Demonstration of the adult dog (Canis lupus familiaris) as definitive host of Sarcocystis aucheniae under natural conditions.

Sarcocystis aucheniae is a coccidian parasite responsible for macrocystic sarcocystosis in South American camelids (SACs), causing economic losses due to carcass condemnation and restrictions on meat commercialization. While the domestic dog (Canis lupus familiaris) had been previously proposed as definitive host of this parasite based on experimental infection of puppies, evidence of adult dogs or other canids fulfilling this role under natural conditions was lacking. This study was thus designed to evaluate the presence of S. aucheniae infective stages in environmental fecal samples of canids. A total of 101 canid fecal samples from alpaca-breeding fields in Arequipa and Puno, in the southern Andes of Peru, were examined by sucrose flotation and microscopy. Sporocysts compatible with Sarcocystis spp. were detected in 17 samples (16.8%). Molecular analysis using a species-specific semi-nested PCR targeting the 18S rRNA gene identified S. aucheniae DNA in eight of these samples. The corresponding canid species were determined by sequencing a fragment of the mitochondrial D-loop region, which confirmed Canis lupus familiaris as the source of all S. aucheniae-positive feces. These findings provide the first field-based molecular evidence that adult domestic dogs naturally excrete S. aucheniae sporocysts, thereby completing the parasite's life cycle in SAC production systems. This discovery has major epidemiological implications, highlighting dogs as key reservoirs in the transmission cycle and as targets for control strategies. Preventing dogs from feeding on raw SAC meat and promoting proper carcass disposal are essential measures to reduce environmental contamination and transmission risk. The results shown in this study attempt to close a critical gap in understanding the biology of S. aucheniae and inform future control efforts in the Andean region.