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Background  Gastrointestinal nematodes (GIN) are ubiquitous in small ruminant farming, representing a major health and production concern. Given their differences in pathogenicity and the current problems regarding anthelmintic resistance, specific diagnosis of GIN is of significant importance. At present, the most widely applied method for this entails culture and microscopic analysis of third-stage larvae, allowing for identification at least to the genus level. Overall, a variety of keys for microscopic analysis have been published, showing substantial variation. Given this fact, this study aimed to produce a practical and updated guide for the identification of infective ovine GIN larvae.    Methods  Using existing keys and protocols, a total of 173larvae of the most common species/genera of ovine GIN from pooled faecal samples from Sardinia (Italy) were identified and extracted, and further individual molecular identification was performed. Morphometric and morphological data as well as high-quality photographs were collected and combined to produce the final guide.    Results  GIN microscopically and molecularly identified during this research include  Trichostrongylus  spp.,  Teladorsagia circumcincta ,  Haemonchus contortus ,  Cooperia curticei , and  Chabertia ovina.  Based on microscopic analysis, 73.5% of the larvae were correctly identified. Based on sheathed tail length, 91.8% were correctly classified into their respective preliminary groups.    Conclusions  It is crucial for the microscopic identification of infectious GIN larvae to examine each larva in its entirety and thus to take multiple characteristics into account to obtain an accurate diagnosis. However, a preliminary classification based on sheathed tail length (resulting in three groups: A, short; B, medium; C, long) was found to be effective. Further identification within group A can be achieved based on the presence of a cranial inflexion, caudal tubercles and full body measurements ( Trichostrongylus  spp. < 720 µm,  T. circumcincta  ≥ 720 µm). Larvae within group B can be differentiated based on sheathed tail morphometry ( H. contortus  > 65 µm,  C. curticei  ≤ 65 µm), the presence of cranial refractile bodies, total body length measurements ( H. contortus  ≤ 790 µm,  C. curticei  > 790 µm) and shape of the cranial extremity. Finally, all characteristics proposed for the differentiation between  Oesophagostomum  spp. and  C. ovina  larvae (group C) were found to have considerable restrictions.    Graphical abstractSupplementary Information  The online version contains supplementary material available at 10.1186/s13071-021-05013-9.

parasites and vectors

Practical guide for microscopic identification of infectious gastrointestinal nematode larvae in sheep from Sardinia, Italy, backed by molecular analysis