I'm very reactive to poison ivy, and lived in places (in the US) with both poison ivy and jewel weed, but the most useful thing I learned was to wash all potentially exposed skin and clothing and gear with soap as soon as possible after exposure. It really does prevent the rash. It doesn't have to be harsh soap, just any soap that cuts oil and wash it away. Dishwashing liquid, hand soap, anything. Urushiol oil is the irritant, and if you can wash it away soon enough, you don't get the rash. Because of knowing this, I never get poison ivy when I know I have been exposed, but the only times I've gotten it as an adult have been when I didn't know I was exposed. Like when I told my foreign friend "See this here? Don't touch it, even in jest, this is what it does, etc, etc," and three days later I got a rash. I just know he couldn't resist touching it, though he denies it to this day.
Maybe jewel weed relieves the itch, I don't know, but I feel sure that wiping jewel weed won't prevent the rash as well as simply washing with soap. Any old soap.
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.
My pie came with a little toothpic holding up this tiny ad: