Hyperthermia can boost innate immune system in juvenile fish - Part 2

A providential temperature effect

Then in 2007, a chance effect happened: The heating system went wrong in one larval tank and the water temperature increased to more than 35 degrees-C. Surprisingly, the larvae – which were suffering an outbreak of VNN – began to recover.

A typical larval barramundi affected by VNN, with clinical signs: red spleen, empty digestive gut. In the center, VNN larvae recovering after heat shock. Digestive gut full and no red spleen. On the right, larvae recovered and has expanded chromatophores and a full digestive tract.

A systematic exploration of this effect on the “big belly disease” and the iridovirus was undertaken, and after many trials and errors, we adapted protocols to each pathogenic situation. One disease-one pathogen was definitely too simple an approach to deal with our fish health issues, aswe were facing a complex system with various pathogens acting at the same time, in synergy in a changing pathobiome that permanently distorted the clinical signs.

Our observations showed that non-lethal thermal shocks were ending the fish mortalities and were initiating an acquired resistance before the next pathogen encounter, in essence a “live vaccination.” 

Boosting the innate immune system

The underlying mechanisms are linked with the anti-stress responses of the heat shock proteins (HSP), which have been evolutionarily retained and passed on from bacteria to higher vertebrates because they are the main defense mechanisms of all the organisms to survive among the constant presence of evolving pathogens.

Innate immunity is the first reaction against pathogens, with the recognition of pathogen-associated molecular patterns (PAMPs) through toll receptors and others. These HSPs are induced at the cell level and have intracellular and extracellular expressions.

Intracellular HSP are companions for other proteins stabilizing cells against pathogen injury in reestablishing proper folding and homeostasis. And extra cellular HSPs are intercellular, signaling molecules that regulate the immune response, which induce specific immunity to infectious agents promoting several biological pathways to control the effect of pathogens.

It appears that the non-lethal heat shock may increase the HSPs effects – as well as those of other biological pathways whose interactions are poorly understood – on pathogens. They can provide the hosts the time needed to react and to counteract their pressure, and they are largely up-regulated when thermal shocks are effected near the lethal temperature, with effects lasting for 24 hours.

From theory to reality: Non-lethal heat shock and the pathobiome

Systematically applying non-lethal heat shock targeting the local community of pathogens can be helpful to many aquafarmers and very cost-effective in comparison to the vaccines, if they are available. And knowing the pathogens is not as important, as the treatment can be effective to control mortality outbreaks early on.

Treated juveniles acquire protection against the local pathogens, and this will be in line with potential mutations or changes in serotypes (groups within a single species of microorganisms, like viruses or bacteria, that share distinctive surface structures). If there are several pathogens acting in synergy, the HSPs can impact all of them, assimilating permanently into the pathobiome.

The non-lethal heat shock has not only an effect on the pathogens and on the hosts but also on all the other organisms present in the culture system, and particularly on all the bacteria from the gut of the organisms, the water or the biofilms. The relations between host and microbiome (all the microbes in a community) are essentials for the health of the organisms, but are complex, and the only way to assess the effects on such a system is the final result: the level of mortality.

At the field level, I advocate the use of this approach to produce fish juveniles of different species like tilapia, barramundi and grouper in the presence of VNN, as well as several iridoviruses and also the Scale Drop Disease (SDD). A recent trial with tilapia juveniles suffering the emerging Tilapia Lake Virus (TiLV) has been positive. And it also appears that when fish are suffering from a virus infection as well as some secondary bacterial effects like fin rot, the heat shock treatment is effective on both.

The objective is to toughen up the fish juveniles before their transfer to grow-out cages, to avoid disease outbreaks following the introduction of naive juveniles into a given pathobiome that is evolving in space and time.

How to do it

The first step is to determine the lethal treatment temperature according to the species and the stage of development of the animals. Water should be pumped directly into the nursery without any treatment (unless some pesticide or heavy metals toxicity is detected), and the juveniles should be immersed in the pathobiome. Water in the tanks should be heated near the fish lethal temperature, which for tropical fish is around 40 degrees-C. A general rule of thumb is about 10 degrees-C above the optimum temperature of the species.

The farmer’s knowledge about the clinical signs of disease outbreaks and the dynamic of the observed mortalities are critical to recognize the early signs of an outbreak, because the correct timing of the treatment is essential to maintain the initial mortality at low levels. Depending on the pathogens and the timing of their emergence, different treatment protocols can be applied at a given frequency for a certain period of time, generally for one week. This is a totally different health management strategy than to just kill/sterilize everything first.

Concurrent with fish experiments, we have also carried out other trials with the same approach for pathogen control in invertebrates, like WSSV in penaeid shrimp and with encouraging results; in oysters for the herpes virus; and with sea cucumbers for Skin Ulceration Disease. However, until now, protection for invertebrates is only acquired for a short period of time and does not last long enough to provide the full protection necessary for commercial production.

Perspectives

On fish like Asian sea bass and tilapia, several viruses and bacteria could be controlled by non-lethal heat shock treatment, resulting in more resistant juveniles.

The benefits of the treatment are seen relatively fast, and the results obtained so far could be applied to other pathogens and other fish species. Feedback from people already using these non-lethal heat shocks on a commercial level are encouraging, producing juveniles resistant to local pathogens.

Currently, there is not enough return on experience because very few farms are using this approach, and for a simple reason: you need to introduce the pathogens in your culture systems, which goes against all accepted biosecurity principles.

My objective is to challenge pathologists on the application of a simple temperature manipulation to boost the innate immune system through the upregulation of the HSPs. Much supporting scientific data are available in the scientific literature, and in fact, hyperthermia treatment is increasingly used in human cancer therapy.