Progress in research on acute hepatopancreatic necrosis disease (AHPND)

A new emerging disease in shrimp, first reported in 2009, was initially named early mortality syndrome (EMS). In 2011, a more descriptive name for the acute phase of the disease was proposed as acute hepatopancreatic necrosis syndrome (AHPNS). Affecting both Pacific white shrimp Penaeus vannamei and black tiger shrimp P. monodon, the disease has caused significant losses in Southeast Asian shrimp farms. AHPNS was first classified as idiopathic because no specific causative agent had been identified. However, in early 2013, the Aquaculture Pathology Laboratory at the University of Arizona was able to isolate the causative agent of AHPNS in pure culture. Immersion challenge tests were employed for infectivity studies, which induced 100% mortality with typical AHPNS pathology to experimental shrimp exposed to the pathogenic agent. Subsequent histological analyses showed that AHPNS lesions were experimentally induced in the laboratory and were identical to those found in AHPNS-infected shrimp samples collected from the endemic areas. Bacterial isolation from the experimentally infected shrimp enabled recovery of the same bacterial colony type found in field samples. In 3 separate immersion tests, using the recovered isolate from the AHPNS-positive shrimp, the same AHPNS pathology was reproduced in experimental shrimp with consistent results. Hence, AHPNS has a bacterial etiology and Koch's Postulates have been satisfied in laboratory challenge studies with the isolate, which has been identified as a member of the Vibrio harveyi clade, most closely related to V. parahemolyticus.

Vibrio harveyi, which now includes Vibrio carchariae as a junior synonym, is a serious pathogen of marine fish and invertebrates, particularly penaeid shrimp. In fish, the diseases include vasculitis, gastro-enteritis and eye lesions. With shrimp, the pathogen is associated with luminous vibriosis and Bolitas negricans. Yet, the pathogenicity mechanisms are imprecisely understood, with likely mechanisms involving the ability to attach and form biofilms, quorum sensing, various extracellular products including proteases and haemolysins, lipopolysaccharide, and interaction with bacteriophage and bacteriocin-like substances.

Vibrio parahaemolyticus, a gram-negative marine bacterium, is a worldwide cause of food-borne gastroenteritis. Recent genome sequencing of the clinical V. parahaemolyticus strain RIMD2210633 identified two sets of genes for the type III secretion system (TTSS), TTSS1 and TTSS2. Here, we constructed a series of mutant strains from RIMD2210633 to determine whether the two putative TTSS apparatus are functional. The cytotoxic activity of mutant strains having a deletion in one of the TTSS1 genes was significantly decreased compared with that of the parent and TTSS2-related mutant strains. In an enterotoxicity assay with the rabbit ileal loop test, intestinal fluid accumulation was diminished by deletion of the TTSS2-related genes while TTSS1-related mutants caused a level of fluid accumulation similar to that of the parent. VopD, a protein encoded in the proximity of the TTSS1 region and a homologue of the Yersinia YopD, was secreted in a TTSS1-dependent manner. In contrast, VopP, which is encoded by a pathogenicity island on chromosome 2 and is homologous to the Yersinia YopP, was secreted via the TTSS2 pathway. These results provide evidence that V. parahaemolyticus TTSSs function as secretion systems and may have a role in the pathogenicity of the organism. This is the first report of functional TTSSs in Vibrio species. The presence of TTSS apparatus gene homologues was demonstrated in other vibrios, such as Vibrio alginolyticus, Vibrio harveyi, and Vibrio tubiashii, suggesting that some other vibrios also contain TTSS and that the TTSS has a role in protein secretion in those organisms during interaction with eukaryotic cells.