Jay Hemdal
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Fish Medic
Determining cause and effect with fish diseases can be a very difficult process. In fact, in some cases, problems with a fish are attributed to a disease when the true issue is some environmental problem and vice versa. Remember, correlation does not prove causation.
Koch Postulate #1:
The microorganism must be found in abundance in all organisms suffering from the disease but should not be found in healthy organisms.
- Organisms found free-living in aquariums are generally not parasitic to fish.
- Organisms found on healthy fish, or on decomposing dead fish may not be parasites
- Organisms found on fish in low numbers are usually not disease-causing
Bradford Hill criteria:
Strength (propagule effect): A small association does not mean that there is not a causal effect, though the larger the association, the more likely that it is causal.
Consistency (reproducibility): Consistent findings observed by different persons in different places with different samples strengthens the likelihood of an effect.
Specificity: Causation is likely if there is a very specific population at a specific site and disease with no other likely explanation.
Temporality: The disease must occur after the cause (and if there is an expected delay between the cause and expected effect, then the effect must occur after that delay).
Biological gradient (dose–response relationship): Greater exposure should generally lead to greater incidence of the effect. However, in some cases, the mere presence of the factor can trigger the effect.
Reversibility: If the cause is deleted then the effect should disappear as well.
Plausibility: A plausible mechanism between cause and effect is generally the case, but that is limited by current knowledge. Meaning – it is unlikely that your fish have a disease that has never been seen before in aquariums.
Koch Postulate #1:
The microorganism must be found in abundance in all organisms suffering from the disease but should not be found in healthy organisms.
- Organisms found free-living in aquariums are generally not parasitic to fish.
- Organisms found on healthy fish, or on decomposing dead fish may not be parasites
- Organisms found on fish in low numbers are usually not disease-causing
Bradford Hill criteria:
Strength (propagule effect): A small association does not mean that there is not a causal effect, though the larger the association, the more likely that it is causal.
Consistency (reproducibility): Consistent findings observed by different persons in different places with different samples strengthens the likelihood of an effect.
Specificity: Causation is likely if there is a very specific population at a specific site and disease with no other likely explanation.
Temporality: The disease must occur after the cause (and if there is an expected delay between the cause and expected effect, then the effect must occur after that delay).
Biological gradient (dose–response relationship): Greater exposure should generally lead to greater incidence of the effect. However, in some cases, the mere presence of the factor can trigger the effect.
Reversibility: If the cause is deleted then the effect should disappear as well.
Plausibility: A plausible mechanism between cause and effect is generally the case, but that is limited by current knowledge. Meaning – it is unlikely that your fish have a disease that has never been seen before in aquariums.
