Filariasis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2], Ahmed Elsaiey, MBBCH [3]

Overview

Filariasis infection occurs when a larva carrying mosquito bites an individual, introducing these larvae into the skin. The larvae then enters the patient's blood through the skin wound and spread to the different sites such as lymphatic vessels, subcutaneous tissues or the serous cavities. At these sites, the larvae matures in a six to twelve months period into the adult filariae which can live up to fifteen years. Reproduction takes place between the male and female adult worms producing microfilariae which are premature organisms with sheath that circulate the blood in case they are settled in the lymphatic vessels. During another blood meal, the mosquito takes up the microfilariae, then these microfilariae lose their sheath within two weeks to be larvae that enter the human body. When a human is bitten by a mosquito, the cycle restarts again. Pathogenesis of the disease depends on number of factors including immune response of the patient, the number of secondary bacterial infections, the accumulation of the worm antigens, release of Wolbachia bacteria from the worm and the genetic predisposition.

Pathophysiology

Pathogenesis

The pathogenesis of lymphedema and its progression to elephantiasis is controversial. Factors involved in the clinical manifestations of filariasis include:

• Immune response of the patient
• The number of filarial and bacterial infection
• The accumulation of the worm antigen in the lymphatic vessels
• The release of Wolbachia bacteria following death of the worm

Factor	Role in pathogenesis
Immune response of the host	There is a strong correlation between the host immune response and lymphedema development. Patients with lymphedema mount a higher immune response when compared to those with microfilariae just circulating in the blood. It is believed that the role of the immune response in the development of the lymphedema leads to inflammation and obstruction of the lymphatic vessels. Infection by filariasis induces cell mediated immunity in response to the filarial antigens. This leads to the production of cytokines and interleukins. High levels of immunoglobulins (IgG1,2,3) have been detected in patients with lymphedema which increases the evidence of the role of the immune response in pathogenesis of the disease.
Secondary bacterial infections	Adenolymphangitis is a result of the inflammation induced by the filarial infection and the immune response. It is believed that it worsens the disease and leads to morbidity. It affects the lower limbs resulting in cord like lesion of the lymphatic vessels and worsening of the filariasis.
Wolbachia bacteria	The round worms causing filariasis are carriers of a kind of bacteria called Wolbachia that is released after the death of the worms. There is a correlation between Wolbachia bacteria and the inflammatory reactions in cases of filariasis especially in the phase of treatment by chemotherapy that ends with lymphedema. Immunologically, serum antibodies released against Wolbachia surface protein may also play a role in the development of the lymphedema.

Genetics

• A mutation in the vascular endothelial growth factor receptor 3 (VEGFR-3) is associated with development of primary lymphedema secondary to dysfunction of the endothelial cells and impaired lymphangiogenesis.
• Mutation in the forkhead transcription factor (FOXC2) also leads to hereditary lymphedema.

Life cycle of filariasis nematodes

In order to understand how filariasis could occur, it is important to know the life cycles of different nematodes causing filariasis. Through this table the important steps in the worms life cycle is discussed as well as the vectors responsible for disease transmission.

Type of filariasis	Causative nematode	Vectors	Life Cycle
Lymphatic filariasis	Wuchereria bancrofti	Culex as C. pipiens Aedes as A. aegypti Anopheles as A. arabinensis Coquillettidia.as C. juxtamansonia	Source: https://www.cdc.gov/
	Brugia timori and Brugia malayi	Mansonia Aedes
Subcutaneous filariasis	Loa loa filaria	Chrysops C. silacea C. dimidiata	Source: https://www.cdc.gov/
	Mansonella streptocerca	Midge (genus Culicoides)
	Mansonella ozzardi	Midge (genus Culicoides)
	Onchocerca volvulus	Blackfly (genus Simulium)
Serous cavity filariasis	Mansonella perstans	Midge (genus Culicoides) Blackfly (genus Simulium)	Source: https://www.cdc.gov/

Microscopic pathology

This video gives a brief explanation on the possible histopathological findings of soft tissue sample of case of filariasis: {{#ev:youtube|67zC7mXigpY}}