Ten years later Manson discovered a second species, Filaria perstans, whose larvae live in the blood.
A, View of the heart of a dog infested with Filaria immitis Leidy; the right ventricle and base of the pulmonary artery have been opened: a, aorta; b, pulmonary artery; c, vena cava; d, right ventricle; e, appendix of left auricle; f, appendix of right auricle.
The adult stage of this form is the Filaria loa found in the subcutaneous tissues of the limbs.
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.
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.
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.