Echinococcus granulosus sensu lato is one of the most important zoonotic parasites causing Cystic Echinococcosis (CE) in humans and animals and is frequently found in rural populations, especially in developing countries, causing significant economic losses (Thompson, 2017). Adult parasites are found in the small intestines of dogs, jackals, wolves and other canids, and their larval hydatid cysts are found in sheep, goats, cattle, pigs and many other domestic and wild mammals, as well as in various organs and tissues, especially in the liver and lung in humans (Simsek et al., 2006). In addition to its zoonotic significance, CE is listed among the 117 notifiable animal diseases of major importance by the World Organisation for Animal Health (WOAH, formerly OIE) (WOAH, 2019).

According to the current classification of the genus Echinococcus, five species are recognized: E. granulosus sensu lato, E. multilocularis, E. oligarthrus, E. vogeli, and E. shiquicus. The E. granulosus s.l. complex comprises several species, including E. granulosus sensu stricto (G1/G3), E. equinus (G4), E. ortleppi (G5), E. canadensis (G6/7 and G8-G10), and E. felidis (Vuitton et al., 2020).

A lipoprotein antigen known as Antigen B (AgB) is secreted from the tegumental cells of the protoscoleces in the hydatid cyst fluid and to a lesser extent from the laminar and germinal layer of the fertile capsules (Siracusano et al., 2012). EgAgB consists of several subunits with varying sensitivity for the serological diagnosis of CE. It is a highly immunogenic protein complex (120–160 kDa), which is subdivided into 8–12 kDa, 16 kDa and 20–24 kDa subunits (Siles-Lucas et al., 2017; Ebrahimipour et al., 2019). AgB is composed of 8 kDa monomers arranged in a lipoprotein structure and displays a high degree of polymorphism. Although AgB is suggested to play a key role in the host–parasite interaction, its precise biological function remains incompletely understood. It has been proposed that this antigen may inhibit host proteases, modulate immune responses, and bind lipids (Virginio et al., 2007; Chemale et al., 2005). The nature and quality of AgB can vary across different intermediate and definitive host species in endemic or hyperendemic regions. Therefore, the serological evaluation of AgB preparations derived from E. granulosus isolates of various hosts should take into account regional variations and local diagnostic capabilities (Pawłowski et al., 2001; Nasrieh et al., 2003).

Recent molecular studies have shown that AgB is encoded by a multigene family comprising at least five gene loci (Pan et al., 2010), designated as AgB1, AgB2, AgB3, AgB4, and AgB5 (Chemale et al., 2001; Arend et al., 2004; Haag et al., 2004). Comparative analyses of the diagnostic potential of antigens encoded by these genes have demonstrated that recombinant AgB2 provides promising results for serological diagnosis (Jiang et al., 2012). Numerous studies have also shown that E. granulosus exhibits considerable genetic diversity, encompassing various strains with variation in host specificity, developmental rate, and human infectivity (Tawfeek et al., 2009). However, the extent to which AgB gene polymorphism contributes to this diversity and influences diagnostic performance remains to be further investigated.

Studies using recombinant AgB have shown that this antigen elicits variable humoral responses among patients (Virginio et al., 2003). This variability is thought to be associated with polymorphisms in the composition and structure of AgB subunits (Monterio et al., 2008), as well as differential expression of genes within the AgB family (Haag et al., 2004). Furthermore, partial sequences of AgB1 and AgB2 have demonstrated strain-specific genetic polymorphisms, which may render these proteins prone to antigenic variation due to their close interaction with host molecules and cells (Rosenzvit et al., 2006). Further characterization of AgB polymorphisms and their impact on antigenicity is therefore essential to optimize serological assays and improve diagnostic accuracy.

Understanding the polymorphism in the AgB2 gene is essential not only to improve the sensitivity and specificity of serological tests for CE but also to gain insights into the parasite's potential mechanisms of immune evasion. AgB2 variants may alter epitope structure, influencing antibody binding and potentially leading to variability in humoral immune responses among hosts. These differences might help identify which antibody isotypes are preferentially stimulated during infection and could also inform the development of more effective diagnostic tools or even guide vaccine design. Therefore, the aim of this study was to characterize polymorphism in the AgB2 gene of E. granulosus s.l. isolates obtained from sheep in Elazig and Bingol provinces of Türkiye, and to investigate the effect of this polymorphism on the host's serologic response. Such insights could contribute to optimizing serological diagnostics and advancing the development of targeted immunotherapeutic strategies for CE.