLER– MICROFILARIA IN PERIPHERAL BLOOD SMEAR

THE NIGHT TRAVELLER– MICROFILARIA IN PERIPHERAL BLOOD SMEAR
#00061797
Author: Dr Rupali Parikh, M.D.Pathology; Dr Lakshmi Vaswani M.B.B.S, D.P.B
Category: Infectious Disease > Parasites > Filarasis
Published Date: 08/07/2018

Filariasis is a common cause of elephantiasis in Coastal Districts of India. The initial manifestations are repeated episodes of fever with chills and rigor, lymphadenopathy which gradually progress to elephantiasis. In India filarial infestation is commonly caused by W. bancrofti and Brugia malayi, the former responsible for almost 98 % of all cases.

                Human beings serve as the definitive host for the parasite and mosquitoes serve as the intermediate host. In the definitive host i.e. in humans, the adult worm lodges in the lymphatics. The adult female parasite is ovoviviparous and gives birth to ova containing microfilariae that circulate in the blood stream. The clinical spectrum of lymphatic filariasis ranges from only peripheral blood eosinophilia to lymphangitis finally terminating in elephantiasis. 

 The microfilariae, or larval stage of W. bancrofti, are sheathed, and range from approximately 245 to 300 µm. As adults, the males range from 2.5 to 4 cm, and the females range from 5 to 10 cm. One end of the round body is blunt, while the other is pointed. Nuclei do not appear at the end of the tail, which is a major difference from other microfilariae. Microfilariae of B. malayi are sheathed like W. bancrofti, and have a very similar shape. However, the nuclei extend nearly to the tip of the tail, a characteristic not shared with W. bancrofti. The only unequivocal means of ascertaining active filarial infection is by demonstrating parasites in host tissue. In the cases of lymphatic filariasis this is most commonly achieved by detection of microfilariae in the bloodstream. Blood samples should be obtained at a time of day consistent with the known periodicity of microfilariae in the specific geographic region, which is between 22:00 and 02:00 hours for nocturnally periodic forms of brugian and bancroftian filariasis.

Detecting microfilaria in peripheral blood with or without Diethylcarbamazine citrate provocation is the common diagnostic modality in suspected cases. However microfilaria has been accidentally detected in fine needle aspirates, aspirated body fluids and even in bronchial washings. Highly sensitive and specific filarial antigen detection assays, both as card test and in ELISA based format are now available for the diagnosis of W. bancrofti infection. This test is positive in early stages of the disease when the adult worms are alive and becomes negative once they are dead. DNA probes using Polymerase Chain Reaction (PCR)test is of high specificity and sensitivity, and detects parasite DNA in humans as well as vectors in both bancroftian and brugian filariasis.