Industry Review
53
Iridovirus in Ghana: A case of partnerships across the globe A new approach of immunotherapy linked with upregulation of anti-stress heat shock protein and other biological pathways for tilapia. By Alain Michel
W
ith travel restrictions during this pandemic, it is the internet that is helping to settle disease challenges. It has been matching scientific knowledge and fielding answers. This is a story of how the internet can be used to help another 6652km away to use upregulation of antistress heat shock protein to arrest the emergence of an iridovirus in tilapia.
Preamble
It is clear that the control of pathogens is key to developing a sustainable aquaculture project at a commercial scale and this is particularly true in tropical conditions where high ambient temperatures are accelerating triggers for diseases. Aquatic organisms, from invertebrates to vertebrates, live in a world full of viruses and bacteria. To survive in such environments, they are equipped with defence mechanisms in their immune system. But this is a learning machine which has to be taught. Fish have an innate immune system, which is the first line of defence. It is mostly a cell system able to react quickly as soon as some cellular receptors are flashing to indicate stressful conditions. But they also have a memory system, an ability to remember the first encounter with a pathogen and to immediately develop a counterattack.
“There is no doubt that pathogens will always be there in the field. The reality is that it is a necessity to learn to live with them.”
One pathogen, one disease?
The idea of one pathogen one disease cannot explain what is being observed at the field level, and the pathobiome concept, which is the pathogens in their environment including the other pathogens, is more relevant. And recently, some genomic research have demonstrated well the direct impact of some viruses on outbreaks of opportunistic bacteria resulting in high mortality. Furthermore, we are always densifying the populations in a limited space, providing the pathogens a nice playground. It should be understood that there is always a strong hostpathogen relationship, with both continually co-evolving. It is an illusion to think we can eradicate them when they are, in fact, a permanent component of the ecosystems.
Live with pathogens
In this article, I focus on a recent case study highlighting the efficiency of heat shock treatment to enhance the immune system of the hosts and to control the mortalities. This is with the tilapia in Ghana where there was an emergence of an iridovirus around the end of 2018 and early 2019.
Emergence of an iridovirus in Ghana: an internet story
In early 2020, a biologist A, in charge of a tilapia farm in Ghana called his friend B running a tilapia farm in Sumatra. A recounted to B of a sudden and huge mortality in the cages of the farm he was working at. During the discussion, B suggested that A call another marine biologist C (me) in Paris who has worked for many years to mitigate outbreaks of viruses and bacteria in a barramundi farm in Indonesia. Following this advice, some hours later A was describing to C the clinical signs he observed in his tilapia. A suspicion of an iridovirus was the outcome, and some days later, it was confirmed by PCR analysis on some samples from Lake Volta. C then advised A of the approach he had developed with the production of barramundi or Asian seabass (Lates calcarifer) juveniles over 10 years. Back then, C faced different kinds of pathogen outbreaks, including an iridovirus type infectious spleen and kidney necrosis virus (ISKNV). C described that he used a non lethal short duration heat shock repeated every day until the mortality was under control. He also suggested that A start preliminary trials on some hundreds of juveniles and fry coming from cages or from the hapa nursery where they were also observing high mortality rate 2-3 weeks after the sex reversal procedure.
History of pathogens in tilapia in Ghana
In mid-2018, a scientific survey funded by the World Bank investigated the health status of the tilapia farms on Lake Volta, mainly in search of a newly emerging virus. At that time, the tilapia lake virus (TILV) was already suspected to be the cause of regular and significant mortalities. However, the study concluded that: • TILV virus was not detected. • Farms are accustomed to these mortalities and no severe impact of social and economic consequences was observed on the local community. • Various bacteria were detected with a predominance of Streptococcus agalactiae. • Biosecurity measures were very poor and hatcheries, being at the top of the production pyramid, have a high potential to spread infectious diseases to all farms.
There is no doubt that pathogens will always be there in the field. The reality is that it is a necessity to learn to live with them.
July/August 2021 AQUA Culture Asia Pacific
