Vertical transmission of disease is perhaps the most important vector of disease transfer that proper biosecurity measures can eliminate. Brood fish generally come from an environment were they have been constantly exposed to pathogens and are therefore highly likely to be carriers of infective agents. Although they are not affecting the adult fish, these organisms can have devastating effects if transferred to the next generation.

The transfer from one generation to the other can happen in two ways: on the egg and in the egg. For a disease like BKD that is transferred in the egg, rigorous screening and discarding of all eggs from positive fish is a harsh but necessary biosecurity measure. However, for many other pathogens the route of infection is on the eggs and then into the fry when they hatch. Such pathogens are easily eliminated with proper egg disinfection.

Egg disinfection is a very specialized biosecurity application and, as such, the aquaculturist should choose an equally specialized product. Salmonid eggs are generally in clean, fresh water, consequently, there is no need for concerns over excessive organic material, but there is cause for concern for the effect a disinfectant may have on the eggs. Therefore, there is no need for surfactants, solvents or detergents to strip away excess organic material. What is needed is something that will kill the pathogen but not the egg or affect the eggshell.

Polyvinylpyrrolidone iodine (PVPI) products have been used for many years for this purpose and are highly effective against pathogens commonly affecting salmonid eggs. However, some of these products contain detergents or strong acids that, although helpful in general disinfection, are not helpful for eggshell quality. When disinfecting eggs with a PVPI solution, aquaculturists should also look for a product with a neutral pH that will not affect water chemistry.

The highly efficacious nature of iodine-based products has led them to being used as general disinfectants. Recent work at research facilities has shown that many iodine products are not effective against some pathogenic organisms in everyday aquaculture applications. However, these products where never designed for general disinfection. In most cases, they were specifically formulated for applications such as disinfecting the udders of dairy cows or fish eggs and many were formulated for use in hospital applications.

Once in incubation after initial disinfection, routine treatment for fungus will also have the beneficial affect of killing anything that might have survived or slipped past initial disinfection.