Further reports (no. 4) on flies as carriers of infection.

Date:
1911
Catalogue details

Licence: In copyright

Credit: Further reports (no. 4) on flies as carriers of infection. Source: Wellcome Collection.

    17/52 (page 15)
    Nature and Life-History of Parasitic Worms. In order to indicate the precise relationship which flies and other insects may bear to the dissemination of parasitic worms, it may be advisable here to recapitulate briefly the principal facts which are known concerning the mode of life and means of transmission of these worms. They belong to three principal classes, namely, Trematodes or flukes, Cestodes or tapeworms, and Nematodes or round-worms. These worms in their adult state live for the most part in the alimentary canal of vertebrate animals. Practically speaking all tapeworms live in the intestine, the great majority of round-worms and flukes live in the intestine, stomach or oesophagus, but certain varieties live in the lungs, the liver, the kidneys, the bladder and the blood and lymphatic vessels. In whatever situation they live, however, they all possess the common characteristic of being unable to multiply without some intermediate external in- fluence. They all produce a large number (in many cases an enormous number) of eggs, but before the latter can grow into adult worms, they must pass part of their life outside their host under certain definite conditions which vary according to the particular kind of worm. In most cases the eggs are conveyed out of the body in the faeces, in some cases in the urine or the expectoration. Some tapeworms throw off a segment containing eggs, which may pass out independently of the faeces. The chief exceptions to this method are the Filaria worms and their allies. With these, how- ever, the present investigation is not concerned. The conveyance of the eggs to the outside is only a short stage in the life of the parasite. Thereafter a more or less lengthy and complicated career awaits them before they are suited to reinfect their original host. This portion of their life history follows one of two broad lines : 1. The egg develops and in time produces a larva, which may be retained in the egg shell or set free, but in either case the larva is ready to reinfect. 2. The egg gives rise to a larva which enters an animal (intermediate host) different from that in which the egg was produced. In this second animal it passes part of its life and, in the event of this animal being devoured by the first, the parasite is enabled to complete its life-cycle and cause infection again. The avenue by which the first animal is infected is in the great majority of cases its food ; in a few instances it may be infected by the larva penetrating its skin. It is evident, therefore, that there must be some means by which the food is contaminated with excremental matter in the first of the above-mentioned categories and, in the second, some means by which the eggs of the parasite are conveyed to the second animal. Water is by far the most im- portant vehicle of transit. A certain amount of moisture is necessary for the development of the eggs although many of them can resist drying for long periods. In closely-associated communities, however, mechanical transit on the feet of individual animals is a common mode of dispersing the eggs. It is here that the agency of flies has to be reckoned with, especially such flies as divide their attention between excremental matter and food-stuffs. The common house-fly is well known to have such habits and it is thus with reason that it has been suspected of conveying the eggs