A bat can carry a hidden infection without giving researchers an easy way to check. This study finds that the bat fly riding on its body can act like a tiny, non-invasive warning signal. Parasites related to Polychromophilus infect bats worldwide, including temperate regions.

In Europe, two main kinds are usually linked to different bat groups, but both have also been found in other bat families. The unresolved question is how often those parasites move beyond their usual bat hosts, especially where different bat families gather together.

The investigation used bat flies to look for these infections instead of taking blood from bats. That works like checking a coat for traces left by its wearer: the fly can carry evidence of what it encountered. The researchers focused on shared roosts, asking whether infections would occur in flies from R.

ferrumequinum alongside Miniopterus, or independently in that host. They also hypothesized that even accidental spillover could introduce both parasite species into each host-and-fly system and let them persist when both bat species were present. The study covered roosting sites in Serbia and Bosnia and Herzegovina, including places shared by the two bat species and places used by only one of them.

The bats were released after processing, while their flies were collected and identified before being checked for parasite DNA. The main parasite was found at six of the nine locations and in all three kinds of bat flies taken from one bat species. The flies also carried several genetic forms of the parasite.

Some individual flies carried evidence of more than one form, pointing to high parasite diversity and efficient spread across the study area. The parasite’s genetic patterns are shared across different bat-fly species rather than sorting neatly by host, suggesting these flies may exchange the same parasite lineages instead of carrying strictly separate ones.

Infection was much more common in the two Penicillidia fly species than in the third fly species, although fewer flies of those kinds were examined. The researchers suggest that larger blood meals or more frequent feeding could help explain the difference.

Together with their ability to carry both European parasite species and their broad range of bat hosts, these flies show how mixed cave communities could allow infections to spill over. A single fly from the second bat species tested positive, but that result has two possible explanations: the fly may have been infected, or it may simply have carried blood from a recently infected bat.

The study therefore cannot confirm either where that infection came from or whether this fly species can pass the parasite on. The researchers also suggest that barriers may limit the parasite's persistence in R. ferrumequinum and in its associated bat flies.

The study adds information about where these parasites occur and how genetically varied they are in European bats and their flies. It also records one possible spillover into a fly associated with a different bat species. Most importantly, collecting bat flies offers an efficient alternative to taking blood from bats, making wider monitoring of bat infections possible with less disturbance to the animals.

Bat flies revealed widespread and genetically varied parasite infections, plus a possible jump into a different bat setting. That gives researchers a practical way to monitor bat health without drawing blood.