Setting: fictional history, kind of a mashup between the roman empire and early turkic peoples plus lots of made-up stuff. This particular scene is in a fairly remote desert, in early spring (very warm days, near-freezing nights). The characters are going to a market town on their own to trade some things, and have no way to contact anyone else.
Characters: Three teenagers, ages 13-15 approximately. The injured/dying one - we'll call her DC - is healthy until the injury. They all have basic first-aid knowledge and have dealt with everyday injuries. One of the other kids has watched her aunt (a healer) work and so is more knowledgeable, but has no special medical training.
Scenario: DC has an accident while riding her horse. The horse breaks a leg and has to be put down. DC ends up with a deep laceration or puncture wound on her arm from the fall, bleeding is stopped with pressure and bandages. They continue moving, but over the next few days DC's wound goes septic and she dies.
What unmistakable visual signs could I use to show untreatable sepsis and/or gangrene? It's a comic, so fever, skin texture, etc. aren't as easily shown. I'm thinking red streaks up the arm and chest, plus nasty necrotic swollen stuff at the wound site. How long would be reasonable to go from injury to death, and at what point would she likely be unable to continue traveling? Does about four days sound right?
Search terms - sepsis, wound infection signs, symptoms infection, gangrene, blood poisoning signs
I'm looking for an accident that will badly injure the horse, but allow the rider to jump or be thrown clear of the fall. Ideally it'd be something that can happen at a walk or trot since they're not going very fast. The horse is a well-trained mare, carrying a fair bit of weight if that makes a difference. I was thinking bad footing or gopher hole makes it trip? Or possibly something spooks it? I can find lots of information on general trail safety, but I'm looking more for the unavoidable-hazard-type thing. If necessary I can use something else, (snakebite?) as long as it's fatal. Once it's hurt, will it be able to get back up? I'd also appreciate any general tips on how it might act or appear during/after the incident.
Search terms - trail safety, horse accidents, horse leg break, fractures riding horses, horse fall injury