Filaria immitis is a dangerous parasite of the dog, the adult worm living in the heart and veins of this animal. It is one of the species which has been clearly shown to undergo its development in the mosquito, particularly in Anopheles maculipennis and Aedes calopus (= Stegomyia). The larval form occurs in the peripheral blood, especially at night. When taken up by mosquitoes they differ from Filaria bancrofti in that they undergo their development in the Malpighian tubules rather than in the thoracic muscles. In about twelve days they have completed their growth in the tubules, pierce the distal end, and pass to the labium. This species occurs primarily in China and Japan, but is also found in Europe and in the United States. It is an especially favorable species for studying the transformations in the mosquito.
Filariæ are also commonly found in birds, and in this country this is the most available source of laboratory material. We have found them locally (Ithaca, N. Y.) in the blood of over sixty per cent of all the crows examined, at any season of the year, and have also found them in English sparrows.
In the crows, they often occur in enormous numbers, as many as two thousand having been found in a single drop of the blood of the most heavily infested specimen examined. For study, a small drop of blood should be mounted on a clean slide and the coverglass rung with vaseline or oil to prevent evaporation. In this way they can be kept for hours.
Permanent preparations may be made by spreading out the blood in a film on a perfectly clean slide and staining. This is easiest done by touching the fresh drop of blood with the end of a second slide which is then held at an angle of about 45° to the first slide and drawn over it without pressure. Allow the smear to dry in the air and stain in the usual way with hæmatoxylin.
Other Nematode Parasites of Man and Animals Developing in Arthropods
Dracunculus medinensis ([fig. 121]), the so-called guinea-worm, is a nematode parasite of man which is widely distributed in tropical Africa, Asia, certain parts of Brazil and is occasionally imported into North America.
The female worm is excessively long and slender, measuring nearly three feet in length and not more than one-fifteenth of an inch in diameter. It is found in the subcutaneous connective tissue and when mature usually migrates to some part of the leg. Here it pierces the skin and there is formed a small superficial ulcer through which the larvæ reach the exterior after bursting the body of the mother.