What is the optimal number of fish to be sampled?

In the case of collecting samples for diagnosis a disease that is associated with clinical sign, mortality, or reduced growth, collect no less than ten fish preferably showing gross disease signs. In the case of inspection, certification, or zone delineation, number of fish to be collected from each lot must be in accordance with a plan that provides 95% confidence that at least one infected fish will be collected if the minimum assumed pathogen prevalence level (APPL) of infection equals or exceeds 5%,however, the level of confidence may vary from one country to the other, therefore, the veterinarian in charge of collecting samples should be aware of regulation, policies, and the nature of pathogen in question. For example, to determine freedom of virus in a lot requires a 2% level of confidence. Table (2) displays the number of fish needed to sample from each lot to achieve the desired level of confidence of laboratory results.

Table 2: Sample number based on an assumed pathogen prevalence level (APPL) in the population of 10%, 5 %, or 2%.

Does fish age determine which organ to collect?

Yes, it does. In the case of fry and yolk sac fry the entire fish is collected after the removal of the yolk sac. For fish measuring 4-6 cm, the entire viscera and kidneys are collected in addition to the brain. For fish over 6 cm, the kidneys, spleen, heart, and brain are collected in addition to other tissues appropriate for the specific pathogen being tested for (e.g., skin and fins in case of herpesviruses or ovarian fluid in case of flavobacteriois).