Microscopic and Molecular Identification of Sarcocystis Species in Wild Brown Rats (Rattus norvegicus) in Lithuania and Latvia

<i>Sarcocystis</i> is a genus of heteroxenous, globally distributed apicomplexan parasites found in reptiles, birds, and mammals. Typically, sarcocysts develop in muscles of intermediate hosts, and oocysts sporulate in intestines of definitive hosts. The parasite's life cycle is based on prey-predator relationships and usually involves two distinct host species. However, some <i>Sarcocystis</i> spp. can complete their full development within a single host species. Brown rats (<i>Rattus norvegicus</i>) are invasive, synanthropic, highly adaptable rodents that are true omnivores and opportunistic feeders. Therefore, it is possible that they can act as definitive hosts of <i>Sarcocystis</i> parasites. This study provides the first comprehensive assessment of <i>Sarcocystis</i> protists in brown rat intestinal samples under natural conditions, combining microscopy and molecular analyses. Of 27 brown rats investigated, 25.9% (7/27) of animals harbored oocysts/sporocysts of <i>Sarcocystis</i> spp. Based on nested PCR and sequencing of four genetic loci (<i>18S</i> rRNA, <i>28S</i> rRNA, ITS1, and <i>cox1</i>), 59.3% of samples were positive for <i>Sarcocystis</i> spp. Parasites identified were genetically similar to <i>Sarcocystis</i> spp. using bird-bird, bird-Carnivora, rodent-Carnivora, or ungulate-Carnivora as their intermediate-definitive hosts. The present study suggests that synanthropic rodents may facilitate cross-ecosystem transmission of these parasites, increasing infection pressure on livestock, companion animals, and wildlife in human-dominated environments. Future molecular and dietary ecological studies are needed to assess the role of synanthropic and opportunistic hosts, such as the brown rat, in the transmission of <i>Sarcocystis</i> spp.