The fast growth and potential of global aquaculture has necessitated the adoption of sustainable and welfare-oriented therapeutics and prophylactic strategies. Knowledge gathered from studies about maternal passive immunity in fish and fish-to-fish passive immunization experiments supports the concept of using therapeutic Abs (of piscine and other vertebrate origin) in aquaculture. Traditional Ab formats (IgG, IgM) are expensive and laborious to produce; however, the introduction of new rAb fragments and single-domain Abs have reinvigorated the concept of passive immunization. This review will focus primarily on farmed salmonids (salmon and trout) within a comparative context and will give an overview of the basic principles and scientific premises for the passive immunization strategy, including existing and emerging Ab therapeutics.

For many years, antibiotics and, now, to an increasing extent, injection vaccination against bacterial and a few viral diseases have become the mainstay of the modern aquaculture industry (1). For reasons not fully understood, development of effective antiviral vaccines has proved difficult. The immunological memory in teleosts appears elusive (2), and the longevity of many successful salmonid vaccines (e.g., against vibriosis/furunculosis) has been attributed to the retention of Ags in the water–oil emulsion (oil adjuvant), creating a depot effect at the injection site (3). This has also resulted in side effects of vaccination like elevated levels of autoantibodies, i.p. adhesions, and chronic i.p. granulomatous inflammation in the fish (3, 4). Such side effects, when combined with the high stress levels endured by fish during handling and anesthesia, have resulted in retarded growth and increased mortality in treated fish, raising questions about animal welfare (5). Injection (i.p.) vaccination as currently applied in salmonid aquaculture is given before sea transfer as a single priming dose without a booster dose to avoid handling stress to the fish. Such vaccination can be applied only to fish of a certain size (>10 g), and it includes crowding the fish in small tanks, anesthesia, handling individual fish, and an appropriate recovery period. For certain bacterial pathogens, immersion vaccination of fish has been found to be effective in inducing a mucosal response. However, immersion vaccines require booster administration and, in addition, effective application routines for cultured fish have not been developed (6).

From an evolutionary perspective, teleost fish have a functioning adaptive immune response that is comparable to higher vertebrates, but they seem to lack class-switch recombination and higher-order affinity maturation that are largely attributable to the lack of germinal centers in these lower vertebrates (7). Fish being ectothermic, the development of their immune system is slow, especially in cold-water species like salmonids. This initial period of vulnerability is not covered by the vaccination approach (8). Although classical molecular components of humoral immunity are present in teleost fish (MHC, TCR, systemic and mucosal Igs) (9), the spectrum of Ig classes is narrow in fish and, at least in some fish species, low-affinity polyreactive IgM Abs predominate (10). Additionally, the teleost Ab response has been described as poorly anamnestic (2). Thus, there appears to be a strategy of these lower vertebrates to prioritize quantity rather than quality.

In contrast to vaccination in which Ags induce an immune response, passive immunization can be defined as administration of extraneous Abs to induce a temporary therapeutic effect against a pathogen. On a broader scale, it can also include therapeutic Abs against immune-related proteins (11) or even hormones (12). In humans, passive immunization strategies are used for treating or preventing various infectious diseases (13), and they constitute an emerging field of interest in aquaculture as well. This review will focus on passive immunization concepts and strategies primarily for farmed salmonids within a general aquaculture context and in light of the aforementioned challenges in vaccination and therapy.

Passive immunity may be acquired naturally through the maternal transfer of adaptive and nonadaptive immune components to offspring. Such transfer is necessary in animals, including fish, during the initial period of vulnerability before immunity is fully developed (14). In invertebrates like mollusks (15) and crustaceans (16, 17), passive immunity pertains to the transfer of non-Ab components. In fish and higher vertebrates, Abs constitute a major proportion of the functional passive immunity that is acquired maternally (13). Although maternal Abs are transferred to the fetus through the placenta in mammals, in almost all teleost fish, Abs are transferred to the yolk (18). In several fish species, including the oviparous salmonids, immunized gravid females have been reported to transfer passive immunity to their offspring (1821). The transfer route of Abs in true viviparous fish, such as surf perch (Neoditrema ransonneti), involves ingestion of maternal IgM present in the ovarian cavity fluid by the fetus (22). Recent findings in the nurse shark (Ginglymostoma cirratum) demonstrate the transfer of monomeric IgM and shark-specific Ig new AgR to the egg and subsequent uptake by the embryo (23). In fish, the duration of protection afforded by passive maternal transfer is ∼3–4 wk posthatch (24).

In most cases, the Abs exported to the yolk sac of fish are monomeric (H2-L2) or even halfmeric (H-L) forms of IgM (18). This may be due to the ease of transportation of monomeric forms across the egg membrane. However, in teleost blood, tetrameric IgM (H2-L2)4 is the most common form of Ab, although a high level of IgM structural variability (halfmer to trimer) exists as a result of differences in disulfide bonding (25). It is unknown whether monomeric or halfmeric forms present in the egg exhibit similar properties of tetrameric IgM, such as agglutination (26). It is interesting to note that, although maternal Abs transferred through the yolk sac reach the blood of the offspring and play an active role against systemic infection, the Abs transferred through egg white, colostrum, or milk, such as IgA or even IgE, appear to function primarily at the intestinal mucosal surface (27). In teleosts, it is not clear whether the mucosa-specialized IgT (28) plays a significant role in maternally transferred immunity. IgT at mucosal surfaces is tetrameric, in contrast to the serum monomeric form, and it binds a larger proportion of enteric bacteria in fish more effectively than does IgM (28). If the IgT molecules are indeed maternally transferred, it would be relevant to identify the type and duration of the protection afforded.

In addition to the prenatal route, the postnatal transfer of Abs through colostrum or milk is an interesting feature in mammalian maternal passive immunity (27). Although the postnatal route is mostly absent in nonmammalian vertebrates, including fish, some exceptions do exist, and they shed light on the potential usefulness of passive immunization as a therapeutic approach. In the discus fish (Symphysodon spp.), the young fry derive nutrition from feeding on parental skin mucus, which is rich in Abs (29). Additionally, the parental skin mucus Ab concentration has been shown to increase significantly in spawned fish and continues to be high until week 4 of the free-swimming fry stage. In tilapia (Oreochromis spp.) infected with the parasite Ichthyophthirius multifiliis, fry that underwent mouth brooding from a mother previously exposed to the parasite showed increased survival rates (20). The fact that many cichlid fish show parental care and the observed association of neonates with parental skin (30) indicate that postnatal transfer of mucosal Abs may be more widespread in teleosts than earlier assumed. Furthermore, if stable tetrameric IgT is indeed present at a high proportion in the fish mucus, these Igs will be available to certain cichlid offspring that show parental associations.

In applied maternal passive immunization strategies, the key elements are timing and quantity of Ab transferred. In the rainbow trout (Oncorhynchus mykiss), maternal passive immunity against infectious hematopoietic necrosis virus was demonstrated to persist for up to 25 d posthatch (31). An immunization schedule that would coincide with the natural peak immunity during vitellogenesis and oogenesis in brood fish has also been suggested, which would make more Ab available to the progeny (18). Other considerations, such as environmental and genetic effects on maternal Ab transfer, are also relevant within the context of applied maternal passive immunization. Although the benefits of maternal passive immunization are many, one must consider the possibility of a “blocking effect” on subsequent vaccination, in which maternal Abs present in juveniles might block the same Ag used in a vaccine and reduce its efficacy (32).

A number of passive immunization studies in fish only qualify as in vivo neutralization tests against the pathogen because the Ab is injected i.p. up to 1 d before the pathogen challenge or, in some instances, together with the live pathogen. For therapeutic considerations, a window has to be identified for each pathogen that includes how quickly and for what duration the passively transmitted Abs are present in the recipient fish (this is discussed further in the last section). Depending on this information, continuous administration of passive Abs may be necessary in some cases. In salmonids, i.p.-injected IgM is demonstrable in serum as early as 10 min (33) after administration, and the uptake is complete in 8 h (34). The Ab could further remain protective for up to 60 d postadministration (33). In other species, the half-life of i.p.-injected IgM is reported to be from 7 to 22 d (3537).

A protective effect of sheep and rabbit IgG has been successfully demonstrated in rainbow trout against Streptococcus spp. (38) and Vibrio anguillarum infections (39). The development of Abs against specific, relevant epitopes is decisive for the attainment of therapeutic or neutralizing effects (40). In addition to the nature of the epitope, the mode of Ab action can be revealed by passive/adoptive-transfer experiments. In an elegant study by Clark et al. (41), the ciliate parasite I. multifiliis was bound and successfully eliminated from juvenile channel catfish (Ictalurus punctatus) by passively transferred murine mAbs or their Ag-binding fragments [F(ab′)2] given by i.p. injection but not by the monovalent Fab fragment alone. However, when secondary goat IgG against murine Fab fragment was injected into the fish, the cross-linking ability of the monomeric Fab fragment was restored. This study demonstrates the inherent drawback of passively administered/derived monomeric Ab fragments, which may be overcome by increasing the Ab valency (using recombinant methods), as will be discussed later in this review.

The valency and class or isotype of the Ab are relevant for its efficacy in passive immunization (42, 43). In mammals, there are five Ab isotypes: IgG, IgM, IgA, IgD, and IgE (44). In teleost fish, IgM, IgD, and IgT/IgZ have been characterized (9). The common Ig monomeric form consists of two H chains and two L chains arranged into two Fab arms (Ag-binding fragments) and one Fc region (crystallizable fragment) separated by a flexible hinge. H chains are typically composed of one V and two to four C regions, whereas L chains consist of one V and one C region. The paired variable fragments (Fvs) of one Fab arm constitute the actual Ag binding site, whereas the Fc region is responsible for interactions with various receptors and effector molecules. The Ig monomers can cross-link into polymer structures, such as tetramers or pentamers (44).

At mucosal surfaces of fish, IgT is assumed to have special functions, similar to its mammalian counterpart, IgA. However, IgM is most abundant at teleost mucosal surfaces, whereas IgT appears to be most induced upon infections (28). IgT and IgM are assumed to be transcytosed to the mucosal surface by the teleost polymeric IgR (45), although evidence also exists for the local presence of Ag-secreting cells in the epithelium (46). For passive immunization strategies, fish Igs would technically be most efficient against fish pathogens; however, the production and extraneous administration of such Abs are not being considered at present. A recent study in pigs explored the possibility of using porcine serum from slaughterhouse sources as a feed supplement for pigs (47). Such an approach might be a possibility in fish, incorporating serum collected from slaughtered fish in fish feed. Fish serum contains high levels of nonspecific natural Abs, and Wang et al. (26) demonstrated the presence of anti–Aeromonas hydrophila activity in embryos of naive zebrafish (Danio rerio), indicating the presence of natural Abs. However, such applications raise the fundamental problem of disease transmission through serum/plasma, which needs to be addressed before they can be considered as therapeutic strategies.

Mammalian IgG is a prime candidate for passive immunization. It is the most abundant Ab in mammalian serum (48) and is the most widely available Ab because of its standardized and commercialized method of production, in the form of mAbs and polyclonal Abs. However, mammalian IgM, with its pentameric structure and resulting multivalency, gives even higher avidity coupled with increased opsonizing efficiency (42). A revolutionary approach to Ab therapy in fish was unveiled by Lorenzen et al. (49); they injected a plasmid encoding a murine single-chain Ab fragment against viral hemorrhagic septicemia virus into rainbow trout. Abs were detected in the trout, and they protected the fish from experimental challenge with viral hemorrhagic septicemia virus 11 d postinjection of the Ab gene construct. The practical aspects of such a strategy (injecting the fish and the time delay for the production of Ab) with respect to passive immunization are not clear. Further, IgY, the predominant Ab found in the egg yolk of birds, is another important Ig class that is suitable for passive immunization because of its availability in chicken eggs (50). That being said, compared with IgG, the F(ab′)2 arm of the IgY molecule is less flexible, possibly making it less effective in cross-linking and agglutinating bacteria (51). In addition, IgY is specifically susceptible to digestive enzymes at low pH (52).

Naturally occurring H chain–only Abs have been found in camelids (53) and, in a different version, in sharks (Ig new AgR) (54). These Abs lack the characteristic L chain that is found in normal Abs. Modification of the amino acids present in the L chain–H chain interface results in a more hydrophilic and stable H chain V region (55, 56). The Ag-binding site is thus confined to one single variable domain of the H chain of H chain–only Abs (VHH) that can easily be produced as recombinant single-domain Abs (sdAbs/recombinant VHH [rVHH]) in prokaryotic expression systems (56) and has been shown to be very effective in passive immunization and therapeutic applications (57). Additionally, camelid rVHH fragments are highly resistant to heat and pH extremes (56, 57). A comprehensive review of the properties of camelid sdAbs and their amenability to passive immunization can be found elsewhere (56, 58).

Ab fragments, such as F(ab′)2, Fv, and single-chain Fv (scFv) derived from typical whole Abs, are also important in passive immunization (59). scFv is a recombinant fusion protein of H and L chain V regions connected by a flexible linker. Ab fragments like these can preclude the effect of the Fc region and consequent inflammatory reactions, which may or may not be desirable (60). However, if Fc-based immune stimulation is required, the teleost Ig Fc can be fused (recombinantly) to sdAbs (61). The properties of Abs that may be relevant in aquaculture passive immunization are listed in Table I.

Table I.
Characteristics of Ig classes and fragments significant for passive immunization in aquaculture
AbStructureValencyMolecular WeightSerum Half-Life (d)Agglutination Efficiency
Mammals IgG Monomer 150 23 Fairly good 
 IgM Pentamer (J chain) 10 970 5–10 Excellent 
Teleosts IgM Tetramer (no J chain) 600–850 12–16 Excellent 
 IgT Tetramer (mucus) 700   
  Monomer (blood) 180   
Birds/amphibians IgY (egg yolk) Monomer 167 1.85 Low (improved in high salt concentrations) 
Ab fragments Fab  50 0.3–0.8 Low 
 scFv  27 0.1 Low 
 sdAb  13 <0.5 Low 
AbStructureValencyMolecular WeightSerum Half-Life (d)Agglutination Efficiency
Mammals IgG Monomer 150 23 Fairly good 
 IgM Pentamer (J chain) 10 970 5–10 Excellent 
Teleosts IgM Tetramer (no J chain) 600–850 12–16 Excellent 
 IgT Tetramer (mucus) 700   
  Monomer (blood) 180   
Birds/amphibians IgY (egg yolk) Monomer 167 1.85 Low (improved in high salt concentrations) 
Ab fragments Fab  50 0.3–0.8 Low 
 scFv  27 0.1 Low 
 sdAb  13 <0.5 Low 

J chain, joining chain.

When Abs interact with pathogens, they bind to the pathogen and may inhibit its spread in several ways. How this complex behaves depends entirely on the nature of the epitope and Ab class involved. The passively applied Ab can result in systemic and localized effects. Ab taken up by the oral route through feed or immersion will primarily target pathogens in the intestine (Fig. 1). A large number of fish pathogens, including viruses, gain entry through the intestinal route (6264), and passively administered Ab in the gut may bind to the adhesive receptors in the pathogen, thereby preventing them from attaching to mucosa (adherence blockade). Abs targeted at epitopes that occur in multiple copies, such as viral coat proteins, can cross-link these epitopes and agglutinate the pathogen (65). This cross-linking or agglutination effect has been demonstrated best with passive immunization against rotaviruses (66) and Escherichia coli (67), where the fraction of agglutinated viral/bacterial load can be measured in the feces.

FIGURE 1.

Mucosal actions of Abs and rVHH. (A) In mammals, plasma cells produce IgA/IgM in response to Ags, such as virus and bacteria, which are transported across the epithelium by polymeric IgR (pIgR). Igs can bind and neutralize/agglutinate Ags in the lumen, within epithelial cells, and in the subepithelial compartment. Within epithelial cells, the Ag–Ab complex degrades by fusion with lysosomes or is expelled back into the lumen. (B) Orally delivered rVHH from camelid H chain Abs may prevent adhesion of Ags to the mucosa. rVHH is expected to be transported across the epithelium by transcellular and paracellular routes because of its small size. Hence, it is possible for rVHH to bind to Ags intracellularly, subepithelially, and in systemic compartments. By linking of rVHH to the Fc region of fish Igs (rVHH-Fc), dimers similar to the camelid H chain Ab are expected to be produced in vivo. Such rAbs may work in a similar fashion to normal Abs, neutralizing and agglutinating Ags. (C) Possible Fc-mediated immune responses of rVHH-Fc, similar to conventional Abs. Phagocytosis by macrophages initiated by opsonization and activation of complement or cell-mediated cytotoxicity, leading to cell lysis.

FIGURE 1.

Mucosal actions of Abs and rVHH. (A) In mammals, plasma cells produce IgA/IgM in response to Ags, such as virus and bacteria, which are transported across the epithelium by polymeric IgR (pIgR). Igs can bind and neutralize/agglutinate Ags in the lumen, within epithelial cells, and in the subepithelial compartment. Within epithelial cells, the Ag–Ab complex degrades by fusion with lysosomes or is expelled back into the lumen. (B) Orally delivered rVHH from camelid H chain Abs may prevent adhesion of Ags to the mucosa. rVHH is expected to be transported across the epithelium by transcellular and paracellular routes because of its small size. Hence, it is possible for rVHH to bind to Ags intracellularly, subepithelially, and in systemic compartments. By linking of rVHH to the Fc region of fish Igs (rVHH-Fc), dimers similar to the camelid H chain Ab are expected to be produced in vivo. Such rAbs may work in a similar fashion to normal Abs, neutralizing and agglutinating Ags. (C) Possible Fc-mediated immune responses of rVHH-Fc, similar to conventional Abs. Phagocytosis by macrophages initiated by opsonization and activation of complement or cell-mediated cytotoxicity, leading to cell lysis.

Close modal

It has been observed that passively administered Abs facilitated nonspecific, innate immune characteristics, such as increased phagocytic activity against the pathogen (68) and improvement of blood parameters (69). These enhanced innate immune reactions are difficult to explain, especially because nonpiscine Abs, such as avian IgY, should not evoke any Fc-mediated reactions, such as complement activation, in fish, and should be examined further. It has also been reported that IgY bound to bacteria in vitro reduced their growth up to 5-fold (69). When bacteria are agglutinated or bound to Ig, bacterial absorption of key nutrients, such as iron (siderophore bound), could also be blocked, leading to growth impairment (70). Another possibility is that the IgY binding to E. coli could have altered the structure of the bacterial cell membrane (71). In a more recent example of the direct action of Abs, trypanolytic camelid rVHH was found to possibly block endocytosis by the parasite Trypanosoma brucei (72). sdAbs may not be effective cross-linkers because of their small size and monovalency, although multiple sdAbs can be recombinantly linked to form diabodies or pentabodies (73) that can improve binding and neutralization efficiency. Camelid sdAbs can be engineered to have Fc fusion tails (74) from fish; thus, it may be possible to realize some of the Fc-mediated functions, such as Ab-dependent cellular cytotoxicity.

The action of therapeutic Abs against intracellular pathogens, such as viruses and certain obligate intracellular bacteria, may sometimes be considered ineffective because Abs are generally considered to be poorly translocated into tissues. Oliver et al. (70) developed IgY against intracellular bacteria Piscirickettsia salmonis affecting salmonids. They preincubated a salmon head kidney cell line with specific IgY for 1 h and then infected the cell line with the bacteria. The cell line remained uninfected for up to 10 d, suggesting a good level of protection. In the case of viruses, even if neutralizing Abs fail to prevent the entry of viruses into cells, a postinternalization block of entry has been proposed (75). This can happen when a virion bound to an Ab is internalized in an endosome and is prevented from exiting, as well as in a postendosomal exit scenario in which Ab-bound virions could be targeted for ubiquitination and subsequent proteasomal degradation. Recently, camelid rVHH that are stable in the cytoplasm have been introduced. These “intrabodies” could also revolutionize the targeting of intracellular pathogens or their products (76).

Uptake of Ags in fish gut has been studied extensively (77); however, unlike in vaccination, where only a peptide region of the Ag is sufficient to trigger immunity, the absorbed Ab in passive immunization should stay functional in its area of action and, therefore, retain its structural integrity during the uptake process. O’Donnell et al. (78) demonstrated the uptake of human γ globulin (HGG) through the posterior intestine of presmolt Atlantic salmon (Salmo salar). Free HGG was detected in serum within 15 min, and it localized in the enterocytes. However, the functional integrity of the absorbed HGG is unknown, because only immunohistochemistry-based detection was used. Similar studies with HGG were undertaken in rainbow trout, which demonstrated localization of HGG in the apical and middle portions of the enterocytes (79). Interestingly, it was revealed that orally delivered Abs that managed to traverse the acidic stomach and protease-rich intestinal lumen could still be affected by lysosomal enzymes, such as cathepsins, during the intracellular uptake process. Nevertheless, the investigators speculated that whole Abs internalized by intestinal epithelial cells may retain their Ag-binding F(ab′)2 fragment intact, which holds significance for an oral passive immunization strategy. It also suggests that Ab fragments or sdAbs may be able to translocate easily into the bloodstream. A different study in rainbow trout suggested that the degree of fragmentation of HGG reaching plasma was very high, regardless of the administration of gastric inhibitors, such as cimetidine or sodium bicarbonate (80). Fujino et al. (81) demonstrated that orally administered rabbit IgG against HRP was taken up by the adult rainbow trout posterior intestinal segment. The undigested, or at least functionally active, IgG was localized in columnar epithelial cells, which appeared to spread to the inner regions over time. The Ab that had been taken up was still functional, because the primary probe (HRP) bound to the absorbed Ab (anti-HRP).

The intestinal uptake of avian IgY in gastric rainbow trout and agastric carp was found to be vastly different. Winkelbach et al. (82) reported that IgY administered through the oral and anal routes in rainbow trout was not subsequently detectable in the blood. Agastric common carp (Cyprinus carpio), lacking acidic stomach and digestive enzymes, such as pepsin, demonstrated an efficient uptake of IgY. Although IgY is more susceptible to pepsin digestion and an acidic pH, this does not seem to be the sole reason for the reduced uptake. A comparative study using a chamber experiment of posterior gut segment also revealed that uptake through the posterior segment was much higher in carp than in trout (83). Although transport of a large molecule, such as Ig, across the epithelium would appear to be through transcytosis in fish (83), the transport of much smaller and hydrophilic molecules, such as camelid rVHH, could also occur through the paracellular transport route involving cellular tight junctions. However, further studies on the uptake and stability of Ab fragments in fish are needed.

Aquaculture is unique in that the aquatic environment, unlike terrestrial farming practices, is less amenable to segregation for individual treatment and care (84). Applied aspects of therapy and vaccination in aquaculture are always a challenge. Like the widely practiced effective vaccination strategies, a standardized i.p. administration of therapeutic Abs would be expensive and difficult in grow-out systems. In addition, for reasons related to fish welfare, injections should be limited.

Therefore, oral administration of therapeutic Abs appears to be the best strategy in aquaculture. This gives high flexibility for repeated therapeutic Ab administration, in the early life stages when the fish are too small for vaccination, as well as when the fish are large in size and near to slaughter and practical limitations prohibit i.p. administration. Moreover, if given orally, the passively transmitted Abs may neutralize Ags in the gastrointestinal lumen, in contrast to i.p.-administered Abs. One often-neglected aspect of oral delivery is that of the resident time of feed/chyme/chyle in the fish gut, because this can vary among species and life stages. For example, the resident time for feed in juvenile rainbow trout is ∼9 h (82). The denaturing effect of stomach acids can be countered by using antacids, such as NaHCO3 (12) or cimetidine (80), in the feed. However, because the increase in pH may affect overall feed digestibility, this may not be a practical approach in aquaculture, and neither is withholding of feed to reduce the level of digestive enzymes (85). Detergents, such as M9, deoxycholate, or Quil-A saponin (86), may help in sloughing off epithelial cells, thinning of mucosa, or even improving absorption through intercellular junctions. Microencapsulation is another possible method to protect the Abs from stomach enzymes and assist in their slow release in an alkaline intestinal environment (87, 88).

As in any other food-production system, biosecurity, including prophylaxis and therapy, is important in aquaculture. It is possible to implement Ab-based prophylactic applications in aquaculture that are founded on previously identified checkpoints akin to a hazard analysis and critical control points system that is practiced widely in the food industry (89). Within aquaculture, hatchery-level operations and recirculatory systems may easily benefit from Ab applications because of the low biomass or smaller grow-out space. Traditional prophylactic methods, such as ozonization and UV irradiation of water, may have drawbacks, including expense and the formation of toxic bromate (90).

Within finfish aquaculture, prophylactic Abs could be used starting from fertilized eggs (91) intermittently washed with specific antifungal Abs, dip treatments for larvae, and disinfection treatments for larval prey (92) (Fig. 2). Additionally, Abs against opportunistic pathogens, such as Vibrio spp., can be of therapeutic value, especially when administered before handling or transportation.

FIGURE 2.

Illustration of key time points in salmon aquaculture relevant for Ab-based prophylaxis. The entire period is susceptible to pathogens, such as bacteria and viruses. Although the adaptive immune system develops around 4–6 wk posthatch, vaccination usually takes place at the age of 6–12 mo, shortly before sea transfer. Transition into the different physiological stages, time of vaccination, sea transfer, and seasonal changes are some of the key time points for Ab-based prophylactic or therapeutic interventions. W, winter.

FIGURE 2.

Illustration of key time points in salmon aquaculture relevant for Ab-based prophylaxis. The entire period is susceptible to pathogens, such as bacteria and viruses. Although the adaptive immune system develops around 4–6 wk posthatch, vaccination usually takes place at the age of 6–12 mo, shortly before sea transfer. Transition into the different physiological stages, time of vaccination, sea transfer, and seasonal changes are some of the key time points for Ab-based prophylactic or therapeutic interventions. W, winter.

Close modal

Therapeutic/prophylactic strategies in large cages, pens, or earthen ponds can be approached under different circumstances. Within the biosecurity realm, a well-timed therapeutic Ab administration could prevent or limit epidemiological spread among different farms within an open body of water. Consequently, mass slaughter (to prevent spread) of valuable fish could be delayed or avoided. For instance, it has been demonstrated that progression of white spot syndrome in shrimp can be delayed using neutralizing Abs against white spot syndrome virus envelope proteins (93). In a vaccinated population, a minor proportion of vaccine-ineffective or disease-susceptible individuals may exist, yet the population may be protected from an epidemic because of “herd immunity.” Mathematical models have been devised that explain the phenomenon (94). For example, in Atlantic salmon farms, the herd immunity threshold has been estimated to be 52% for infectious salmon anemia (95) and 66% for pancreas disease (96). Although therapeutic Abs are not as long-lasting as in a vaccine-based immune response, the mode of function can be assumed to be the same. Therefore, in the case of a prevaccinated population, Ab therapy can significantly increase the herd immunity threshold and provide a better level of protection. Passive immunization of a nonvaccinated population or against a disease for which no vaccines exist can be more complicated, because the duration of protection offered is relatively short. The question of whether the Ab promotes shedding of the pathogen (97) into the environment also needs to be addressed. This is especially important, because the possibility of reinfection could be high in a population with a waning immune response. In such a situation, continuous administration for a predetermined period may be necessary to sustain the levels of Ab and preclude reinfection (98). One of the key assumptions for using therapeutic Abs within grow-out aquaculture systems is that they can provide protection during the period required for the development of the adaptive immune response toward a disease. This is an interesting premise, but it needs to be approached carefully because it has been demonstrated in mammals that vaccination is ineffective in the presence of protective (maternal) Abs against the same target; a major factor to consider is the masking of Ag epitopes by therapeutic Abs and inhibition of B cell stimulation (99). The impact of such a possibility has to be verified experimentally in aquaculture before we can speculate about its potential benefits.

Globally, the pace of development of therapeutics in aquaculture has been lagging as a result of the lack of any major technological advances. Nevertheless, although Ab-based therapy can be called a traditional approach, evolving Ab formats have made the option much more versatile and effective. Novel techniques, such as targeted bacteriophage therapy (100) and RNA interference–based antiviral therapy (101), are also under consideration within aquaculture. Although these techniques hold a lot of promise, they carry the potential risk for virulent gene transfer, development of phage resistance (102), and even issues related to genetic manipulation.

Like vaccines, polyvalent therapeutic Abs can be developed and administered simultaneously against a panel of multiple pathogens. Also, with regard to culturing of shellfish, which have no adaptive immune system (103), passive immunization is a sustainable and effective therapeutic solution, especially in the shrimp-production cycle, which spans a maximum of 6 mo. A major factor that has been overlooked in the development of therapeutic Abs in aquaculture is the creation of a knowledge base for pathogenesis and pathogenic epitopes. To create highly effective therapeutic Abs, a detailed understanding of the structural proteins of the pathogen will be beneficial. Additionally, the pharmacodynamic aspects of therapeutic Abs need to be explored more thoroughly to develop passive immunization as a reliable strategy. For instance, selecting for optimal combinations of key residues in the variable domain of rAbs may improve the affinity and, hence, the ability to neutralize the target (104). Such advanced knowledge of fish/shellfish immunology and pathology may pave the way for targeting of immune molecules of aquatic animals with therapeutic Abs to realize disease-free, low-stress culture systems, ultimately aiming for sustainability and better animal welfare.

This work was supported by the Research Council of Norway through “The Mucosal Surfaces of Atlantic Salmon: A Site for Viral Entry and Possible Passive Immunization” Project 225065/E40.

Abbreviations used in this article:

Fv

variable fragment

HGG

human γ globulin

rVHH

recombinant VHH

scFv

single-chain Fv

sdAb

single-domain Ab

VHH

variable domain of the H chain of H chain–only Ab.

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The authors have no financial conflicts of interest.