Re-reading the OP, talk of water splashing. To me, that is a indication the waste hose is not against the hydrant. I would think that anything on the hose would go with the flow, not upstream to the hydrant. And I would think the amount of nasty on the outside of hose would be limited. What is inside should stay inside, until it gets to the end. The OP said nothing about it, was the end still in the drain? To me, that would be the only issue.
Now a dumb truck driver's thoughts. As long as we have walked other animals have also dumped on the ground. And our bodies have adapted to that mess. Of course, more of us on the ground, washing and other cautions are needed. But, I do not worry much about what is in most semi-wild areas like parks or campgrounds. Now a lawn? A crop field? Do you know what chemicals have been spread on that? And then to think that most chemicals are produced based on "Will it sale?" not "Is it safe?"