Check out the photo below. This was the result of an insignificant insect bite on my 15-year-old's arm a couple of months ago. He noticed a tiny bite on his wrist in the morning (he's a fit healthy teenager so no earthly reason to do anything about it - we all get the odd insect bite and it's no big deal). But at 8.30 that night, when he took his jumper off, he noticed a red line running up his arm, almost to his elbow. It just looked like a scratch. Half an hour later the “scratch” was 1cm wide, inflamed, no longer looking like a scratch. The bite itself was 4cm x 5.5cm, raised, red, hot to touch.
This is really serious and you need to act quickly once a bite gets to this stage. Even just the round, raised, hot bit at his wrist isn't a good sign and particularly with a baby or younger child I would have been racing off to the doctor with just that. But once the redness starts tracking up the arm, you shouldn't even be leaving it a few hours.
A course of antibiotics and it was on the mend in no time at all. But just be aware of this type of thing. It may well have been a whitetail spider (we do get them in the house from time to time) but it's actually irrelevant what the bite was from. Even a simple mozzie bite can turn really nasty. The red tracking is a nasty sign of infection from whatever caused the bite.
So the message here: insect bites, or any break in the skin - normally not a big deal. But keep an eye on these things. If the bite becomes increasingly inflamed, swollen, hot to touch, you should be seriously thinking about a GP visit. Once the tracking starts, as shown in the photo, you don’t want to be wasting any time. We were taken VERY seriously by the doctor - sent home with antibiotics and told that if it wasn’t already improving by the next day we were to return for IV antibiotics. That’s how seriously they take these things. And this was in a fit, healthy teenager. So be even more vigilant with a baby or very young child.
One other thing here: elevation seems to be key. My son had his arm in a sling for the next couple of days and elevation seems to make a huge difference in halting the tracking. It’s true also for elderly people who get cellulitis in their lower legs. Antibiotics are important for this, but keeping the limb elevated is a big part of the fix.