Many cisterns are infested with Guinea worm (filaria medinensis, Gm.).
Although several species belonging to the second class occasionally enter the bodies of water snails and other animals before reaching their definitive host, they undergo no alteration of form in this intermediate host; the case is different, however, in Filaria medinensis and other forms, in which a free larval is followed by a parasitic existence in two distinct hosts, all the changes being accompanied by a metamorphosis.
Filaria medinensis - the Guinea worm - is parasitic in the subcutaneous connective tissue of man (occasionally also in the horse).
Ten years later Manson discovered a second species, Filaria perstans, whose larvae live in the blood.
The adult stage of this form is the Filaria loa found in the subcutaneous tissues of the limbs.
It is invariably the result of some cause acting generally, such as renal disease, valvular defect of the heart, or an impoverished state of the blood; while a mere oedema is usually dependent upon some local obstruction to the return of blood or lymph, or of both, the presence of parasites within the tissue, such as the filaria sanguinis hominis or trichina spiralis, or the poisonous bites of insects.
In the case of filariasis due to Filaria bancrofti, which is common throughout the Tropics, the embryos of the parasite are disseminated by various Culicinae and Anophelinae (Culex pipiens in Queensland; C. fatigans in the West Indies; Myzomyia rossii in India; Pyretophorus costalis in a large portion of tropical Africa; &c.).
Six or seven species of mosquitoes are also the intermediate hosts of Filaria immitis, which infests the right auricle and pulmonary artery of the dog, and occurs throughout the tropics, in southern Europe, the United States of America, and elsewhere.