Aquatic organisms in marine and freshwater systems are threatened by fungal and fungal-like diseases globally, and these pathogens are especially dreaded in aquaculture, Forschungsverbund Berlin (FVB) said in an announcement. However, these diseases also pose a threat to amphibian biodiversity.

There are few approved chemical means for combating these pathogens, FVB said, and many have unwanted side effects.

Now, scientists at the Leibniz-Institute of Freshwater Ecology & Inland Fisheries (IGB) — a component institution of FVB — are proposing alternative biological concepts to control fungal disease in a more environmentally friendly way.

Some fungal and fungal-like diseases produce small infectious stages known as zoospores that swim in water to look for new hosts. They can infest fish and amphibians but also algae and seaweeds that are produced for human consumption.

"The damage caused by these diseases is considerable. Few chemicals are approved for prophylaxis but are expensive, harmful to the environment and often ineffective in the long run, which makes proper application very difficult, especially when used in species protection," said Dr. Thijs Frenken, lead author of the study from IGB.

FVB reported that 80 million tons of fish produced globally come from aquaculture, and its share in human protein diet is expected to increase.

Diseases are the largest cause of economic losses in aquaculture, FVB said, noting that at least 10% of all hatched salmon in the aquaculture industry succumb to zoosporic diseases. For example, infections with this fungal disease lead to production losses of at least $6.5 million per year just on Scottish salmon farms.

Efficient and sustainable ways of controlling fish diseases are paramount to the future success (and economic viability) of the aquaculture industry, FVB said.

"We have to go back to the basics and start applying our ecological understanding of these organisms to limit spread of infections," Frenken said.

The researchers proposed seven biological concepts for protecting aquatic organisms against zoosporic diseases that may be less harmful and more sustainable than chemical methods:

1. Prevent or reduce transmission (control of distribution pathways and vectors). Animal and plant species can spread pathogens. Close contact between different populations -- for instance, by migrations -- can increase the risk of spreading pathogens.

2. Increase the diversity of host species. The so-called monoculture effect applies, where genetically homogeneous populations are more susceptible to infectious agents. Making host populations/communities more diverse can limit spread of infections.

3. Vaccinate and immunize. Vaccinating fish against viral or bacterial diseases is a common practice in aquaculture. No vaccines against fungal-like diseases currently exist, but this could be a promising avenue.

4. Stimulate the defense and production of antifungal peptides by the host. When parasitic pathogens enter the host, host cells die, and peptides are released. These signaling substances induce an increased immune defense in the neighboring cells.

5. Use probiotics. Probiotics can inhibit the growth of parasitic zoospores and also can prevent zoospores from attaching to the host by forming surface-active substances. Probiotics have already been successfully tested in fish as a treatment for zoosporic infections.

6. Hyperparasitism. Introduce another parasite that infects (and eliminates) the target parasite.

7. Use "parasite eaters." Eating parasites is very common practice in nature. Other microscopic organisms in the water (zooplankton), for example, can "graze" on parasitic fungi.

"The constantly changing environmental conditions have a great influence on the parasite/host interaction. These dynamics must also be incorporated into the planning of protection and therapy concepts. We hope that our work will stimulate the further development of alternative biological control strategies. Much more work is needed before we can safely implement these methods into natural habitats without incurring unforeseen risks," IGB researcher and head of the study Dr. Justyna Wolinska emphasized.

FVB is the largest non-university research institution in Berlin, Germany. It comprises eight institutes that conduct research in the fields of natural, life and environmental sciences.