Some considerations on the phenomena of parasitism amongst protozoa / by E.A. Minchin.

  • Minchin, E. A. (Edward Alfred), 1866-1915.
Date:
[1910]
Catalogue details

Licence: Public Domain Mark

Credit: Some considerations on the phenomena of parasitism amongst protozoa / by E.A. Minchin. Source: Wellcome Collection.

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    alternation of hosts and a complicated life-cycle, which has been most completely studied in the case of the malarial parasites of man, transmitted by the agency of mosquitoes. 6. In some cases the parasite may penetrate the ovary of its host, pass into the ova, and thus infect the embryo and the next generation of the host. Transmission of this kind is known in a certain number of cases: for instance, in the ‘‘pébrine” disease of silkworms caused by Nosema bombycis. It is never the sole method of transmission, but is always supplementary to other methods. To turn now to the methods by which Protozoan parasites penetrate into new hosts. We find four known methods, which can be summarised very briefly after what has been stated already. 'The commonest is the method of casual or contamina- tive infection, where the host infects itself by taking up the propagative phases of the parasite accidentally from its surround- ings. Most usually infection takes place by way of the mouth, with the food; but it may be effected by the respiratory organs. Other modes of infection are the contagious, as in dourine already mentioned; the inoculative, as in malaria and other diseases caused by blood-parasites ; and the so-called hereditary method, as in Nosema bombycis and other cases. From the foregoing summary of the methods by which Protozoan parasites are propagated and spread from one host to another, it is clearly seen that there are only very few cases in which it is of direct advantage to the parasite to cause the death of the host, and even then only when the propagative phases of the parasite are fully matured. In cases where the parasite is disseminated by its host being devoured by another animal, it is necessary for the propagation of the parasite that the host should die a violent death, but not through the agency of the parasite; on the con- trary, the interests of the parasite are best served by the host remaining in good health until the fatal moment arrives, unless it be supposed that some weakening of the host renders it more liable to become the prey of its enemies. Blood-parasites, trans- mitted by the inoculative method, are not perceptibly benefited by the host’s illness, and certainly not by its death. It is, however, very necessary for this method of dissemination that the required
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