The embryos of three species of Filaria infest the blood of man, chiefly in the tropics. One embryonic species is found in the blood of infested patients by day, one by night, and one during both day and night. The length of the embryos varies from 1 ∕ 75 to 1 ∕ 125 inch, and its width from 1 ∕ 3000 to 1 ∕ 3500 inch. The night embryo, which is the most common, is produced by the Filaria Bancrofti. This adult worm infests the lymphatic system of man, sometimes reaching a length of three to four inches. Its embryos may obstruct lymphatic vessels, causing obstruction of the flow of chyle (hence originates chyluria), and elephantiasis, in which enormous swelling of the legs and other parts ensues.
The nocturnal migration into the lymphatic vessels, and thence into the blood of the embryo of the F. Bancrofti, is an adaptation to the nocturnal habits of a particular mosquito (culex pipiens or ciliaris). When the mosquito bites an infested person, his proboscis removes some embryo filariæ, which are quickly transferred to its stomach. Some of these escape digestion, develop within the mosquito, and when the mosquito dies in water they bore their way out, and are subsequently swallowed by man.
It is essential, therefore, in order to prevent this disease to boil or efficiently filter all drinking water, and to prevent the access of mosquitoes to water. Persons infested with filariæ should sleep inside mosquito nets, in order that they may not, when bitten by mosquitoes, spread the disease.
Tape-worms are found infesting the alimentary canal of man. Each has a double phase of existence. In the first, the characteristic head, or scolex, along with a bladder-like body, lies embedded in the solid tissues of an animal; in the second, the strobilus or tape-worm, occupies the alimentary canal of another animal. The tape-worm consists of a number of flat segments, each of which is capable of producing a large number of eggs, from each of which a six-hooked embryo is developed. The segments escape from the alimentary canal, and their ova are discharged and scattered broad-cast. These eggs are swallowed by another animal, the hooked embryo escapes from its case, migrates into the solid tissues, and there produces a scolex. When the host is eaten by another animal or by man, the scolex enters the alimentary canal, loses its bladder-like body, and developes a chain of segments. It follows from the above that two distinct hosts are necessary to complete the cycle of existence of these creatures, one being commonly a herbivorous, and the other a carnivorous animal. Thus:—
| Cystic Form. | Tape-worm Form. | |
| Cysticercus Cellulosæ in the muscles of the pig | becomes | Tænia Soluim in the alimentary canal of man. |
| Cysticercus Bovis in the muscles of the ox | „ | Tænia Mediocanellata in the alimentary canal of man. |
| Cænurus Cerebralis of the sheep’s brain | „ | Tænia Cænurus in the alimentary canal of the dog. |
| Echinococcus of man, etc. | „ | Tænia Echinococcus in the alimentary canal of the dog. |
The cysticercus cellulosæ has been already described (page [23]). The cystic form of the dog’s tape-worm (echinococcus) is a most dangerous parasite for man. When the egg of the dog’s tape-worm is swallowed by man, the embryo escaping from this egg burrows from the alimentary canal, and forms large cysts, chiefly in the liver, but occasionally in the lungs, brain, and other organs. For the removal of these, surgical interference is required. This form of cyst is commonly known as a hydatid. It is most frequently seen in Iceland and Australia, though not uncommon in this country. Its frequency depends largely on the number of dogs, and on the facility with which the ova of their tape-worms can gain access to water.
The adult Tape-worms are usually derived in man from eating meat containing the cystic form. The cysticercus of the pig produces Tænia Solium; that of the ox, the Tænia Mediocanellata.
These are the two most common forms of tape-worm in man. The minute head of T. Solium has four suckers and a double row of hooklets, 28 in number; while the head of T. Mediocanellata has four suckers but no hooklets. The segments of T. Solium are smaller than of T. Medioc., and the structure of the segments of the two is somewhat different.
Preventive Measures.—In avoiding the various Entozoa described, it is important (1) to carefully avoid all underdone meat. The eating of smoked sausages, or of meat which is not cooked throughout, is a common source of tape-worm and of trichinosis.
(2) All vegetables should be thoroughly washed: this is especially important in the case of water-cress, lettuce, etc., which are eaten raw.