The same thing has been observed by Macculloch in the Roman Campagna, where, he says (p. 124), it is remarked that, "if the laborers cut down certain plants (a bushy thistle chiefly), a fever, that would otherwise not have occurred, is the consequence."
In opposition to the mosquital origin of malarial disease it is known that numerous mosquito wounds may be inflicted without the occurrence of malarial disease; but this is by no means incompatible with the theory. We do not yet know whether the poison be mosquital saliva, or whether the fever-producing element be a bacillus with which the puncturing proboscis of the insect may be loaded at the time of inflicting its wounds. The scratch of a lancet will not produce vaccinia, unless the instrument be charged with vaccine matter; the puncture-needles of Pasteur would be harmless and impotent, did he not load them with infecting bacteria; so with dog-bites and hydrophobia, etc.
Nay, it may even turn out that, under certain circumstances, mosquito-bites shall even be protective against malarial disease, for as Pasteur and others are able to produce, artificially, "attenuated culture-fluids," the inoculation of which, while producing slight symptoms, protects from more serious phases of disease, so may there exist in nature naturally "attenuated" fever-poison fluids, the inoculation of which, by mosquital puncture, may produce trivial symptoms, and thus protect from more decided attacks of veritable fever. What product of man's art has not been anticipated by Nature? Hardly any.
In the absence of direct experiments with the mosquito as a fever producing agent, I have endeavored to ascertain if the geographical distribution of the insect had any relation with that of malarial disease. But the insect and the disease are both so wide-spread that it is difficult to find any locality entirely exempt from either. Tasmania, Singapore, New Zealand, Ceylon, and the Dismal Swamp of North Carolina, as well as the bog-country of Ireland, are said by some medical writers to be entirely exempt from fevers. Others discredit this statement. I do not know who is right; but I have endeavored to ascertain whether insects of the mosquito tribe were or were not rife in these localities. On this point there seems to be little scientific knowledge available. Of the insects of Singapore—a locality in which the absence of fever seems to be generally admitted—I have been able to find no account. With relation to Tasmania (Van Diemen's Land), where exemption from fever is again generally admitted, I have only been able to find, in the Capitol Library, one reference to its insects. This is from the work of James Bischoff (London, 1832, p. 33), who, quoting from Widowson's "Present State of Van Diemen's Land," says: "The insects are not so numerous or so annoying as in most other tropical countries. The ant, the mosquito, and a common green fly are chiefly seen. The mosquito does not sting so severely as in hotter climates."
