IntroductionSince the late 20th century, preserving biodiversity has become increasingly prominent. However, the idea of parasite conservation has gained a lot of attention, scientifically and in the media, only recently (Carlson et al., 2020; Windsor, 2021; Brian, 2023; Truter et al., 2023). Preserving parasites from rare and/or endangered wildlife host species may constitute a dilemma to conservationists (Stork & Lyal, 1993; Windsor, 1995, 2021). Additionally, the restricted amount of economic and human resources available for conservation limits the number of species that can be targeted by conservation plans. While many species receiving conservation attention are large and charismatic (Martín-López, Montes & Benayas, 2007), parasites are small and commonly negatively perceived, hence receiving less support for their preservation. Also, the interest of the parasite species may conflict with that of its host (e.g. when a parasite poses a threat to an endangered host species), raising the question “which of the two species “deserves” conservation efforts?”.Given the extent of the current extinction crisis and limited conservation resources, it may sound tempting to ignore parasites in conservation programs. Alongside (mostly unconscious) inactivity in parasite conservation, there are many instances of intentional parasite removal, directly or by drug administration, during conservation actions targeting their hosts (Duncan, 1989; Phillips & Scheck, 1991; Laubscher & Hoffman, 2012; Moir et al., 2012). This practice is relatively common and presumably seems justified to its executors because of the extinction risk of the host species, while consequences for parasite species are not considered. The most illustrative example is that of the critically endangered California condor (Gymnogyps californianus ) and its highly host-specific louse species (Colpocephalum californici ), where all remaining condors were captured and deloused, leading to the presumed extinction of this louse (Snyder & Snyder, 2000; Dunn, 2009). Similarly, the tick Amblyomma tuberculatum specific to the threatened gopher tortoise (Gopherus polyphemus ) is regularly removed before tortoises’ relocation to new sites, increasing the tick’s extinction risk. Even though ignored by conservation actions, parasites provide crucial ecosystem services (e.g. linking food webs, regulating host populations, reducing impact of toxic pollutants (Sures, 2003; Lafferty et al., 2008; Hatcher, Dick & Dunn, 2012)). Parasite extinction is not good news: their ecosystem services will disappear (Wood & Johnson, 2015), potentially impacting many free-living species (other than the hosts; Dobson et al., 2008). Beside their role in ecosystems, parasites also hold an intrinsic value, being part of genetic and species diversity and representing a (large) portion of evolutionary history (Lymbery & Smit, 2023). Parasites are expected to face similar threats as their hosts, such as pollution, habitat loss/fragmentation, and introduction (and invasion) of exotic species. In addition, a parasite’s life cycle may also be disrupted by the decline or extinction of its host species, leading to a risk of co-decline or co-extinction (Strona, 2015). To preserve the crucial role of parasites in ecosystems, it is therefore needed to face the challenge posed by the “parasite conservation dilemma” and find ways to include parasites in conservation actions without negatively affecting the conservation of other species (Stringer & Linklater, 2014; Carlson et al., 2020; Windsor, 2021; Lymbery & Smit, 2023). A challenge here is that artificial conditions in ex situ breeding programs can promote parasite outbreaks (Nath et al., 2021), in which case the parasites may become a threat to the survival of the target species.The European weatherfish (Cobitidae: Misgurnus fossilis ) – the only weatherfish native to Europe – is critically endangered in Flanders (Belgium), where it is the target of a breeding and reintroduction program by the Research Institute for Nature and Forest (INBO, Linkebeek, Belgium). The European weatherfish is a demersal freshwater fish native to central and eastern Europe, north of the Alps, and northwestern Russia (Kottelat & Freyhof, 2007; Hauer, 2020). It inhabits slow-flowing or stagnant rivers with dense vegetation and muddy bottoms (Kottelat & Freyhof, 2007). Populations of European weatherfish decreased in large parts of its native range (Hanel & Lusk, 2005) because of habitat loss (wetlands drainage, dam construction, weeding and dredging of channels), pollution, and invasion of two congeners from Asia (Northern weatherfish, M. bipartitus , and Oriental weatherfish, M. anguillicaudatus ). It is currently classified as extinct in Switzerland (OFEV / CSCF, 2022), nearly extinct in Denmark (Møller, Beck & Carl, 2012), critically endangered in Belgium and Austria (Zulka & Wallner, 2006; Maes, 2014), endangered in the Czech Republic, Germany, Slovenia (Lusk, Hanel & Lusková, 2004; RGN, 2022; Freyhof et al., 2023), near threatened in Slovakia (Koščo & Holčík, 2007), and vulnerable in Croatia, Hungary, the Netherlands and Poland (Keresztessy, 1996; Mrakovčić et al., 2008; Witkowski, Kotusz & Przybylski, 2009; Spikmans, 2014). The European weatherfish is a high priority species according to the Vienna Conservation Regulation and it is listed in Appendix III of the Bern Convention and in Annex II of the Habitats Directive 92/43/EEC.In 2023, a parasitosis outbreak hit juvenile weatherfish in the breeding facility of the Research Institute for Nature and Forest (INBO). Although the issue was solved by modifying rearing conditions, we assessed infection parameters of M. fossilis from the Flemish breeding program and characterised (both morphologically and genetically) their ectoparasites. We hypothesize that the housing conditions specific to juveniles, where mortality was observed, may be associated with a higher parasite infection prevalence and intensity compared to adults. To test whether these parasites are native in Belgium (similarly to its host) and to investigate whether their abundance has changed over a historical time frame, we also assessed infection parameters of M. fossilis from a historical collection and identified their ectoparasites. The findings have implications in the light of including parasites in conservation programs of their hosts.