The Life Cycles of Parasites

Parasites as Agents Beneficial to Man.

Many parasites are distinctly detrimental to man himself or to the animals or plants on which he depends ; others are of no economic importance to him; but there are also forms which may be beneficial. They may be considered under the following headings: (I) The utilization of parasites as therapeutic agents is based upon the fact that acute attacks of fever are often beneficial to a host suffering from chronic and fatal disease. It has been found that general paralysis in man (caused by the spirochaete of syph ilis) subsides when the host is artificially inoculated with spiro chaetes of relapsing fever or with malarial parasites.

(2) Biological control of injurious organisms by means of para sites. Parasites play an important role in nature in checking the increase of numerous species of animals and plants which possess a great power of reproduction. The importance of parasites in attacking organisms detrimental to man has been recognized only in recent years. It has been found possible to protect these para sites, to cultivate them, to transfer them from one region to an other, and to use them as a powerful weapon directed against or ganisms harmful to man. A striking example of biological control by means of parasites of an injurious insect is afforded by the Hawaiian sugar cane leaf-hopper (see ENTOMOLOGY).

(3) Caprification or pollination of fig flowers by parasites. It is known that in the Smyrna variety of fig the receptacles contain only female flowers and pollination is brought about by a small gall forming insect (Blastophaga) which is parasitic in the wild caprifigs containing both male and female flowers.

(4) Parasites causing the formation of pearls. Pearls are formed as a reaction of the mantle epithelium of the oyster around some foreign body which acts as an irritant. In most cases the foreign body which stimulates the formation of the pearl is the larval stage of a parasitic worm. Thus, "The most beautiful pearl is nothing else but a brilliant sarcophagus of a worm" (Dubois).

(5) Useful products and substances of commercial value pro duced by parasites. Scale insects or Coccids, which live as parasites upon a great variety of plants, are responsible for the production of several substances of commercial value, such as cochineal dye stuff, the stick-lac (see LAC) from which shellac and a red "lac dye" are prepared, and the objects called "ground pearls" (q.v.).

Parasitism in the Past and Present Time.

Parasitism can not be considered as a mode of life only recently acquired by va rious organisms. Although parasites, owing to their adaptation, could hardly develop characters favourable for fossilization, a number of remains have been found revealing the existence of parasitism in very old geological periods. There is some evidence

that bacteria already existed in the pre-Cambrian period. Para sitic or "spot" fungi have been found in the remains of plants in as early as Carboniferous time. Fossil specimens of Rhinoceros show evidence of infection with Actinomyces (lumpy jaw). Par asitism of sea snails (Gastropods) upon sea-lilies (Crinoids) be gan in Silurian, became common in Devonian and reached a climax in the Carboniferous period. Insect galleries and galls have been found in plants from Cretaceous and Tertiary rocks. It is quite possible that parasites also played an important role in the ex tinction of several forms of mammals. Tsetse flies (Glossina) which transmit Trypanosoma brucei causing the "nagana" disease in mammals, in early Tertiary times were much more widely dis tributed and even reached the region of Colorado.

Man from very early times probably suffered from most of the parasitic diseases from which he suffers now. Calcified eggs of Bilharzial parasites have been found in the kidneys of Egyp tian mummies of the loth Dynasty (about 1200 B.C.) ; that is in the country in which Bilharziasis is so very common now. Tu berculosis in the form of Pott's disease was also found in a mum my of the priest of Ammon from an Egyptian cemetery dating from the 21st dynasty— I too B.C. Parasitic diseases have un doubtedly played an important role in the history of man. They undermined his health, lowered and dissipated his energy, re duced or destroyed the stock of animals and plants upon which he depended, and were often the direct cause of enormous losses of human life. Such diseases, especially malaria and hookworm, were in a great measure responsible for the disappearance of many human tribes, and for the fall of many very highly civilized empires. They played an important part in the history of military conquest as well as in the history of the commercial expansion of man. As recently as the 14th century Europe was swept by epi demics of plague which destroyed a quarter of the population. Parasites have hindered several human efforts in modern times. Thus, the construction of the St. Gothard tunnel was seriously hampered by hookworm disease ; and the first enterprise of the construction of the Panama canal failed utterly after great loss of life from malaria and yellow fever. It was only when these diseases had been conquered in the Panama zone that the work could be brought to a successful conclusion. (See also PESTS. )