Fish health is key to maintaining sustainable production over time. This section is intended to serve as a general guide for maintaining healthy stocks.
Good fish health results from the interface between adequate biosecurity, sanitation, animal welfare, husbandry, proper housing and equipment usage, nutrition, immunization, and general disease prevention, control, and monitoring.
This section is intended to serve as a general guide for maintaining healthy stocks. Fish reared in different environments have slightly different requirements. Since maintaining good fish health depends on many factors, the details for proper procedures should be provided by a fish health professional.
This section is intended to be used only as a reference for fish health management. The end user is responsible for the use or misuse of this information and Troutlodge does not accept liability for the results of the use of this reference information.
Figure 1. Effect of vaccination on the amount of antibiotic used
Total sales, in tons of active substance, of antimicrobial veterinary medicinal products for therapeutic use in farmed fish in Norway in the period 1981 - 2014 versus fish produced with biomass (Slaughtered)
A basic understanding of the process of how diseases occur in fish populations is needed to adequately design a fish health program aimed at the avoidance of such disease conditions. In its most basic sense, fish health results from the interactions between three main factors: host animal, any pathogenic organisms present, and the environment in which they interact (Figure 2).
Even though pathogens may be present in a system, unless they encounter a susceptible animal in an environment conducive to their establishment within that animal, they may coexist without a disease occurrence. Any disturbance or imbalance in any of these three factors, however, tips the scales towards a disease condition.
How diseases develop?
Figure 2. Relationship of pathogen, host and environment.
Disease occurs when the three interact to bring the host in contact with pathogens in an environment conducive to pathogen growth.
There are many diseases that can affect trout at all ages. Some diseases are vertically transmitted (from broodstock fish to egg and from egg to alevin) whilst others are laterally transmitted (from fish to fish). Many diseases of trout can cause very high mortality making it uneconomic to continue farming.
[Important]: It is essential to ensure that your egg supplier can certify freedom of disease in the eggs and any certification should be provided by an independent authority.
It is not within the scope of this hatchery guide to give detailed information on the many diseases or possible treatments and only the most common diseases are covered below. Diseases can generally be broken down into 3 categories.
1. Viral diseases such as:
2. Bacterial diseases such as:
3. Environmental Diseases such as:
The descriptions of the various diseases below is only a brief outline and the advice of professional veterinary or fish health managers should always be sought. Many diseases can only be properly identified in a laboratory.
IHN - Infectious Haematopoietic Necrosis
Infectious haematopoietic necrosis (IHN) is an infectious viral disease of trout. Most if not all salmonid species are susceptible to the virus, with fry and small fingerlings becoming infected very readily, and becoming more resistant as they mature.
The infection is often lethal and the mortality rate can be 100% in fry. Those fish that survive an outbreak of IHN can become carriers of the virus, providing a reservoir of infection. In addition, infected juveniles will shed IHN virus particles in the faeces, urine and external mucus.
IPN - Infectious Pancreatic Necrosis
Infectious pancreatic necrosis (IPN) is an infectious viral disease that affects numerous species of fish including trout in many different parts of the world. The causative agent, infectious pancreatic necrosis virus (IPNV), has also been isolated from shellfish. The disease is also sometimes called “pin-head” disease as the bodies of young fish become emaciated given them a pin appearance.
IPNV is a very robust virus and can be transmitted in fresh and salt water, on equipment, and can survive in silage waste and in the gut of birds and mammals, allowing it to be transmitted in faeces. Movement of equipment from infected sites should be avoided, and mortalities and other wastes should be regarded as highly infectious.
Apart from the pin head appearance clinical observation may be characterised by a swollen abdomen and faecal casts (mucous) trailing from the vent. Internally there may be pancreatic necrosis.
Although there is currently no registered treatment for IPN work is well progressed on developing a vaccine against IPN.
VHS - Viral Hemorrhagic Septicemia
Viral haemorrhagic septicaemia (VHS) is historically a disease of farmed rainbow trout in fresh water in continental Europe. However, disease outbreaks in the marine environment have recently been recorded in other marine species. VHS causes high mortalities and seriously affects production. Typical outbreaks occur in spring and autumn at temperatures generally below 14°C resulting in an acute to chronic disease. All ages of fish can be affected although young fish are more susceptible.
A wide range of clinical signs may be observed. These include pale gills, dark body colour, fluid in the body cavity, bulging eyes and in some cases an intermittent period of erratic spiralling behaviour and rapid respiration. Haemorrhage is often evident in the eyes and skin, and also in the viscera.
The virus can be transmitted by diseased fish, by non-symptomatic carriers, and can be found in gonadal fluids of broodstock. Birds, blood-sucking parasites and equipment may also be a source of infection.
VHS is an OIE (World Organisation for Animal Health) notifiable disease. As with many viral diseases of fish, there is no specific treatment or cure for VHS
BKD – Bacterial Kidney Disease.
Bacterial kidney disease (BKD) is a chronic bacterial disease first reported in wild Atlantic salmon populations in Scotland in 1933.
Outbreaks can occur throughout the year, but generally accompany rising water temperatures in the spring. Infection can result in significant mortalities in trout and nearly all age groups of fish can be affected, although the disease is rare in very young fish. Losses are generally chronic, occurring over an extended period.
Clinical signs of BKD is variable and ranges from a complete lack of clinical signs to fish exhibiting protruding eyes, darkening of the skin and haemorrhage at the base of the fins. The gills may appear pale and anaemic and internally there may be fluid accumulation in the abdominal cavity and enlargement of the kidney.
Whilst there is currently no treatment for BKD the impact of the disease may be minimised by management strategies including good hygiene, reducing stress, quarantine of infected stocks, culling of infected stocks or total hatchery depopulation followed by disinfection.
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