Acute Hepatopancreatic Necrosis Disease (AHPND) is among the most devastating bacterial diseases in shrimp aquaculture, causing severe and continued global economic losses. The Type VI Secretion System (T6SS), a bacterial nanomachine involved in interbacterial competition and virulence against hosts, is reported to be present only in strains of Vibrio parahaemolyticus that cause AHPND. This study examined this association in further detail in Vibrio parahaemolyticus PH1339 and PH1273 and V. campbellii PH1401 and PH1409 isolated from Philippine shrimp farms with AHPND outbreaks. Whole genome analysis reveals that these strains encode a number of T6SS clusters (T6SS1, T6SS2, and T6SS3). Importantly, comparative genome analysis revealed that the T6SS1 cluster was present exclusively in AHPND-causing strains (PH1339 and PH1401), whereas the T6SS2 was found in all strains, and the T6SS3 was present only in strain PH1401. The T6SS1 core genes are highly conserved, but the effector and immunity (E/I) loci exhibited low intra- and interspecies sequence identity. Having a wide variety of E/I loci allows bacteria to interact with their hosts, providing different toxic payloads that are crucial to bacterial competition and virulence.
To further examine the association between AHPND and T6SS, we screened 54 publicly available Vibrio genomes and found that T6SS1 is detected in all AHPND strains (encoding pirAB⁺), including V. campbellii. These findings suggest that the acquisition of the pirAB genes is dependent on the presence of the T6SS1, highlighting its potential role in AHPND virulence. The T6SS1 gene cluster could then be used as a genetic marker to identify Vibrio strains associated with AHPND, thus improving disease detection and surveillance.