The Increasing Incident Rate Zoonotic Infections Among workers of the Aquaculture Industry in the Eastern - Mediterranean Region

Mycobacterium affects a very wide range of marine species which are cultured in the Mediterranean region, including European Sea Bass (Dicentrarchus labrax) and Sea Bream. (Sparus aurata)

Mycobacterium marinum infection in fish has traditionally been associated with a chronic, very slow-progressing disease in various aquatic environments. However, recent reports from the Eastern – Mediterranean region indicate a rapidly expanding shift towards a much more acute and aggressive form of the disease, particularly in intensive and recirculating marine aquaculture systems (RAS).

The implications of this escalating problem are very significant, affecting not only fish health and performance, but also posing a zoonotic risk to humans working in the aquaculture industry.

The Fish are not Alright

Mycobacterium marinum infections in fish often manifest in subtle ways initially.

Early signs of disease are non – specific, and may include decreased growth performance, increased values of feed conversion ratio (FCR), persistent external parasite infestations, secondary bacterial infections, superficial ulcers and discoloration.

As the disease progresses within the culture facility, more severe symptoms such as distended abdomen, loss of appetite, external hemorrhages, fin erosion, and exophthalmia (bulging eyes) may become apparent.

In some cases, fish may die without exhibiting any external signs.

One of the earliest indicators of Mycobacterium involvement in developing Sea Bass and Sea Bream disease outbreaks is the lack of response to standard antibiotic treatments, or failure of vaccination programs.

Additional early signs of an underlying Mycobacterium marinum infection may be recurrent external parasite infections shortly after treatment.

As the pathogen establishes itself within the culture environment, increasing levels of mortalities due to Mycobacterium infection are typically accompanied by characteristic changes in internal and external organs.

External signs of disease include the development of large granulomatous legions on the base of the fins, on the operculum, or on the base of the fish’s tail.

A Sea Bream (Sparus aurata) with large granulomatous lesions caused by Mycobacterium marinum.

Internally, symptomatic fish will be characterized by splenomegaly (enlarged spleen) and hepatomegaly (enlarged liver), as well as the presence of granulomas (nodular inflammatory lesions) on or within the spleen, kidney, and liver.

Enlarged spleen and liver in a sea bass due to Mycobacterium marinum infection, showing no external clinical symptoms.

Diagnosis

Early diagnosis of Mycobacterium marinum infections in Mediterranean marine species will be based on careful evaluation of the history of a progressive disease, often characterized by decreased growth performance, increased FCR and lack of response to antibiotic therapy or failure of vaccination programs.

Once the disease has been established, a post-mortem examination will reveal

nodules in the internal organs ;in particular ,the kidney ,spleen ,and liver.

Culture of Mycobacterium spp. is usually a very lengthy process, requiring advanced skills and infrastructure of the diagnostic lab. As such, in most suspected cases there

is no attempt to culture the organism.

The definitive diagnosis for Mycobacterium marinum is obtained by an Acid-fast stain test of symptomatic tissue (such as Spleen, Liver or Kidney) or by molecular detection using polymerase chain reaction. (PCR)

Acid-fast positive stain of a suspected spleen sample from a European sea bass, highlighting vibrant red-stained bacteria amidst a blue-stained background.

RAS: A Breeding Ground for Mycobacterium

Recirculating aquaculture systems, (RAS) while offering numerous advantages for intensive fish production, seem to create particularly favorable conditions for clinical expression of Mycobacterium marinum.

In RAS, the traditionally chronic nature of the disease very frequently may escalate to an acute and very aggressive outbreak, characterized by very high morbidity and mortality rates.

Recirculated aquaculture systems provide conditions conducive to the proliferation of infectious agents, highlighting the need for effective management and monitoring.

Re-circulating aquaculture systems favor the growth of Mycobacterium marinum organisms and spread of the disease due to higher densities of fish when compared to other culture systems, higher organic loads, favorable temperatures, and the buildup of sediment and subsequently pathogens in tanks, pipes and filtration components of the system.

Outbreaks in RAS systems of both Sea Bass and Bream will often be enhanced due to nutritional deficiencies and suboptimal water quality parameters.

All these factors can compromise the fish's immune system, making them more vulnerable to infection and the rapid spread of disease.

The Human Cost

Mycobacterium marinum is a zoonotic pathogen, meaning it can be transmitted from animals to humans.

The primary route of infection is through cuts or scrapes that come into contact with contaminated water or fish.

The most common symptom in humans is a slowly developing nodule at the site of entry, often on the hands or upper arms. In more severe cases, the infection can spread to deeper tissues, joints, and bones, leading to significant complications.

A close-up view of a hand showing a slow-growing nodule, a common symptom of Mycobacterium infection, at the knuckle area.

In recent years, there has been a notable increase in reported cases of cutaneous Mycobacterium infections in the Eastern - Mediterranean region, particularly among individuals working in the aquaculture industry.

This rise is likely attributable to a combination of factors, including increased awareness, improved diagnostic capabilities, and the expanding host range of the pathogen to include numerous species of ornamental fish.

Post-surgical recovery following a deep Mycobacterium infection, showing sutures on the palm after intervention and ongoing antimicrobial treatment.

Prevention and Control

Given the lack of effective vaccines and treatments for Mycobacterium infections in fish, prevention is paramount. Strict biosecurity measures, including thorough disinfection protocols and quarantine procedures for new fish are essential. In RAS,

maintaining optimal water quality, minimizing stress, and providing optimal nutrition are crucial for enhancing fish immunity and reducing the risk of disease outbreaks.

Early detection and aggressive culling of suspected carriers are also vital for limiting the spread of the disease. Rapid response to secondary infections and external parasite infestations can help mitigate the impact on fish health and welfare.

For humans working in the aquaculture industry, practicing good hygiene and wearing protective gear when handling fish or cleaning tanks can significantly reduce the risk of infection.

Prompt medical attention should be sought if any suspicious skin lesions develop.

The Road Ahead

The increasing incidence of zoonotic Mycobacterium infections in Mediterranean aquaculture is a cause for concern. Addressing this issue will require a multi-faceted approach involving improved biosecurity, enhanced diagnostics, and greater awareness among both fish health professionals and aquaculture workers. Research into the development of effective vaccines and treatments for Mycobacterium infections in fish is also urgently needed.

By prioritizing prevention and control efforts, the aquaculture industry can safeguard both fish health and human well-being, ensuring a sustainable and prosperous future for this vital sector.