106 MISC. PUBLICATION 631, U. S. DEPT. OF AGRICULTURE 



The number of larval stages has been the subject of much disagree- 

 ment, but evidence very strongly indicates the presence of only three 

 stages (Knipling (70), Patton (107)). Others believe that there are 

 four or five, or, in other words, that the larva molts once or twice in 

 the course of its travels from the point of entrance to its final resting 

 place on the back. 



Larva. — The young larva is a soft -bodied, robust, fusiform grub, 

 somewhat broader behind than in front, and rather densely covered 

 with small spines, which seem to thin out, however, as the stage de- 

 velops. The spines are more densely set toward the anterior margins 

 of the segments. The mature grub (figs. 56 and 57) is a tough- 

 skinned creature that can hardly be crushed by pressure when squeezed 

 between the thumb and index finger ; it is rounded at both ends, and 

 bears rows and isolated islets of spines which can easily be felt by the 

 fingers as roughened areas. The mouth hooks, which are well 

 developed in the first stage, become rudimentary after the first molt. 



Etiology and Pathogenesis. — Numerous records are on hand of at- 

 tacks on man by Hypoderma bovis, H. lineatum, and H. diana. Most 

 case histories reveal some association with cattle during the summer 

 or fall preceding the attack. Parasitism is accompanied by severe dis- 

 comfort, and the results may be serious or even fatal. Adults are 

 often attacked, but the number of cases of hypodermyiasis among 

 children is proportionately very high. 



As in cattle, ingress is probably through the skin. The first symp- 

 toms usually occur during the winter months. Abdominal pains or 

 stomach disorders may be experienced; and as the larva wanders 

 through the subcutaneous connective tissues, severe discomforts, itch- 

 ing, pains, and cramps may be felt. The wanderings of the larva 

 may be extensive and rapid ; they are usually in an upward direction, 

 although the larva may temporarily pass down an arm or leg and 

 then retrace its course. The path of the larva may be traced by the 

 localized painful or swollen areas, or by indefinite reddish lines ; but 

 since, unlike Gasterophilus, its burrows are through the loose, subcu- 

 taneous connective tissues, it does not leave nearly so definite a trail. 



When the larva is ready to molt, it produces indurated swellings, 

 usually on the upper part of the chest or back, or on the head or neck. 

 Since conditions in the human body are abnormal for the larva, it 

 may move toward the surface several times in an attempt to find 

 just the right situation for the formation of the warble. The swelling 

 then becomes domelike and an opening develops through which the 

 larva may be expelled. If the larva is forced out of the warble, a 

 quantity of serous or pustular fluid is also expelled. This fluid has 

 been produced by the action of the spines of the larva and of the micro- 

 organisms that enter from the air ; it is upon this fluid that the larva 

 feeds during its later stages. 



The pain and discomfort accompanying parasitism may be severe 

 and the patient may be temporarily unable to use an invaded limb. 

 An apparently accelerated night activity of the larva may make sleep 

 difficult or impossible. Sometimes the results are more severe. In 

 the case of a boy in Montana, who was parasitized by seven larvae of 

 H. lineatum, almost complete paralysis of the lower extremities re- 

 sulted, and a year later it had not completely cleared up; this may 



