Moved from ID Request.

Fever following envenomation is a sign that the immune system is taking an outside threat seriously, and generally suggests that the “adaptive” immune system is being turned on. As venom reaches the lymph nodes, the immune cells there may react (within in couple of hours) by dividing repeatedly. The lymph node will grow in size, and if it does this quickly, can cause an ache in the area involved. (The major lymph nodes are located in the arm pit if bitten on an arm, the groin if bitten on a leg.) It is rare for this to happen without some sign of inflammation at the site of the bite. For a temperature to go up within a couple of hours after envenomation, I would be surprised if there wasn’t some ache in a lymph node, but this is not a well studied phenomenon. The most painful lymph node ache that I have been told about (and was intrigued enough to try it out on myself) was from a puss caterpillar envenomation. It was very uncomfortable.

I see fever without aching lymph nodes most commonly in kids that are very sensitive to mosquito bites, but others describe it with inflamed tick bites. Again, those temperatures go up along with big reactions at the site of the bite. The slightly elevated temperatures are often accompanied by general tiredness for those kids. Any arthropod venom (and I include saliva as a venom) can cause this low grade immune activation, and it is not something to be worried about in and of itself. If there are other signs, away from the site of the bite, then that would potentially be concerning and it would be a good idea to get checked out by an allergist with experience in unusual envenomation reactions in that case.

If you have any pictures of the bite, I’d love to see them: venom@tulane.edu.

http://bugguide.net/node/view/26206

Genus Trachelas. They're not known to have a medically significant bite.