I have never heard of anything like this. Perhaps the manufacturer could tell you how frequently this is reported, and whether they know why. The original studies focused on ticks, but could not incriminate them, nor stable flies, house flies, deer flies,etc. As I say, E. risticii is a strange one, and I am currently trying to get ticks to transmit it. The fecal-oral route is a very promiscuous way of perpetuation.
Cholera is the classic example of the explosive nature of fecal-oral transmission.
This is a very interesting observation that should be followed up. The literature is full of these seemingly unrelated cross-protective observations. For example, giving mice glucan, zymosan, Listeria, or BCG protects them against fatal murine malaria. None of these things are even closely related to protozoa. But they all stimulate a T cell response which seems to have broad antimicrobial activity.