Vaccines may be the biggest tool in the fish health toolbox

With the current race to end the pandemic and recent approval of new COVID vaccines, an article on aquaculture vaccines just seemed to make sense! Vaccines have been around since the late 1700s and have revolutionized medicine by minimizing or even eliminating some diseases. It is not surprising that everyone is hoping a vaccine will be the cure to COVID-19 and the ticket back to normalcy.  Vaccines to prevent or minimize disease in humans, livestock, or even pets, are common but aquaculture vaccines are not used as widely and have been challenging to develop. Aquaculture is diverse and with a limited number of licensed vaccines available, producers often ask why or if they should vaccinate their fish. In this article, I provide a basic overview of what a vaccine is, types of aquaculture vaccines, vaccine delivery, herd immunity, and when to utilize a vaccine as a tool in your fish health management program.  

What is a Vaccine?
Vaccines consist of substances that usually mimic a pathogen or part of a pathogen to “trick” the animal’s immune system into thinking it is being attacked. The hallmark of an immune response in any animal is the ability of immune cells to distinguish between “self” and “non-self” or foreign invaders and eliminate the invader from the body. A vaccine has the advantage in that once the immune system is activated it develops certain proteins called antibodies that circulate in the blood and can specifically recognize and bind to a pathogen to inactivate or eliminate it from the animal. Therefore, vaccines prevent major disease outbreak before they occur. In aquaculture, vaccines (if available) should be part of a disease management program along with other tools such as antibiotics (used to treat bacterial diseases), other therapeutics (which can be applied water), feed additives (that may stimulate immunity in general), biosecurity, and optimum rearing conditions that minimize stress on the animals.    

Types of Aquaculture Vaccines
There are many types of fish vaccines but relatively few types are available for commercial use as fully licensed vaccines. The most common are bacterins consisting of killed or attenuated bacterial pathogens. The killed vaccines are safe, relatively simple to produce, and for some diseases can be quite effective. An attenuated vaccine contains a pathogen that is still alive but its ability to cause disease (or sickness) in an animal has been eliminated. These often are more effective than killed vaccines because a live pathogen can stimulate a stronger immune response in the animal.  Other vaccines that utilize parts of a pathogen (e.g. sub-unit, recombinant, DNA, or RNA) are less common but some are available and more are being developed by researchers. One example for aquaculture would be the APEX-IHN vaccine, a DNA vaccine licensed in Canada, and of course in humans the new Pfizer or Moderna COVID vaccines that use genetic material (i.e. mRNA) that when injected result in the production of the main coronavirus surface protein by your cells. You therefore see part of the virus without ever being exposed to it and in turn your body is triggered to respond to the “foreign” substance, produce antibodies that recognize and bind to this protein, and ultimately neutralize the real virus if and when it is encountered. 

Delivery of Fish Vaccines
Aquaculture vaccines can be delivered differently than those used for humans and other animals, where almost all are delivered by injection. Although there are many injectable vaccines used for fish, these are primarily utilized for high value species or when losses to a specific disease occur at larger sizes (e.g. when Atlantic salmon smolts are moved to seawater). For fish that are susceptible to a target disease during juvenile stages or the cost of injection is prohibitive, immersion vaccines are the most common. Immersion vaccination is less stressful on fish and can be administered to fish during earlier life stages. Another method is oral vaccination where the vaccine is mixed in or top-coated on the feed. This is the easiest and least stressful way to vaccinate fish but is usually less effective because the vaccine components must pass through the gut and can be degraded during digestion in the stomach. 

Herd Immunity
So, what does herd immunity mean and how does this relate to aquaculture vaccines? Herd immunity is when much of a population (animals, humans, fish, etc.) is immune to a specific disease. When this occurs, the pathogen that causes that disease (virus, bacteria, etc.) is unable to persist in a population of vaccinated animals and the disease is eliminated. This usually requires a large portion of the susceptible population to be vaccinated to see this desired effect. For aquaculture, where large populations of fish are in the water, it can have a dramatic effect especially on pathogens that require a host to survive (such as most viruses). It may not eliminate a pathogen if it persists in nature or within carrier animals, but if the population is vaccinated, the number of diseased animals shedding the pathogen into the water column is reduced. This can not only reduce disease on the farm but limit the potential spread to other susceptible fish in the environment. 

Why should I vaccinate my fish?
Vaccines have been and continue to be the most desired tools for prevention of severe disease outbreaks and their development has transformed human and animal health. In most cases, the decision to use an aquaculture vaccine is a financial one. When cost savings from the anticipated improvement in survival outweigh vaccine cost, then you should vaccinate. However, other reasons related to cost but also quality should be considered. Vaccines can improve growth performance and low-quality product concerns (e.g. deformities in surviving fish) by minimizing disease outbreaks. If antibiotics are commonly used to control specific diseases, vaccination reduces the potential development of antibiotic resistant bacterial strains. Public perception also favours vaccine use over antibiotics, and this can translate to higher value products and improved marketing opportunities. Bottom line is that if a vaccine is available it should be included as a tool in the “Fish Health Toolbox” for any aquaculture producer. 

For my next topic, I’ll be more than happy to answer questions or if you want advice regarding fish health, drop me a line at kcain@uidaho.edu.  

Ken Cain is a Professor at the University of Idaho who teaches courses on Fish Health Management, Aquaculture, and Wild/Hatchery Fish Interactions. Ken’s passion for fishing led him to a lifelong career in aquaculture and research focused on fish immunology and vaccine development. He continues to be motivated by a desire to solve problems that challenge the industry and impact fishery resources.