OUR KNOWLEDGE OF INSECT VECTORS OF DISEASE. 



161 



Notes on Table V. — -Diseases of Helminthal Origin. 

 With the exception of possible infection with certain helminths resulting from 

 the carriage by flies of helminth ova from faeces and subsequent deposition of the 

 ova on food, the insect-borne helminths all undergo part of their life-history in the 

 body of the insect vector. Thus the adult Filaria bancrofti live in the lymphatic 

 glands. The ova find their way into the blood stream, where on hatching they are 

 known as microfilariae, and some are taken up from the blood when a mosquito 

 bites a person harbouring the organisms. These, if they have entered the stomach 

 of Culex fatigans, or other intermediate host, soon make their way to the thoracic 

 muscles of the mosquito, where they undergo definite metamorphosis. When this 

 is complete (usually in from 16 to 20 days) the worms make their way into the 

 mosquito's proboscis, and when next it pierces the skin of some victim the filariae 

 burst through the proboscis-sheath and make their own passage through the skin, 

 from which they soon travel to some lymphatic gland, w r here they become sexually 

 mature, and the cycle is repeated. Similarly Dipylidium caninum passes part of its 

 life-history in the rat flea, and becomes sexually mature in the dog or man. The 

 ova are ingested by the larval flea, and infection by the cysticercoid stage follows 

 the accidental ingestion of the flea by the definite host. 



Table V. 



The more Important Insect-borne Diseases of Helminthal Origin. 



Organism. 



Host. 



Disease. 



• 



Vector. 



Dipylidium caninum 



Man and 

 dog. 



Taeniasis (tape- 

 worm). 



Dog louse (Trichodectes 

 latus),* dog flea (Ctenoce- 

 phalus canis).* 



Ova of certain helminths 



Man 



Helminthiasis (para- 

 sitic worms). 



Flies.* 



Microfilaria bancrofti 



Man 



Filariasis (elephan- 

 tiasis). 



Mosquitoes (Culex, Stego- 

 myia, Anopheles).* 



Loa loa 



Man 



Calabar swellings 



Horse-flies ( Chrysops dimi- 

 diata).* 



Filaria immitis 



Dog 



Dog filariasis 



Mosquitoes (Culex, Ano- 

 pheles).* 



Notes to Table VI. — Diseases directly attributable to Insects and Acarina. 

 In each case the disease results from the damage done by the insects and Acarina 

 in adopting existence upon the body of the host and living upon its tissues. The 

 time spent upon the host may cover the whole life-history of the parasite and many 

 subsequent generations, as with Sarcoptes scabiei and the Pediculi, or may only 

 embrace part of the parasite's development, as with the larvae of flies causing myiasis. 

 In either case the injury to the host may be so extensive as to cause death from the 

 loss of vital tissue, or the injury itself, although insignificant, may indirectly cause 

 death to the host by providing a suitable path for invasion by pathogenic micro- 

 -organism 8. 



