Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

Resume and Discussion of Experimental Findings

The principal positive result of the work has been the experimental reproduction of the lesion produced by the black-fly in characteristic histological detail by the use of preserved flies. The experimental lesions not only reproduced the pathological pictures, but followed a clinical course, which in local symptomatology especially, tallied closely with that of the bite. This the writer interprets as satisfactory evidence that the lesion is not produced by any living infective agent. The experiments performed do not identify the nature of the toxic agent. Tentatively they seem to bring out, however, the following characteristics.

1. The product of alcoholic extraction of flies do not contain the toxic agent.

2. The toxic agent is not inactivated by alcohol.

3. The toxic agent is not destroyed by drying fixed flies.

4. The toxic agent is not affected by glycerin, but is, if anything, more active in pastes made from the ground fly and glycerin, than in the ground flies as such.

5. The toxic agent is rendered inactive or destroyed by hydrochloric acid in a concentration of 0.25%.

6. The toxic agent is most abundant in the region of the anatomical structures connected with the biting and salivary apparatus (head and thorax).

7. The toxic agent is not affected by a 0.5% solution of sodium bicarbonate.

8. The toxic agent is not affected by exposure to dry heat at 100° C. for two hours.

9. The toxic agent is destroyed or rendered inactive in alkaline solution by a typical hydrolytic ferment, pancreatin.

10. Incomplete experimental evidence suggests that the activity of the toxic agent may be heightened by a possible lytic action of the blood serum of a sensitive individual, and that the sensitive serum itself may contain the toxic agent in solution.

These results, as far as they go (omitting No. 10), accord with Langer's except on the point of alcoholic solubility and the effect of acids. The actual nature of the toxic agent in the black-fly is left a matter of speculation.

The following working theories have suggested themselves to the writer. First, the toxin may be, as Langer believes in the case of the bee, an alkaloidal base, toxic as such, and neutralized after injection by antibodies produced for the occasion by the body. In such a case the view that a partial local fixation of the toxin occurs, which prevents its immediate diffusion, is acceptable. Through chemotactic action, special cells capable of breaking up the toxin into harmless elements are attracted to the scene. Their function may be, on the other hand, to neutralize directly, not by lysis. This would explain the rôle of the eosinophiles in the black-fly lesion. If their activities be essential to the destruction or neutralization of the toxin, one would expect them to be most numerous where there was least reaction. This would be at the site of a bite in an immune individual. A point of special interest for further investigation, would be the study of such a lesion.

Second, it is conceivable that the injected saliva of the fly does not contain an agent toxic as such. It is possible, that like many foreign proteins, it only becomes toxic when broken down. The completeness and rapidity of the breaking down depends on the number of eosinophiles present. In such a case immunity should again be marked by intense eosinophilia.

Third, lytic agents in the blood serum may play the chief rôle in the liberation of the toxic agent from its non-toxic combination. An immune individual would then be one whose immunity was not the positive one of antibody formation, but the negative immunity of failure to metabolize. An immune lesion in such a case might be conceived as presenting no eosinophilia, since no toxin is liberated. If the liberation of the toxin is dependent upon lytic agents present in the serum rather than in any cellular elements, a rational explanation would be available for the apparent results (subject to confirmation) of the experiment with sensitive and immune sera. In this experiment it will be recalled that the sensitive serum seemed to bring out the toxicity of the ground flies, and the serum itself seemed even to contain some of the dissolved or liberated toxin. The slowness with which a lesion develops in the case of the black-fly bite supports the view of the initial lack of toxicity of the injected material. The entire absence of early subjective symptoms, such as pain, burning, etc., is further evidence for this view. It would appear as if no reaction occurred until lysis of an originally non-toxic substance had begun. Regarding the toxin itself as the chemotactic agent which attracts eosinophiles, its liberation in the lytic process and diffusion through the blood stream attracts the cells in question to the point at which it is being liberated. Arriving upon the scene, these cells assist in its neutralization.

The last view presented is the one to which the author inclines as the one which most adequately explains the phenomena.

A fourth view is that the initial injection of a foreign protein by the fly (i.e., with the first bite) sensitizes the body to that protein. Its subsequent injection at any point in the skin gives rise to a local expression of systematic sensitization. Such local sensitization reactions have been described by Arthus and Breton, by Hamburger and Pollack and by Cowie. The description of such a lesion given by the first named authors, in the rabbit, however, does not suggest, histopathologically at least, a strong resemblance to that of the black-fly. Such an explanation of many insect urticariæ deserves further investigation, however, and may align them under cutaneous expressions of anaphylaxis to a foreign protein injected by the insect. Depending on the chemical nature of the protein injected, a specific chemotactic reaction like eosinophilia may or may not occur. Viewed in this light the development of immunity to insect bites assumes a place in the larger problem of anaphylaxis.