**1.3 Pathology**

LF is a chronic and persistent disease, but the majority of infected individuals remain asymptomatic and do not develop clinical symptoms. However, LF can cause a broad spectrum of clinical manifestations including the most severe forms seen in patients with elephantiasis or hydrocele. The most common symptoms are lymphedema of the legs, lymphangitis, elephantiasis, and only in *W. bancrofti*-infected individuals, hydrocele [21, 22]. Interestingly, even asymptomatic patients display some degree of subclinical disease with microscopic haematuria/proteinuria, dilated lymphatics and presence of scrotal lymphangiectasia. The most prevalent symptoms are caused by the presence of worms and their products in the lymphatic system, which leads to the induction of endothelial cell proliferation and lymphangiectasia, the dilatation of lymphatic vessels. Furthermore, host proteins such as vascular endothelial growth factors, matrix metalloproteinases and angiopoietins are involved. Studies have demonstrated that lymphedema development is associated with genetic risk factors and nucleotide polymorphisms for genes encoding for the proteins mentioned above [23–25]. The dilated lymph vessels and the associated impaired lymph function leads to lymph fluids no longer being pumped against gravity, resulting in elevated tissue pressure. Progression of lymphedema can take many years during which leg skin thickens and loses elasticity, develop deep folds accompanied with development of dermatosclerosis and skin lesions. The lesions additionally can lead to secondary bacterial and fungal infections, further accelerating the development of chronic and severe lymphedema and elephantiasis [26–28]. Hydrocele is a common clinical symptom in men infected with *W. bancrofti*. Hydrocele is characterized by

#### *Biology of the Human Filariases DOI: http://dx.doi.org/10.5772/intechopen.102926*

fluid accumulation inside the tunica vaginalis and swelling of the groin and scrotal area. Acute hydrocoel is a result of worm death, both naturally and medically, leading to temporary clogging of the lymphatics due to disintegrating worms [29], whereas chronic hydrocoel develops after years of infection due to impaired lymph transport. A rare, but serious manifestation of LF is the so-called tropical pulmonary eosinophilia (TPE). TPE is caused by immunological hyperresponsiveness to MF in the lungs, associated with coughing and wheezing, extremely high eosinophil counts and high levels of serum IgE. TPE can lead to further manifestations such as lymphadenopathy as well as spleno- and hepatomegaly [30].
