Gill problems account for a large proportion of production losses in the sea phase. Like other species, fish are dependent on having sufficient access to oxygen, and it is the gills that ensure that they can utilise the oxygen dissolved in the water they live in.
Many infection agents and particles in the water can harm the gills and thereby be a threat to the welfare, health and survival of the fish.
Gill problems in the saltwater phase are often spoken about as a multi-factor disease, and are connected to a range of infectious and non-infectious factors. This means in cases with reduced gill health, you often find a variety of causes, but in other cases it may be that no cause will be found at all. This means that prevention of gill problems is the key with operational measures together with general strengthening of fish health and the immune system.
Review of specific gill problems:
As gill problems often involve multiple factors, we often talk about gill problems using terms such as "autumn sickness", "chronic proliferative gill inflammation" and "epitheliocystis", which actually points to when symptoms occur, and how the types of symptoms have more than one cause. AGD – or amoebic gill disease - is however a disease that is primarily related to one specific agent – the amoeba Paramoeba Perurans, but this is also seldom found in isolation.
AGD – Amoebic Gill Disease
|Facts:||AGD is caused by the amoeba Peramoeba Perurans, on its own or together with other gill pathogens. The amoeba settles over the surface of the gill and causes irritation, mucus formation and inflammation. In severe cases, this can lead to the fish not obtaining enough oxygen and suffocating.|
|Southern Norway, Central Norway. Most common around autumn when the water temperature and salinity are at their highest, occurs sporadically throughout the year.|
|Symptoms:||Gills covered in mucous, listless fish, reduced appetite, poor stress tolerance, increased need for oxygen, reduced tolerance for low oxygen levels during delousing, transportation, and similar.|
|Diagnosis:||Appearance of white spots on the gills, especially the 2nd gill arch. PCR|
|AGD is treated most effectively with bathing treatment in freshwater. This is more gentle than bathing in hydrogen peroxide, although this can also be effective.|
Chronic gill inflammation/ Chronic proliferative gill inflammation/ Epitheliocystis.
Chronic gill inflammation is a state of inflammation that can lead to damage to the gill tissue and thereby reduced respiratory capacity. The inflammation can be caused by a range of different agents, which can mean the symptoms can be very variable.
- The disease has been compared with lung inflammation in humans, in whom it is primarily a viral infection that paves the way for bacteria.
- The inflammation is often characterised by tissue proliferation.
- In this case is it often known as PGI, or Proliferative Gill Inflammation.
- In other cases, cysts can be found in the epithelium, when it is called epitheliocystis.
There is still much we don't know about gill inflammation, and extensive research is being carried out to uncover causal connections and thereby possible treatment methods.
Agents associated with gill inflammation:
- Branchiomonas cysticola – especially associated with epitheliocystis
- Microsporidal infection – especially associated with autumn sickness. Spread by salmon lice.
- Non-infectious contributors: algae, jellyfish, hydroids etc.
Throughout Norway. Most common around the autumn when the water temperature and salinity are at their highest, sporadically throughout the year Number of cases not recorded as gill disease is not reportable.
Symptoms: listless fish, reduced appetite, poor stress tolerance (increased need for oxygen), reduced tolerance for low oxygen levels (delousing, transportation, etc.). More susceptible with algae blooms.
How can feed be used in disease control? Promotes resistance to infections through a thicker and more effective mucous layer. Strengthens the immune system of the fish against primary and secondary infection. Increases the antioxidative ability of the fish
Our feed: Intro and Primo
Gill health and circulatory health
Heart health for farmed salmon has many challenges such as malformations and infections. These can weaken the heart's ability to compensate for the increased need for oxygen in demanding situations. If the fish is also suffering from gill disease, this may result in reduced oxygen intake, and further stress on the circulatory system. In situations where the fish is exposed to exertion and reduced access to oxygen, this can lead to circulatory failure and death from being handled.
To ensure that the fish are robust with good circulatory health, a good layer of mucous and a strong immune system can have a preventive effect against gill problems.