Post by Brad Richardson.— Disease outbreaks in aquaculture can often ultimately end in mass mortality events. However, few people every really consider the dynamics of what is happening during a disease outbreak. This can things such as the actual transmission of a pathogen from one individual to another, the management actions of a pond owner, different rates of mortalities, etc. All of these factors play into the length and magnitude of an outbreak.

Below is a simplified representation of the progression of an Aeromonas hydrophila disease outbreak in farm-raised Channel Catfish (Ictalurus punctatus). In this image, the boxes represented "number of individuals" in each state (susceptible, infected, recovered, or dead). The red arrows indicate how the different variables relate to one another; and the circles represent modifiers such as the rate of transmission or mortality rates.

Combining models, such as the one above, with knowledge about the system, we can start to simulate different conditions and predict how the population of fish will respond to a disease outbreak. Using the model from the image above, I set some basic rules for how I want the system to behave based on regular management strategies of catfish farmers. For instance, I set a rule that if more than 100 dead fish were seen on the pond in a given day, then the farmer should switch to medicated feed (shown as "Medicated" in the model above). After setting my starting constraints, I ran a simulation to see how the pond and fish population would respond over a 7-day period. The output from the simulation is shown in the graph below. As you can see, the farmer switched to medicated feed (yellow, line 4) 2 days after the disease outbreak began; however, fish continued to die from the disease for another day or so (pink, line 2). This makes sense because individuals that are heavily infected will still likely die before the medication in the feed can fight off the pathogen.

This is just one small example of the power of these models and the types of questions that we can begin to explore with them. In the future, I will continue to build on this model and explore different strategies that farm managers may be able to use to limit the number of fish lost to Aeromonas hydrophila disease outbreaks, and, potentially, how different strains of the disease behave within the catfish aquaculture ponds.