Protecting aquatic species: strategies for pathogen management

The hidden costs of disease in aquaculture: beyond mortality

Weakened productivity is linked to reduced profitability. Although the exact cost of disease is unknown, it is widely accepted to be in the tens of billions of dollars every year. Mortality is the most obvious loss in profit, but there are also many hidden costs associated with disease, for example:

• Diagnostics & veterinary expertise
• Medicines and chemical treatments
• Time & labor
• Lost growth performance
• Feed losses & increased FCR
• Market access & consumer reputation
   

Aquaculture practices drive pathogen proliferation

Intensive aquaculture practices such as antibiotic usage, disinfection and even feeding can favor the proliferation of pathogenic bacteria (Figure 2). This is because many pathogens are r-strategists, which means they have high growth rates, especially in non-competitive environments and can dominate in unstable conditions. The opposite, K-strategists, including many probiotics, grow slower but dominate in stable conditions. This means that a disease management plan is required throughout the production cycle. This should include continuous pathogen surveillance and prioritizing proactive strategies to avoid pathogenic outbreaks, or reactive measures such as antibiotics.

Figure 2: Examples of how current aquaculture practices promote r-selection, increase the probability of opportunistic bacteria (A) Disinfection of intake water. (B) Addition of feed to the system.

Source: Vadstein et al. 2018

Antibiotic reduction in aquaculture 

In line with other protein sectors, antibiotics have traditionally been used to treat bacterial infections in aquaculture. However, their overuse leads to antimicrobial resistance and consequently the industry has many initiatives to reduce its reliance, in favor of preventative healthcare. For example, in addition to meeting Aquaculture Stewardship Council (ASC) criterion, the Sustainable Shrimp Partnership (SSP) in Ecuador also expects its members to use zero antibiotics. Similarly, Norway have grown their salmon production volumes, whilst reducing their consumption of antibiotics by 99%. The Chilean salmon industry has also managed to reduce its antibiotic usage by 50% since 2017, although to a lesser extent. In 2022, Chilean salmon reported to use 458.6 g of antibiotics for every ton of Atlantic salmon produced (CSARP, 2022).

Feed additives as valuable health management tools

Functional feeds are an important part in a robust health management program, but there is no silver bullet. Consequently, multiple active components with complementary modes of action must be explored. Thes include, but are not limited to:

• Direct inhibition: Many probiotic bacteria, organic acids and essential oils impact pathogen growth and survival.
• Competitive exclusion: utilizing beneficial bacteria (probiotics) to compete with pathogens for much needed resources e.g., nutrients, adhesion sites.
• Membrane damage: Gram-negative pathogens have an outermost layer of lipopolysaccharide. This gives them greater mitigation against antimicrobials and can pump out unwanted substances, like antibiotic metabolites. Breaking this membrane causes stress to the pathogen, disrupting the membrane functionality and reducing overall virulence.`
• Quorum quenching: quorum sensing is a means of bacterial communication and is linked to pathogen virulence. Quorum quenching breaks these channels, disrupting virulence and biofilm formation.
• Toxin degradation: endotoxins can be produced by several pathogens, which cause great damage to the animal. This is the case in Acute Hepatopancreatic Necrosis AHPND (Acute Hepatopancreatic Necrosis Disease), where a number of Vibrio spp. produce pirAB toxins which attack the hepatopancreas.

Salmon vs salmon rickettsial septicemia 

Salmon rickettsial septicemia (SRS), caused by the etiological agent Piscirickettsia salmonsis, has been the most important infectious disease in the Chilean salmon industry since the 1980’s, costing >300 million USD per annum. It is estimated, >95% of all antibiotics used in Chile are used to treat SRS. 

In a recent study, Biotronic® Top3, an enhanced acidifier was able to significantly reduce the mortality of Atlantic salmon after a 65-day Piscirickettsia salmonsis cohabitation challenge (55.0 ± 7.9% vs 72.2 ± 13.9%, respectively; P = 0.0064). In the same trial, it was calculated that the enhanced acidifier was able to reduce the probability of a mortality event by nearly 40%.