A cosmopolitan parasite of <i>Rattus</i> in the Galápagos rodents raises conservation concerns

We draw attention to a recent study that identified the invasive nematode Mastophorus muris, typically associated with Rattus, infecting two endemic Galápagos rats on islands where no invasive rodents have been previously recorded. This unexpected finding raises concerns about undetected rodent introductions and highlights the urgent need for population monitoring and captive breeding efforts to prevent the extinction of these unique species, which represent distinctive lineages of Neotropical rice rats. A recent study by Gardner et al. (2025) investigated helminths of Galápagos rodents and identified the cosmopolitan nematode Mastophorus muris—commonly found in Rattus and Mus—parasitizing two endemic rodent species: Nesoryzomys fernandinae on Fernandina Island and Aegialomys galapagoensis on Santa Fe Island (Figure 1). These findings are concerning as the origin of M. muris in these rodents is unknown since there is no prior direct or indirect evidence of invasive rodents on either island. Moreover, these parasites and hosts were collected in 1999, and no subsequent field studies have been conducted on the rodents of Fernandina and Santa Fe islands since then (Gardner et al., 2025). In general, Galápagos rodents remain poorly studied, with the most comprehensive ecological data being decades old (Clark, 1980; Harris & Macdonald, 2007b). These circumstances highlight the urgent need for research-informed conservation action to prevent the potential extinction of these unique island rodents. Mastophorus muris is a spirurid stomach nematode known to parasitize the world's most common invasive rodents, including Rattus rattus, R. norvegicus, and Mus musculus. In addition, M. muris requires insect arthropods, such as cockroaches, as intermediate hosts for larval development, and it frequently parasitizes native rodents after contact with invasive rodents (Jost et al., 2024; Quentin, 1970; Rojas & Digiani, 2003). In the Galápagos, in addition to being present in Fernandina and Santa Fe Islands, M. muris has also been identified in Rattus rattus on Santa Cruz and San Cristóbal Islands (Gardner et al., 2025). Three plausible hypotheses may explain the presence of M. muris in the endemic rodents on Fernandina and Santa Fe Islands. (1) There are currently undetected introduced rodents present on both Fernandina and Santa Fe Islands. (2) Historically, invasive rodents colonized Fernandina and Santa Fe Islands, but they failed to establish. (3) No invasive rodents ever reached Fernandina and Santa Fe Islands; instead, infected intermediate hosts (i.e., arthropods) arrived by natural dispersal or were inadvertently introduced by humans (e.g., through tourism activities). These three probable scenarios have important implications for designing and implementing conservation actions to safeguard these unique rodents from extinction. A monitoring program is urgently needed to assess rodent populations' health, including the impact of M. muris, and detect possible invasive rodents. Given the unpredictability in how quickly encounters with invasive rodents and their parasites may drive endemic rodents to extinction, further action is also required. This may include the establishment of captive breeding colonies for N. fernandinae, A. galapagoensis, and the few other surviving endemic rodents of the Galápagos to provide insurance populations in the worst-case scenarios where the endemic species collapse due to interactions with introduced rodents and their parasites (Trillmich, 1986). The introduction of invasive rodents is a well-documented catalyst for the decline and extinction of endemic rodents in oceanic islands across the world, including the Canary Islands, the Galápagos, and the West Indies (Harris, 2009). The main mechanisms involve direct competition and disease transmission (Smith & Banks, 2014). On Santiago Island in the Galápagos, there is evidence that Rattus rattus outcompetes the endemic Nesoryzomys swarthi through aggressive interactions (Harris & Macdonald, 2007a), and competition with Rattus rattus likely contributed to the extinction of at least three species of Galápagos rodents (Brosset, 1963; Dowler et al., 2000). While disease transmission by R. rattus is increasingly recognized as a major factor in native rodent declines, the available evidence is sparse, probably due to limited efforts in holistic sampling and specimen archival of both rodents and their parasites (Galbreath et al., 2019). A notable finding is the extinction of Rattus macleari on Christmas Island, which was likely caused by infection with the cosmopolitan parasite Trypanosoma lewisi of the introduced R. rattus (Wyatt et al., 2008). Other parasites such as gastrointestinal helminths, like Mastophus muris, could also have detrimental effects on immunologically naïve hosts, but further research is needed to evaluate this possibility. The unexpected presence of Mastophorus muris in isolated endemic rodents underscores critical knowledge gaps and the urgent need for conservation interventions, including permanent population monitoring and captive breeding, to ensure the survival of these unique species. Jadyn Hartwig: Writing—original draft (lead). C. Miguel Pinto: Conceptualization, writing—original draft, writing—review and editing. Thanks to Violette Fonrose for preparing the figure, and Boris Herrera for editing it. Special thanks to Dolma Alonso, Scott Gardner, and Nicolás Moity for reading this manuscript. This publication is contribution number 2750 of the Charles Darwin Foundation for the Galapagos Islands. The authors declare no conflicts of interest. All data underlying this letter are available as part of the article by Gardner et al. (2025).