Page 109 - Medical Parasitology_ A Textbook ( PDFDrive )

102 10 Nematodes: Roundworms

Pathogenesis and Clinical Features
The pathogenesis of W. bancrofti infection is dependent on the immune system and
inflammatory responses of the host. Infection may present as asymptomatic, inflam-
matory (in acute phase) and obstructive (in chronic phase). Asymptomatic phase
may consist of high microfilaraemia. In the endemic regions, patients may show no
overt symptoms of lymphatic filariasis. In the inflammatory (acute) phase, the anti-
gens from the adult worms elicit inflammatory responses. It is characterized by high
fever, lymphatic inflammation (lymphangitis and lymphadenitis) and lymphoe-
dema. These symptoms subside after 5–7 days. Other symptoms that may occur
include orchitis and epididymitis. The obstructive (chronic) phase is caused by
blockage of lymph vessel and lymph nodes by the adult worms. The affected lymph
nodes and vessels are infiltrated with macrophages, eosinophils, lymphocytes and
plasma cells. The vessel walls become thickened and the lumen narrowed, causing
lymph stasis and lymph vessel dilatation. It may cause granuloma formation, with
subsequent scarring and even calcification. Inflammatory changes damage the
valves in lymph vessels, further aggravating lymph stasis. Increased permeability of
lymph vessel walls lead to leakage of protein-rich lymph into the tissues. This pro-
duces the typical hard pitting or brawny oedema of filariasis. Fibroblasts invade the
oedematous tissues, laying down fibrous tissue, producing the non-pitting gross
oedema of elephantiasis. Chronic lymphatic filariasis is also characterized by lymph
varices, lymph scrotum, hydrocele and chyluria (lymph in urine). Involvement of
the genitalia and chyluria are characteristics of W. bancrofti infection and not of B.
malayi infection. Microfilariae are not normally present in the chronic phase.
Elephantiasis affects men mainly in the legs, arms and scrotum. In women, the legs,
arms and breasts are affected. Elephantiasis in B. malayi infection involves the leg
below the knee. Incubation period is about 8–12 months.
Adult filarial worm contains endosymbiotic bacteria, Wolbachia spp. which has
a role in the pathogenesis of its infection and is also essential for worm fertility. It
has become a target for antifilarial chemotherapy.
In tropical pulmonary eosinophilia (TPE), there is a failure in the suppression of
immune response to microfilarial antigens so that microfilariae are filtered out and
destroyed in the lungs with allergic inflammatory reaction. TPE is a hypersensitivity
reaction to filarial antigen. Patient may present with low-grade fever, loss of weight
and pulmonary symptoms such as dry nocturnal cough, dyspnoea and wheezing.
Children and young adults are more commonly affected in endemic areas. There is
persistent eosinophilia. Chest X-ray shows changes similar to miliary tuberculosis.
It is associated with a high level of serum IgE and filarial antibodies. Serological
tests are usually strongly positive. Microfilariae are absent in peripheral blood. The
condition responds to treatment with diethylcarbamazine (DEC), which acts on
microfilariae. This condition may be caused by W. bancrofti, B. malayi, or by some
animal filaria species.

Diagnosis
1. Microscopic examination
Detection of microfilariae in thick blood film, chylous urine and hydrocele
fluid stained with Giemsa. It is best to collect ‘night blood’ samples between 10
pm and 2 am. When the microfilariae density is low, concentration techniques