Vibriosis in Aquaculture
What is the problem?
Sudden outbreaks of diseases of microbial origin in valuable aquatic farmed species (fish and crustacea) are causing significant economic consequences for the aquaculture industry worldwide, due to increased mortality and reduced performance. Furthermore, medical treatments of sick animals raise scepticism of consumers about aquaculture’s quality and credibility.
One of the most devastating disease for aquaculture, caused by an infection of bacterial species is called Vibriosis.
Vibrio ssp, such as Listonella anguillarum (formerly Vibrio anguillarum), V. splendidus, V. ordalii, V. harveyi, V. vulnificus, V. parahaemolyticus, V.alginolyticus, V. salmonicida, V. campbelli , V. fischeri, V. damsella, V. pelagicus, V. orientalis, V. ordalii, V. mediterrani, V. logei are bacteria responsible for this most common disease in aquaculture.
In addition, these microorganisms can become a serious threat for human health, as they accumulate in the reared animal’s flesh.
Antibiotic treatments to reduce the negative impacts of vibriosis, however, can lead to undesirable side effects, such as toxicity to the reared animals but also increase the environmental impact. Furthermore, it was reported that some pathogenic Vibrio strains are resistant to several antibiotics. This leads the modern aquaculture to a growing need for new effective prophylactic methods to reduce the use of antibiotics with their undesirable side effects.
Characteristics of Vibriosis
Vibrio-related infections frequently occur in hatcheries but can affect all stages of life in fish and shrimp. Vibrio spp. are gram-negative ubiquitous bacteria. They are present in surrounding water but also are part of the natural microflora of healthy animals and become opportunistic pathogens when natural defense mechanisms are suppressed. Stressors, such as poor water quality, inappropriate diet composition, overcrowding, presence of other micro- or macro-organisms and abrupt temperature changes, are the most important factors triggering immunosuppression and finally vibriosis outbreaks.
The bacteria can use the skin, gills and gastrointestinal tract as the portal of entry, but most commonly the infection develops by penetration through wounds in the animal`s body surface, which in fish is the skin, together with the mucus, and in shrimps is the exoskeleton (cuticle). The integrity of these physical and chemical barriers is directly affected by gut health.
Clinical signs of Vibriosis
Most often, the disease starts with external changes and if untreated, the infection may become systemic leading to increased mortality. Vibriosis in fish usually starts with dermal ulceration, followed by systemic infections and septicaemia. The infection manifests in lethargy, anorexia, abnormal swimming, ulcerative and haemorrhagic skin lesions, abdominal distension, exophthalmia or “pop-eye”, gill necrosis, darkened skin, and death.
In shrimp systemic vibriosis typically results in lesions of the cuticle, cloudy muscle tissue, formation of septic nodules in the lymphoid organ, heart, gills, hepatopancreas, antennal gland, nerve cord, telson and muscle, brown or black lesions on the cuticle, appendages or gills, tail necrosis. Affected postlarvae may often display brown gills, cloudy and/or septic hepatopancreata – characterised by atrophy of the hepatopancreas with multifocal necrosis and haemocytic inflammation, loss of the epithelium of the midgut, and many more.
Potential control and prevention measures against vibriosis
Since Vibrio spp. need adherence and multiplication in the host, a combination of following control and prevention actions should be implemented to prevent the disease development:
- Good water and feeding management,
- reduce stress factors in farm such as high stocking densities, temperature, salinity and handling during the sizing, vaccination etc,
- Routinely monitor the farm for early diagnosis
- Implementation of effective biosecurity procedures:
- Resistance breeding,
- Restocking with resistant species,
- introduction of fish fry from disease-free hatcheries and/ or disinfection of eggs and larvae
- Antibiotic treatments only if inevitable, to prevent resistance
- Enhancing host gut integrity and defence system by pre-/probiotics and gut health stabilizing feed additives such as phytogenic feed additives. While facilitating growth, reducing stress and Vibrio count, they are also safe to animals, human and the environment.