Most diseases in aquaculture are an interaction between:
- Host
- Environment
- Pathogen (an organism capable of causing disease such as viruses, bacteria, fungi, parasites)
Adverse environmental conditions such as low dissolved oxygen, high levels of ammonia interacting with the host can lead to non-infectious diseases. There are some pathogens that can cause diseases to occur without the host being exposed to environment stress. These types of pathogens are obligate pathogens.
Obligate Pathogens
Obligate pathogens must infect a host in order to survive. The majority of pathogens that cause the most significant economic impact and pose the greatest threat to aquaculture are caused by obligate pathogens. Crayfish plague is an example of an obligate pathogen caused by a fungal disease that is capable of causing disease even in optimum environmental conditions. Read more about Crayfish plague here.
Exotic diseases are diseases caused by a pathogen that does not normally reside in a particular area and/or the population of concern has no previous experience with it. Introduction of an exotic disease can lead to epizootic outbreak (i.e. affecting a large number of animals within a geographical location) because the host has no previous experience with the pathogen so it’s immune system doesn’t have the ability to handle the disease.
The herpesvirus which caused pilchard deaths in 1995 and 1998 from Carnavon, Western Australia to Noosa Heads, Queensland is an example of the devastating impact exotic diseases. In 1998 this disease wiped out 70 % of West Australian wild pilchard stocks.
Notifiable Diseases
In Australia, each State and Territory has it’s own list of ‘notifiable’ animal diseases that contain diseases of either national or State/Territory concern. If you suspect your stock is infected with a notifiable disease you are legally required to immediately notify their relevant state or territory animal health authority. Notifiable disease list for Western Australia can be downloaded here!
The Role of Stress in Disease
Most diseases in aquaculture aren’t cause by obligate pathogens or exotic disease but by an interaction between the host, pathogen and the environment. Environmental stress decreases the host’s resistance to disease and increases the host’s susceptibility to pathogens that wouldn’t normal cause problems.
Reducing stress is the key in preventing most disease outbreaks in aquaculture. Read this article on the role of stress in fish disease and leave a comment to answer the following questions:
- What impact does stress have on fish?
- How do fish cope with stress?
- How can you prevent stress in fish?
5 responses so far ↓
The overall impact is it lowers a fish’s immune sytem and its tolerance to other Diseases that the stock may encounter in its natural or man made enviroment.
Fish respond in severla ways,they release (battle ready)hormones into thier system they try to adjust to thier changing environment as well as having stress hormones trying to counter act what is happening to them.All of this would effect the stock in general especially if the source of stress was effecting all the fish currently being held,so if any diseases were to eneter the water system this would in turn take advantage of already weakend fish from the stress they are already are trying to cope with.
The first and foremost would be to ensure everything that you as a Fish Farmer can be done is being done,for example good work practices good management of stock and the holding tanks,procedure in place to ensure the water quality remains within the optimum levels at all times and if it falls outside those levels a back up system to get it back on track as fast as possible.
Teach any staff if you are at that level or practice this yourself as well good hygien and when transporting fish enure you have all the required equipment and tanks etc are of a good size,the less amount of stress that you cause to the fish the better.Try to keep the fish in an enviroment as close to thier narual one or the water if possible.Feed the fish the correct amounttoo much feed may increase other water parimeters that you dont want to have increased.
Stress is one of the main contributors to disease in fish. Stress is defined as physical or chemical factors that cause bodily reactions that may contribute to disease and death. Fish subjected to stress conditions trigger a reaction in the fishes body which results in a series of changes to the fish’s body systems. This is a natural response in order for the fish to adapt to the stress condition but can only last for a period of time and eventually energy reserves are depleted and the fish will be prone to infection by disease. These bodily changes include a blood sugar increase initiated by the adrenal gland to produce a burst of energy to prepare the fish for an emergency situation. As well, the inflammatory response (a defence used to fight against invading organisms) is blocked by the same hormones released from the adrenal gland. Water balance of the fish (Osmoregulation) is disrupted, increasing its energy requirements for osmoregulation. Respiration increases, and reserve red blood cells are released into the blood stream.
One of the easiest ways to prevent disease in your fish is to minimise stress. We can do this by maintaining good water Quality e.g. monitor and maintain ph, ammonia and nitrite within optimum levels, keep adequate levels of dissolved oxygen and avoid overstocking. Minimise handling of fish and avoid damage to their skin, scales and protective mucus membrane. Adopt and use sterile and safe practices when handling and transporting fish. Do not over feed or under feed your fish, overfeeding will only lead to poor water quality and under feeding will weaken the fishes immune system.
@Scott and Peter
Both excellent answers.
Majority of the cases fish pathologist are sent relate to diseases caused by stress. Often the key factor is the farmers not appreciating the optimium water quality parameters for their species. Just simple things like monitoring the ammonia, nitrite, pH, alkalinity and nitrate in closed systems.
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