6.5 Disease Outbreaks in Aquaculture: Understanding and Management

Disease outbreaks in aquaculture can vary significantly in their presentation and impact on livestock. In this section, we will explore the characteristics of disease outbreaks and the essential components for diagnosing and managing them effectively.

Recognizing Disease Outbreaks

Disease outbreaks can differ in terms of their onset speed, the total number of affected animals, and the duration of the outbreak. The “typical” disease presents as something other than the routine or usual behaviors such as feeding, swimming, or change in body appearance. Frequent, at least daily, observation of the livestock can detect a disease outbreak appearing in the tank, pond, or aquarium. Prompt attention to anything that is abnormal gets the best results from treatment. A crucial step in managing such outbreaks is obtaining an accurate diagnosis or at least a list of potential causes. Without a proper diagnosis, treatment becomes a guessing game, prognosis becomes uncertain, and the cost of recovery for livestock can become prohibitively high. Disease outbreaks are a leading cause of aquaculture operations facing financial difficulties, often leading to bankruptcy.

The Four Critical Components of Sorting out the Diagnosis

Just as there are four categories of potential disease agents, there are also four critical components and facts essential for diagnosing disease: history, examination, testing, and actions taken. Maintaining this order is crucial for effective diagnostic efforts. Gathering information on the history of the disease is instrumental in guiding the examination of affected fish. Even in emergencies, a partial history and a brief facility tour can provide valuable insights. Testing is essential but becomes purposeless without a suitable history and examination.

Epidemiology and Disease Patterns

Understanding the epidemiology of pathogens in fish populations and other animals is vital for making accurate diagnoses and treatment plans. Epidemiological information, when combined with historical data and examinations, can assist in determining the cause of an outbreak and devising appropriate interventions.

Analyzing Disease Curves

Figure 6.6 represents a hypothetical collection of cumulative mortality disease curves, illustrating various patterns of mortality in affected populations. A difference to notice is the slope of the line. The steeper or more vertical the line the shorter the time to get the accumulated number of mortalities. The more gradual slopes represent more time and the height of the curve displays the loss of livestock to that time.

• Line 1, the “peracute” curve, depicts a sudden and rapid buildup of clinical signs and deaths, often linked to environmental factors. For example, a sudden drop in oxygen levels can lead to a swift increase in clinical signs and mortality. This sort of event takes little time to become a dramatic and serious loss.
• Line 2 is more sloped, with infections growing quickly over time, but less rapidly than in peracute cases. Line 2 is considered “acute” for infectious diseases. Pathogens may affect different members of the population with varying speeds in the development of clinical signs and mortality.
• Line 3 represents a “subacute” disease occurrence, characterized by a gradual rise in mortality in certain fish in the population.
• Line 4 shows a “chronic” disease pattern, where problems develop gradually over an extended period.

The data collected to create these graphs would be beneficial for planning or predicting disease outbreaks. Farm managers can watch the production parameters displayed in the changing graphical line as a prelude to oncoming events. Early detection of the line trends may be a reason to call a veterinarian, increase sampling for disease monitoring, or review of assay protocols.

Factors Contributing to Disease Outbreaks

Several factors or variables contribute to disease outbreaks: number of organisms, the infectivity and virulence of the pathogen, the host’s susceptibility, and environmental stress.

Number of Organisms

The population of pathogens taken in by animals is a critical indicator of the pathogen’s potential outcome. This includes the concept of infectious dose, where the minimum number of microorganisms required to make an animal sick is exceeded. Some animals may be infected without showing clinical signs. Uneven distribution of the pathogen throughout the tank, net pen, or raceway could also lead to some livestock not getting infected.

Infectivity of the Pathogen

Infectivity is the ability of a pathogen to establish an infection. More specifically, infectivity is the extent to which the pathogen can enter, survive, and multiply in a host. Another definition defines infectivity as a disturbance in the balance between host resistance and bacterial virulence. This is different than the “infectious dose” concept in the previous paragraph. Many diseases are adapted to infect specific hosts and will begin to make new strains to increase infectivity and how they infect their host.^[1] Infectivity is not a constant with all strains of a pathogen. Strain differences can affect infectivity and pathogen distribution. It would not be uncommon to have pathogens become resistant to antibiotics and disinfectants, or broader range of environmental temperatures.

Virulence

The ability of a pathogen to produce disease, or virulence, is not uniform among all strains of a pathogen. Viruses can be notorious in genetic variability due to their rapid replication, generation of large populations, and high mutation rates.^[2] An example of viruses high in genetic variability would be RNA viruses such as groups of the Rhabdoviridae affecting fish.^[3] Most pathogens have some mechanism of attenuation or loss of virulence that may hypothetically increase the host’s and/or environment’s survivability. Genetic variability can lead to increased virulence in certain strains over time, potentially affecting vaccinated animals.

Host Susceptibility

An individual animal’s susceptibility to disease, or their likelihood of infection, can vary with age, nutrition, and other husbandry factors. Young and mature animals may react differently to the same pathogen dose.

Environmental Stress

Changes in the environment, such as temperature and daylight length, can stress animals and impact their immune system, making them more susceptible to infections.

Probability of Disease

Estimating the probability of disease involves considering the factors mentioned above. The Theobald Smith Model provides a simplified equation for calculating disease probability from an assortment of factors:^[4]

[latex]\large{ \text{probability of disease} = \frac{\text{(# agents } \times \text{ virulence of agents)}}{\text{(resistance of host)}}}[/latex]

Theobold’s Model has limitations, as not all species are susceptible to every pathogen, and subclinical infections can occur.

Understanding the complexities of disease outbreaks and the factors influencing them is crucial for effective disease management in aquaculture. It requires a multi-pronged approach that integrates history, examination, testing, and actions taken to mitigate the impact of disease outbreaks on aquatic populations.