Trade in frozen milt from farmed fish and the risks of transmitting infectious disease

This is the key message in a risk assessment conducted by the Norwegian Scientific Committee for Food and Environment (VKM) on behalf by the Norwegian Food Safety Authority.

Milt as a source of contamination

Some pathogens are known to be transmitted via the sexual products (eggs and milt) of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss). Surface disinfection of fertilized eggs, the main trade product, is a standard procedure to prevent infections. The only known use of cryopreserved milt is fertilization of eggs.

"Disinfection of milt is not possible without compromising its viability. Milt may contain pathogens originating from the broodfish. In addition, environmental contamination may also occur, either when fish undergo sexual maturation or during stripping and extraction of milt," says Espen Rimstad who has led the work for this scientific opinion.

Documenting freedom from disease

Fish diseases are listed both nationally and internationally to establish standards for the control and prevention of these. Having approved, disease-free status for the diseases listed in EU and Norwegian legislation is mandatory in all farms with broodfish that produce milt.

"For infections involving several organs it is possible that the pathogen may also be present in milt. In such cases, testing tissue samples other than milt is applicable. In the risk assessment we emphasize that specific testing of milt cannot guarantee the absence of infection or disease," says Rimstad.

Certain pathogens require extra attention

Any viral or bacterial agent present in mucus and water may potentially contaminate milt, before or during extraction. The number of potential infectious agents, including opportunists, that could be present is therefore large. Yet five pathogenic agents were, singled out as highly relevant for disease transmission through cryopreserved milt.

- The agents Renibacterium salmoninarum (which causes Bacterial Kidney Disease) and Infectious pancreatic necrosis virus can be transmitted vertically, that is from parent to offspring. Both infections may also be transmitted between fish. It is therefore important that the broodfish is free of these agents, according to Rimstad.

In addition, Piscirickettsia salmonis, Flavobacterium psychrophilum and Infectious hematopoietic necrosis virus, were also assessed as highly relevant infectious agents.

Knowledge gaps

Studies on transmission of pathogens via milt are sparse. Information is missing on whether milt infected with pathogens infect eggs at fertilization. Laboratory experiments on vertical transmission rarely reflect real-world data. There is no information available about the presence of almost all listed and non-listed pathogens in milt, preventing a more complete assessment of the risks.

VKM´s working group

The following members comprised the working group:

• Espen Rimstad, VKM-member in the Panel on animal health and welfare, chair of the working group
• Edgar Brun, external expert at the Norwegian Veterinary Insititute
• Duncan Colquhoun, external expert at the Norwegian Veterinary Insititute
• Niels Jørgen Olesen, external expert at the Danish Technical University
• Dean Basic, projectleader, the VKM secretariat

The composition of the working group was based on relevant scientific expertise in areas such as fish health, virology, bacteriology, pathology, epidemiology as well as experience with risk assessments.

The Panel on Animal Health and Welfare is responsible for the assessment.