By Ge Gao and Gloria Van

Introduction

In 1894, Emmerich and Weibel documented the first outbreak of Aeromonas salmonicida in trout fish. This bacterial pathogen found in marine and fresh-water environments is well-known for causing furunculosis (Figure 1). Furunculosis is a disease that commonly affects salmonid fish such as salmon, trout, and whitefish. At first, scientists believed A. salmonicida was an exclusive pathogen to salmonids. Now we understand that this bacterium is capable of causing furunculosis and several other diseases among salmonids and non-salmonids species. For over 100 years, this bacteria has globally caused declines in fish hatcheries and extreme financial losses. 

Disease

The severity of A. salmonicida depends on many factors; environmental conditions, its interactions with other organisms, stress levels of fish, and the strength of fish immune systems. Ultimately, A. salmonicida is a clever pathogen that targets fish with wounds or weak immune systems.

They enter fish through their skin, gut, or gills. Once inside their host, they replicate themselves in phagocytic cells such as macrophages. Macrophages are important cells of the immune system that are responsible for digesting and destroying pathogens. They continue replicating and quickly spread across the brain, kidneys, liver, and spleen. Eventually, A. salmonicida leads to furunculosis by suppressing the response of leukocytes. Leukocytes are white blood cells, necessary immune cells that fight off infection and disease. Infected fish showcase a variety of internal and external symptoms such as swollen skin lesions, skin abscesses under the skin called furuncles, intestinal inflammation, and lack of energy/movement (Figure 2).

The disease is highly contagious and spreads through the water column and direct contact with infected fishes or its eggs. Cases are most frequently reported in fish hatcheries but can occur in wild populations. Unfortunately, most infected fish will die, some without showing any symptoms at all. Human infections are very rare as the optimal growth temperature of A. salmonicida ranges from 18-24°C.

Epidemiology

A. salmonicida has a worldwide distribution, most commonly causing furunculosis among fish hatcheries. Canada’s Department of Fisheries and Oceans reported 298 cases of furunculosis between 2013-2017.

Controlling this bacterium is challenging. Some countries have succeeded, while others continue to struggle. In 1963, Switzerland and Germany successfully eliminated all traces of furunculosis through strict regulations and increasing the frequency of their inspections. They continue to maintain these standards to avoid any re-introduction of furunculosis. Australia and New Zealand took a different approach by avoiding the importation of any new fish or eggs. On the other hand, furunculosis is still a problem in the United States and many other countries in Europe, such as Sweden and Denmark.

Cases in Salmonids are more likely to be reported and researched due to their popularity and economic importance. As our knowledge of A. salmonicida continues to grow, we understand that it can infect almost every fish species. For instance, regions where the bacterium was previously absent, such as Japan and regions of mainland Asia have since reported A. salmonicida cases. Therefore, it is likely that furunculosis cases are much higher because they are under-reported and under-investigated.

Virulence Factors

The mechanisms of A. salmonicida are complex and consist of multiple virulence factors. The three main factors are the additional layer (A-layer), extracellular products (ECP), and a Type III secretion system (T3SS).

1. The A-layer is an additional layer on the cell surface encoded by a gene named vapA, and is a complex protein structure composed of lipopolysaccharides (LPS) and other proteins (Figure 3). The A-layer is responsible for macrophage cytotoxicity resistance, helping A. salmonicida attach to fish macrophages and shields it from the fish’s immune system defense mechanisms. Therefore, it helps the bacterium to replicate inside the fish undetected. The A-layer and the bacterial capsule are similar in their functions. The capsule protects the bacteria from being destroyed by immune cells and inflammation. One key difference between the two is that the capsule is made of polysaccharides and/or proteins.

2. ECP are secreted molecules such as proteins and enzymes. One example is proteases which are enzymes that degrade proteins. Proteases assist the growth of the bacterium by stealing nutrients from the fish. Stealing nutrients leads to skin lesions which are abnormal appearances of skin that appear discoloured and patchy. A. salmonicida produces many different types of proteases, one being AspA. AspA degrades muscle tissue which causes furuncles, skin abscesses under the skin filled with pus. 

3. T3SS inserts toxic proteins inside fish cells which results in cell lysis. Cell lysis is when the outer membrane breaks down, and intracellular components like DNA, RNA, and proteins are released. This process creates an imbalance in the cell environment which often leads to cell death. T3SS has three major structures:  (1) A secretion apparatus that assists the delivery between protein toxins and their effector, (2) a needle-like channel through the bacterial outer membrane and the host cell membrane, which allows the pathogen to inject their toxic proteins (effectors) into the host cell cytosol, and (3) a translocation tool responsible for the physical movement of the toxins, transferring them from the needle into the fish cell (Figure 4).

Treatment

Prevention is one of the most important treatment strategies as infected fish are highly contagious and hard to control. Scientists have tried many strategies such as selective breeding, vaccinations, and antibiotics. 

Selective breeding is when parents are specifically picked to produce offspring with advantageous traits for survival and reproduction. Since 1954, scientists have worked on breeding fish that are resistant to diseases caused by A. salmonicida. The results were mixed. They observed that when some fish developed slowly, they were more resistant to developing the disease. However, many fish have very different responses to selective breeding so, this strategy is considered somewhat effective but not enough by itself.

Vaccine development for furunculosis began in the 1940s. Many different types of commercialized vaccines were produced. Unfortunately, none were considered effective in controlling furunculosis. Not until a breakthrough occurred in the late 1900s using oil-adjuvant injection vaccines. This type of vaccine contains an ingredient, oil-in-water, which creates an even stronger immune response in fish against A. salmonicida. Oil-adjuvant vaccines are generally accepted and applied in the fish industry. 

In 1952, antibiotics were introduced to treat furunculosis. By the late 1990s, they were the most widely used treatment. Similar to vaccines, there are a wide range of antibiotics available. Antibiotics are effective but there are ongoing concerns about antibiotic resistance. 

All treatments have their advantages and disadvantages. There is no standard treatment that works for all cases of A. salmonicida. The best solution remains to be prevention which is why new approaches for vaccine development continue.

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