Acute hepatopancreatic necrosis disease (AHPND), a relatively new farmed penaeid shrimp bacterial disease originally known as early mortality syndrome (EMS) has been causing havoc in the shrimp industry. The shrimp production in AHPND affected regions has dropped to ~60%, and the disease has caused a global loss of USD 43 billion to the shrimp farming industry. The conventional approaches, such as antibiotics and disinfectants, often applied for the mitigation or cure of AHPND, have had limited success.
Additionally, their usage has been associated with alteration of host gut microbiota and immunity and development of antibiotic resistance in bacterial pathogens. In this review, the most important findings of AHPND are highlighted, discussed and put in perspective, and some directions for future research are presented.
In the past, chemical and antibiotics have been commonly used in the shrimp culture system to control bacterial diseases including AHPND. However, the excessive and indiscriminate use of antibiotic has resulted in the development of antibiotic-resistant microbes, which may have potential risks for consumer health globally.
“Probiotics, phage therapy, use of plant-based compounds and environmental manipulation might be applicable in shrimp culture system to control the AHPND (alone or in combination).”
Bacterial diseases have brought socio-economic and environmental unsustainability to the shrimp aquaculture industry during the last decades. Vibriosis, an important bacterial disease, caused by opportunistic Vibrio spp. continues as the most serious threat to shrimp farmers in the region.
Apart from “classical” vibriosis, some Vibrio spp. are also responsible for causing acute hepatopancreatic necrosis disease (AHPND), originally known as early mortality syndrome (EMS).
AHPND, having a devastating impact on the shrimp aquaculture industry, develops quickly, starting approximately 8 days post stocking and severe mortalities (up to 100%) occur within 20–30 days. Hence, in this review at first an overview of the current knowledge on acute hepatopancreatic necrosis disease (AHPND) is given, including the disease associated gross signs and histopathology changes.
Later, the current status on management/ mitigation solutions for acute hepatopancreatic necrosis disease (AHPND) with respect to shrimp aquaculture is summarized.
Acute Hepatopancreatic Necrosis Disease (AHPND)—An Overview
The shrimp production in AHPND affected regions has dropped temporarily to ~60% and has resulted in collective losses exceeding an estimated USD 43 billion across and in Mexico. AHPND affects multiple species of shrimp including commercial species, Penaeus monodon, Litopenaeus vannamei and Macrobrachium rosenbergii and crustacean model Artemia franciscana.
AHPND is characterized by severe atrophy of the shrimp hepatopancreas accompanied by unique histopathological changes at the acute stage of disease. Furthermore, as disease progress massive sloughing of hepatopancreatic or digestive tract epithelial cells in the absence of any accompanying pathogen can be observed within approximately first 30 days of shrimp post-larvae stocking.
Gross Signs and Histopathology of AHPND
At the initial phase, shrimp exhibits signs of damage in the hepatopancreas and in the gut there is partial or total absence of food. During the acute phase, no bacterial cells are observed in the AHPND-affected tissue, which suggests that AHPND causing bacteria secreted binary toxins might be responsible for mediating AHPND in shrimp at later stage of infection.
“The AHPND-affected shrimp exhibit signs of anorexia and lethargy with empty digestive tract and loss of tissue pigmentation. The hepatopancreas becomes atrophied and whitish in appearance.”
During the terminal phase, the damage of tissue is mostly done by PirAVP and PirBVP toxins. However, bacterial proliferation at the site of damage is caused by secondary bacterial infections, possibly by a vibriosis.
Causative Agent of AHPND
The AHPND is caused by specific strain of bacteria, e.g., Vibrio parahaemolyticus, V. punensis, V. harveyi, V. owensii, V. campbelli and Shewanella sp. that contains pVA1 plasmid. The PirAVP and PirBVP are the primary virulence factor of AHPND causing bacteria that mediates AHPND etiology and mortality in shrimp.
Supplementation of PirABVP toxin has significant antagonistic interaction on in vivo virulence of V. campbellii, V. parahaemolyticus, V. proteolyticus and V. anguillarum strain in the same model. One of the factors that might interfere with virulence of Vibrio spp. is the digestive physiology of the host animal.
The binding of PirABVP toxin with epithelial cells of the digestive tract, might have induced immunological response in brine shrimp larvae, which subsequently prevents the attachment and entry of pathogenic bacteria and decreases the in vivo virulence of Vibrio species. Therefore, it appears that PirABVP toxins will not always aggravate vibriosis.
Vibrio parahaemolyticus as a Causative Agent of AHPND
In shrimp aquaculture, V. parahaemolyticus is an important aquatic pathogen and several strains are capable of causing acute hepatopancreatic necrosis disease (AHPND) and other important disease resulting in significant economic losses.
AHPND-causing pVA1 plasmid reported to contain two plasmid mobilization genes and a group of transfer genes for conjugation, the plasmid has been reported to mobilize to other Vibrio strains and even non-Vibrio spp. These processes explain the huge possibility of conversion from non-pathogenic to pathogenic AHPND strain that positively enhance the spread of AHPND.
It is also remarkable to mention that all human pathogenic V. parahaemolyticus strains produce thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH), as the main virulence factors.
Control and Management of Acute Hepatopancreatic Necrosis Disease (AHPND)—Current Status
The prophylaxis measures to control AHPND mainly focus on pond management (aeration, feeding, etc.) and disinfections before shrimp post-larvae stocking. The conventional approach applied so far in the mitigation or cure of V. parahaemolyticus AHPND strains, such as interrupting feeding or application of antibiotics and disinfectants has had limited success.
“Most of the therapeutic and control measures developed mainly targets AHPND-causing V. parahaemolyticus.”
However, the presence of AHPND-causing pVA1 plasmid encoding the binary toxins named PirAVP and PirBVP in non-Vibrio parahaemolyticus and even on non-Vibrio species has generated concerns since the management measures used to control a particular AHPND causing bacterial strain may not be useful and can generate unwanted economic pressure to farmers.
Therefore, the management measures adopted, based on presence or absence of PirABVP toxins in the shrimp and aquaculture system, can be more suitable to control and eradicate AHPND from shrimp culture system.
Probiotics have emerged as promising alternatives for improving disease resistance in farmed shrimp against AHPND. The beneficial effect of probiotic microorganism is generally influenced by several factors related to rearing conditions under larger scale, survival ability until reaching the gastrointestinal tract of the host, method of administration, dosage, probiotic strain and shrimp species.
Maintaining a biological balance among bacteria and algae in aquaculture ponds and gastrointestinal tract of shrimp is one of the ways to reduce the effect of AHPND in shrimp. Probiotics can participate in establishing a balance of gastrointestinal microbial flora, improving the digestive functions and immune system and increase the survival of L. vannamei against the pathogenic V. parahaemolyticus AHPND strain.
The emergence of bacterial antibiotic resistance problem in animals and humans, the use of phages as a therapeutic agent (shows an effective bacteriolytic activity) is advantageous as it is natural and relatively inexpensive, without serious or irreversible side effects reported to date.
Jun et al. results showed that the pVp-1 phage can infect 90.9% of V. parahaemolyticus AHPND strains and further demonstrates bacteriolytic activity against three strains, known to be highly pathogenic. Following prophylactic and therapeutic treatment, pVp-1 phage- treated shrimps exhibit significant recovery from AHPND histopathological lesions.
These results highlight that phage could be suitable for prophylactic and/or therapeutic use against AHPND-causing V. parahaemolyticus.
Plant-Derived and/or Natural Compounds
In recent years, plant-based compounds are identified to possess the property of inducing heat shock protein within the animal in a non-invasive manner. These compounds/ molecules are also commonly called as heat shock protein inducers (Hspi).
Functionally, the protective function of Hsp70 is documented to be due to its molecular chaperone activity maintaining protein homeostasis by protecting the nascent polypeptides from misfolding, facilitating co- and post-translational folding, assisting in assembly and disassembly of macromolecular complexes and regulating translocation.
Therefore, natural compounds/molecules can be used to induce Hsp70 production in host and provide protection against biotic and abiotic stress.
Aquatic bacteria are often subjected to fluid shear and hydrodynamic forces, created by either natural factors or anthropogenic activities such as the use of aerators and pumping devices frequently used to enhance shrimp productivity. AHPND V. parahaemolyticus strain has two phenotypic forms and shaking condition determines the existence of phenotypic form.
Hence, designing methods that can induce phenotype switching in AHPND-causing V. parahaemolyticus in an aquaculture setting will open the possibility for effective management of AHPND in shrimp farming, without necessarily removing the AHPND-causing bacteria from the culture system
Growing shrimp in a biofloc system can be a promising alternative strategy to improve environmental conditions and health status of cultured animals.
The basic principle of the biofloc system is to recycle waste nutrients, in particular, inorganic nitrogen resulting from uneaten feed and feces into microbial biomass, which can be used in situ by the cultured animals or be harvested and processed into feed ingredients.
Apart from serving as protein and lipid sources these aggregates flocs can contain microbe-associated molecular pattern (MAMP) and microbially bioactive components such as carotenoids, vitamins, glutathione, antioxidants and minerals, which nutritionally modulate the shrimp health and immune response and result in better growth performance and increased resistance against pathogenic microbial infections.
The above-mentioned management practices including probiotics, phage therapy and plant-derived compounds have shown promising results to control the outbreak of AHPND in shrimp. Pre-stocking and post-stocking measures, including evaluation and screening of the health status of post-larvae, feed quality assessment and disinfection of input materials is helpful to control AHPND in shrimp farms.
“Apart from management measures, the polyculture system has been identified as a potential strategy to control AHPND in shrimp farms.”
The results demonstrated that the presence of predator fish, multiple shrimp species or high stocking density in culture system contribute to increased risk of AHPND infections. However, alternative approaches like polyculture, water ageing (≥ 7 days long) and delay in feeding after stocking were likely to promote protection against AHPND in shrimp.
Conclusions and Future
The AHPND affected shrimp show unique histopathological changes, including massive sloughing of hepatopancreatic epithelial cells without any accompanying signs of a pathogen, which demonstrates the involvement of bacterial secreted binary PirAVP and PirBVP toxins in inducing AHPND.
Moreover, recent studies have demonstrated that, apart from PirABVP toxins, the AHPND associated strains have other specific virulence factors that might be involved in virulence of AHPND-causing bacteria and disease pathology.
The above-mentioned management approach discussed in this review, including, probiotics, phage therapy, use of plant-based compounds and environmental manipulation might be applicable in shrimp culture system to control the AHPND (alone or in combination).
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “NECROSIS HEPATOPANCREÁTICA AGUDA (AHPND): VIRULENCIA, PATOGÉNESIS Y ESTRATEGIAS DE MITIGACIÓN EN LA ACUICULTURA DEL CAMARÓN” developed by: VIKASH KUMAR – ICAR-Central Inland Fisheries Research Institute (CIFRI) – Ghent University, SUVRA ROY – ICAR-Central Inland Fisheries Research Institute (CIFRI) – Ghent University, BIJAY KUMAR BEHERA – ICAR-Central Inland Fisheries Research Institute (CIFRI), PETER BOSSIER- Ghent University Y BASANTA KUMAR DAS – ICAR-Central Inland Fisheries Research Institute (CIFRI).
The original article was published on JULY 2021, through MOLECULAR DIVERSITY PRESERVATION INTERNATIONAL (MDPI) under the use of a creative commons open access license.
The full version can be accessed freely online through this link: https://doi.org/10.3390/toxins13080524.