Some fish pathogens may be more common in or adapted to a given species, genus, or family; however, many are ubiquitous and have broad host ranges. Environmental factors, including type of rearing system and production method, influence pathogen variety and prevalence within a given unit and facility. Many pathogens can infect fish regardless of rearing system and production method, but the following sections describe pathogens that may be more problematic under certain conditions.
Most production ponds are outdoor and earthen (dirt or clay lined) and, over time, become high in organic loading as a consequence of feeding, fish growth, waste accumulation, and other related processes. In Florida, smaller ponds facilitate periodic cleaning, whereas much larger catfish ponds in the southeast are logistically much more difficult to clean. Although fish in production ponds are certainly susceptible to a wide spectrum of parasites, high organics favor sessile ciliates, including Heteropolaria/Epistylis, Apiosoma, and Ambiphyra, which attach to fish but feed on bacteria and other nutrients in the water column. Other protist parasites that thrive in rich, organic environments include the trichodinids (eg, Trichodina, Tripartiella), and Tetrahymena. Uronema, the marine counterpart to Tetrahymena, also thrives in highly organic waters.
Fish cultured in outdoor ponds are much more susceptible to infection by metazoan parasites with indirect life cycles because of the presence of final hosts and intermediate hosts that permit completion of the life cycle. These include digeneans, myxozoa, nematodes, and cestodes. The presence of birds and snails facilitate infestations by digeneans, requiring fish and snails as intermediate hosts and aquatic birds as the final host. Bolbophorus damnificus is an important digenean parasite of channel catfish, the final host of which is the American white pelican, and the intermediate hosts are the ram’s horn snail and the fish. Centrocestus formosanus, another important digenean parasite infecting the gills of numerous fish species, uses a wading bird, such as the green heron or great egret, or the cone snail (Melanoides tuberculata) to complete its life cycle.
Oligochaetes, annelid worms, are also much more prevalent in highly organic waters. Henneguya ictaluri, a myxozoan parasite and the causative agent of proliferative gill disease in channel catfish, requires the oligochaete Dero digitata to complete its life cycle. Eustrongylides, a nematode parasite that infects wading birds as adults, has an indirect life cycle that may involve either an oligochaete worm and a fish or just a fish. Juvenile, pond-reared fish can be adversely affected when heavily infected with this organism. Other fish, reptiles, or amphibians may serve as paratenic hosts by feeding on infected fish and carry or transport the parasite.
Bacterial diseases, such as those caused by Edwardsiella ictaluri and Streptococcus spp, as well as viral diseases may be more difficult to control once they infect a pond population because of logistical challenges with pond disinfection.
Recirculating aquaculture systems (RASs), by contrast, are situated within enclosed facilities with more limited access by potential final or intermediate hosts. Parasites with direct life cycles are more common and dangerous in an RAS, because RASs used in production tends to have greater fish densities and by definition recycle water, which results in closer fish-to-fish contact and greater buildup of parasite numbers within the system. Once a parasite infects a fish within an RAS, it becomes magnified, and disease can spread rapidly. Ichthyophthirius multifiliis (“ich”), Cryptocaryon irritans (“salt water ich”), and Amyloodinium (marine velvet disease) can spread rapidly within an RAS. Similarly, egg-laying monogeneans such as Dactylogyrus sp and Neobenedenia sp, the capillarid nematodes (which have direct life cycles), and microsporidia also can spread rapidly within an RAS.
Bacterial and viral diseases are also much more problematic in RASs, partly because microbial flora can be much less diverse and skewed than in more natural pond systems. Mycobacteriosis and streptococcosis have caused significant morbidity and mortality in RASs, which can favor their growth, amplification, and formation of reservoirs. Certain environmental conditions promote mycobacterial growth, including warm water temperatures, low dissolved oxygen levels, acidic pH, high soluble zinc, high fulvic acid, and high humic acid. Many of these conditions—especially the low dissolved oxygen, low pH, and an organically rich environment—are present in intensive aquaculture systems. Closed conditions and higher stocking densities enhance the potential for spread and amplification. Biofilms on the tanks, in pipework, and in filtration systems, as well as organics, inaccessible or overlooked mortalities, and detritus, can serve as reservoirs.
Open ocean net pens, or cage culture systems within large reservoirs or lakes, have their own unique challenges. Under ideal conditions, “open” access through the net or cage structures is intended to make use of the vast surrounding water body and allow for dilution of solid and dissolved wastes. However, this “open” access also facilitates direct or indirect contact with pathogens endemic to the water body. Smaller wild fish or other organisms may directly enter and intermingle with the farmed species, or pathogens themselves may flow directly into the structure through the water or on other vectors. The higher densities found in net pens and cages can then magnify the disease through close contact and facilitate spread.
Salmonid producers, during marine grow-out phases, can have significant losses caused by sea lice and infectious salmon anemia, among others. Neobenedenia and other monogeneans are a major problem in warmwater marine systems. Bacterial diseases such as vibriosis are common in net pens, and emerging viral diseases, including those caused by iridoviruses (including megalocytiviruses) and betanodaviruses, are of increasing concern in marine open ocean systems because of potential for transmission from wild populations.