DNA (Deoxyribonucleic acid) Vaccine Technology in Aquaculture/ Fisheries

By:- Bijayalaxmi Nongmaithem / Ajit Kumar Ngangbam *

INTRODUCTION

Aquaculture involves the farming of any aquatic organisms which includes fish, molluscs, crustaceans and other aquatic plants. It is the production of aquatic animals and aquatic plants under controlled conditions. This type of farming implies some scientific intervention in the rearing process to enhance production for example maintaining regular stocking, proper feeding and protection from predators.

Aquaculture is the fastest growing animal food-producing sector with per capita supply increasing from 0.7 kg in 1970 to 7.8 kg in 2006 with an average annual growth rate of 6.9 percent. The production level shows a huge change from 1 million tonnes per year in the early 1950s, to 51.7 million tonnes production in 2006 with a value of US$78.8 billion.

World aquaculture is dominated by the Asia–Pacific region, which accounts for 89 percent of production in terms of quantity and 77 percent in terms of value. China remains the largest producer in the world, with reported fisheries production of 51.5 million tonnes in 2006.

The share of China is 69.6 per cent in production and 51.2 per cent in value. India has emerged as the world's second largest producer of farmed fish but its total production is merely one-twentieths that of China and the share of India, on the other hand, is 7 per cent only in total global production.

The fishery sector needs to increase its global production and efficiency to meet the increasing consumer needs for fish and other related products. Infectious diseases have been a major hindrance to the development and profitability of fisheries sector. From this point of view, vaccines offer the most efficient way to control infectious pathogens as with compared to other treatment and the current vaccine products have only been successful against only some fish pathogens.

These are mostly bacterial, and there are still several important diseases, mainly of viral and parasitic origin, for which no prophylactic treatment exists. DNA-based vaccination could overcome various problems which are normally associated with traditional methods of immunization.

DNA vaccines as when compared to traditional vaccines have several practical and immunological advantages that make them very attractive for the fishery sectors. Immune responses to DNA vaccines have only been poorly characterized in fish because of the limited knowledge of the fish immune system and the less number of researches on DNA vaccine.

Other than optimizing the efficiency of DNA vaccines, many other important issues, such as safety and production cost will be determinants for the potential application of this DNA vaccines technology in fish farms.

DNA VACCINES TECHNOLOGY IN FISHERIES

DNA vaccines can also be considered as the advanced or the third generation vaccines which are made up of a small, circular piece of DNA that is genetically engineered to produce one or two specific proteins from a micro-organism. The vaccine DNA is injected into the cells of the body, where the host cells reads the DNA and converts it into pathogenic proteins.

Since these proteins are recognized as foreign, when they are processed by the host cells and displayed on their surface, the immune system is alerted, which then triggers a range of immune responses. The concept of DNA vaccines was developed from failed gene therapy experiments.

The first demonstration of a plasmid-induced immune response was when mice inoculated with a plasmid expressing human growth hormone elicited antibodies instead of altering growth. DNA vaccination is the method of administrating the gene encoding the vaccine antigen rather than the antigen itself.

The Subsequent expression of the antigen by cells in the vaccinated hosts triggers the host immune system and illicit both the cell mediated and humoral mediated immune response. The primary function of DNA vaccines, a bacterial plasmid DNA containing a construct for a given protective antigen, is to establish the specific and long-lasting protective immunity against diseases where conventional vaccines fail to induce protection.

Although development of DNA vaccines has been attempted for various pathogens in a number of different fish species, the DNA vaccines against the infectious haematopoietic necrosis virus (IHNV) and viral haemorrhagic septicaemia virus (VHSV) remain the most efficient and also the most extensively analysed to date.

The vaccines against rhabdovirus diseases in fish have given some of the most promising results. The development of DNA vaccines against some fish pathogens like Aeromonas sp. is in progress. DNA vaccines against other types of fish pathogens have so far had limited success.

The most efficient delivery route at present is Intramuscular injection, and suitable delivery strategies for mass vaccination of small fish still need to be developed. In terms of safety, no adverse effects in the vaccinated fish have been observed till date.

As DNA vaccination is a relatively new technology, various theoretical and long-term safety issues related to the environment and the consumer remain to be fully addressed, although the risks should not be any greater than with the commercial fish vaccines that are currently used.

The present classification systems lack clarity in distinguishing DNA-vaccinated animals from genetically modified organisms (GMOs), which could raise issues in terms of licensing and public acceptance of the technology.

SAFETY OF THE DNA VACCINE

From the safety point of view, three important aspects must be considered; this includes the environment, the consumer and the vaccinated animals. In almost all of the experimental and clinical DNA vaccination experiments conducted so far no serious side effects on the host individual have been reported.

DNA vaccines is based on purified plasmid DNA carrying only a single gene from the pathogen are found to be non-infectious and are unable to replicate within the vaccinated host so there is no risk of transferring the actual disease with the vaccine. DNA vaccines are very much safer than conventional vaccines, i.e. inactivated whole virus, with or without oil adjuvant, or attenuated live virus.

Other factors that make DNA vaccines preferable are that inactivated whole virus vaccines may contain unknown impurities and trace amounts of inactivating agents, while live attenuated vaccines pose a risk of infection by mutants or may revert to virulence. Investigations to date suggest that the injected plasmid DNA does not integrate into the genome of the host cells.

The chance of integration into the germ line is in all probability an even rarer event. Till now no DNA vaccines have been licensed yet, the major challenge remaining is to develop an appropriate set of regulatory requirements for this DNA vaccine.

The Food and Drug Administration in the United States of America and the European Agency for Evaluation of Medical Products have prepared some guidelines concerning DNA vaccines in general and veterinary DNA vaccines in particular. Several relevant issues, such as requirements on composition and safety testing, are covered, but no specific restrictions in terms of application of DNA vaccines are given.

CONSTRUCTION OF DNA VACCINE