BIOASSAY MODELS FOR PATHOGENICITY STUDY OF ACUTE HEPATOPANCREATIC NECROSIS DISEASE (AHPND) CAUSED BY Vibrio spp.

Sridevi Devadas*1, Sanjoy Banerjee1, Fatimah Md. Yusoff1, Subha Bhassu2, and Mohamed Shariff Mohamed Din1
 
1Laboratory of Marine Biotechnology, Institute of Bioscience, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia.
2Genetics and Molecular biology (ISB) and Centre for Biotechnology for Agriculture (CEBAR), 50300, University Malaya, Kuala Lumpur, Malaysia.
gs45950@student.upm.edu.my

Acute hepatopancreatic necrosis disease (AHPND) is caused by a unique strain of Vibrio parahaemolyticus (VPAHPND), carrying one or more extrachromosomal ~70kbp plasmids (pVA1) that encode homologues of the Photorhabdus insect-related (Pir) binary toxins (PirAvpand PirBvp). However, two other non-V. parahaemolyticus species, Vibrio harveyi and Vibrio campbellii have also been identified as carrying pVA1-like plasmids, indicating that these plasmids can be detected in other Vibrio species. It is crucial to perform bioassay challenges in order to determine the pathogenicity and to verify Koch's postulate of an isolated strain associated with the disease. Hence, this paper presents an overview of bioassay models for pathogenicity study of the AHPND causing-Vibrio spp. To facilitate future experimental work on AHPND and other similar shrimp diseases.

The VPAHPND is determined as a causative agent of AHPND through an immersion (Model A) and reverse gavage (Model B) treatments. It has been shown that immersion and reverse gavage resulted in 100% mortality accompanied by typical AHPND pathology. Shrimps subjected to reverse gavage treatment survive longer than those that undergo immersion treatment. However, infected shrimps from both treatments recorded similar AHPND histological lesions.

Outbreak in Mexico was confirmed using the immersion model together with the per os (Model C) treatment. The per os method using contaminated pellet resulted in 100% mortality and showed the presence of AHPND histological lesions within 3 days of the treatment. Moreover, there have been indications that ingestion of the VPAHPND strain could be the natural route of infection - in which case oral feeding may well produce more reliable results than the reverse gavage method.

No detailed model of the cohabitation is available for AHPND. However, the cohabitation model (Model D) is suggested based on existing experimental designs from other shrimp diseases. Prior to the cohabitation challenge, it is important to design appropriate routes of infection (Model A/B/C) to infect naïve shrimp before using them in transmitting the disease.

Intramuscular Injection (IM) is not recommended for AHPND bioassay since the method fails to reproduce AHPND in the challenged shrimp.