Veterinary Research Communications, 27 Suppl. 1 (2003) 471–479© 2003 Kluwer Academic Publishers. Printed in the Netherlands

Emerging Pathologies in Aquaculture:

Effects on Production and Food Safety

C. Ghittino, M. Latini, F. Agnetti, C. Panzieri, L. Lauro, R. Ciappelloniand G. PetraccaIstituto Zooprofilattico Sperimentale dell’Umbria e delle Marche V ia Salvemini 1,06126 Perugia, Italy

ABSTRACT

Infectious diseases represent a limiting factor for the further development of Italian aquaculture. The

recent introduction and spreading of new pathogens, along with the global climatic change, has contrib-

uted to a considerable decrease in trout production. Emerging pathologies in rainbow trout culture

include viral diseases, e.g. infectious haematopoietic necrosis (IHN), bacterial diseases, such as lacto-

coccosis and visceral flavobacteriosis, and parasitical diseases, e.g. proliferative kidney disease (PKD).

Higher mortality rates in trout fry and fingerlings are generally induced by visceral flavobacteriosis and

IHN, while the main losses in large trout during the warm season are due to lactococcosis and PKD.

Mariculture has at present a better sanitary status compared to trout culture, but a rapid dissemination

of pathogens, including zoonosis agents, is envisaged also for seabass and seabream. Emerging patholo-

gies in sea bass include VNN, pseudotuberculosis, streptococcosis and tuberculosis. Seabream is much

more resistant and is mainly affected by novel V ibrio infections and enteromyxidiosis. A good sanitary

management of fish farms is essential for avoiding or limiting losses caused by emerging pathologies.

Transmission of zoonosis agents to man, through the consumption of cultured fish, is very remote in

Italy. On the contrary, transmission of Streptococcus iniae, V ibrio vulnificus andMycobacterium marinum

by means of improper manipulation of infected fish, could represent a potential hazard for fish farmers

and fish processors, as well as for people preparing fish meals.

Keywords: disease, fish culture, seabass, seabream, trout, zoonosis

INTRODUCTION

Fish culture has long been a well-established industry in Italy (Ghittino, 1983). As aconsequence of global warming and the appearance of new diseases, the productionof the main cultured freshwater fish species, rainbow trout (Oncorhynchus mykiss), hasrecently decreased, being around 40 000 tons per year. On the other hand, intensivemariculture has considerably increased, with a yield that reached 20 000 tons in 2002,comprising 60% of seabass (Dicentrarchus labrax) and 40% of seabream (Sparusaurata).The major threat to Southern European freshwater and marine fish farming isrepresented by infectious diseases, such as viral, bacterial and systemic parasiticalconditions (Ghittino, 1985).Emerging pathologies in trout farming include infectious haematopoietic necrosis,rainbow trout fry syndrome, Gram-positive coccal infections and proliferative kidneydisease (PKD), while the main sanitary problems in seabass and seabream farming

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are viral nervous necrosis, novel V ibrio infections, pseudotuberculosis, Gram-positivecoccal infections, tuberculosis and enteromyxidiosis (Roberts, 2001).

MATERIALS ANDMETHODS

Over the past 3 years health monitoring was performed in Italian trout farms andmariculture plants. Sick fish were sampled and forwarded for diagnosis to the FishDisease Laboratory, by the State Veterinary Institute located in Perugia, Italy.Fish were subjected to necropsy and fresh parasitological examination, followed bybacteriological and virological analysis. Organic lesions were evaluated with normalhistology.Bacteriological examination was carried out on ordinary and selective media(skimmed acetate agar for flavobacteria, TCBS agar for V ibrio representatives,Loewenstein–Jensen for mycobacteria). Identification was obtained by phenotypiccharacterization.Virological examination was performed on EPC and RTG-2 monolayers.Identification was obtained through the indirect fluorescent antibody test.

RESULTS

V iral diseases

Infectious haematopoietic necrosis (IHN)

The etiological agent of IHN, a Rhabdovirus, was introduced in Europe at thebeginning of the 1980s and since that time the disease has spread in many areas,including Italy (OIE, 2000, 2002). The spread was enhanced by the fact that broods-tock may act as carriers and vertically transmit the virus. Disinfection of eggs withiodine does not totally guarantee against the transmission of IHN. For these reasonsseveral European trout hatcheries became infected by introducing infected eggs.Disease outbreaks appear with water temperature below 15°C. Unlike for viralhaemorrhagic septicaemia (VHS), large trout are seldom affected by IHN, which istypical of juveniles. Infected fry usually show delayed yolk sac reabsorption andsudden mortality up to 100%. Fry and fingerlings reveal severe muscular haemor-rhaging, associated with an anaemic condition. As in the case of VHS, a typicalnervous symptomatology becomes evident in fingerlings at the end of outbreaks.No medical prophylaxis is at the moment available to control IHN. Prevention isbased on the application of hygienic measures, such as void for sanitation and quaran-tine (Ghittino, 1985). Hatcheries should be isolated from ongrowing facilities andsupplied only with well water. Tanks should be dried and disinfected before startingnew cycles. Eggs should be purchased only from companies certified free from IHN

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and disinfected with iodine prior to their introduction in the hatchery. In those troutfarms devoted to egg production a constant health surveillance and monitoringshould be carried out on broodstock fish. In case of IHN onset in hatcheries orbroodstock facilities, stamping out is recommended.IHN is included in list B of OIE and is a notifiable disease in the EU (OIE,2000, 2002).

Viral nervous necrosis (VNN)

The Nodavirus responsible for VNN was introduced in the Mediterranean in the1990s (OIE, 2000, 2002). Vertical transmission of the agent is strongly suspected, sincethe majority of outbreaks are reported from hatchery systems.Both juvenile and large seabass may suffer from the disease with water temperatureabove 18°C. If larvae become infected the consequence is a mortality up to 100%, butserious losses may also be present in ongrowing facilities (20–30%). Diseased fish areusually blind, show a typical nervous symptomatology and die in an asphyxialmanner. Histological examination shows pathognomonic neuronal degeneration,associated with degeneration of the retina granular layer.No medical prophylaxis is available to control VNN. Prevention is based onhygienic measures and consists mainly in avoiding the introduction of fish ofunknown health status into the farm (Pavoletti et al., 1998). Hatcheries should besupplied only with sterilized water. New broodstock should be kept in quarantine inseparate tanks and checked for the presence of infectious agents; discharge water mustbe chlorinated. An appropriate drying and disinfection of tanks is indicated beforeeach fish transfer. If juveniles have to be purchased, they should come from farms thatare subject to methodical VNN monitoring. In case of VNN onset in a hatchery or anursery, stamping out measures are suggested.

Bacterial diseases

Rainbow trout fry syndrome (RTFS)

RTFS, or visceral flavobacteriosis, commonly but erroneously known as myxobacter-iosis, is the most common bacterial disease that can be found in Italian trout hatcher-ies (Ghittino and Pedroni, 2001). It is caused by Flavobacterium psychrophilum, anorganism highly pathogenic for juveniles that, despite therapy, give raise to averagelosses of 20–25% (Austin and Austin, 1999). Fry affected by visceral flavobacteriosisshow hydrops and delayed reabsorption of the yolk sac, while fingerlings up to 5grams reveal lethargy, slight anaemia and remarkable spleen enlargement.Attempts to control this condition rely mainly on the delivery of medicated feed(antibiotics) and the application of medicated baths (disinfectants). Due to the limited

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number of effective antibiotics, therapy is often problematic: resistance is frequent,therefore continuous relapses are unavoidable. No vaccines are available to preventvisceral Flavobacteriosis. Prophylaxis is based on the application of hygienic precau-tions, such as void for sanitation before starting new cycles, routine disinfections andgood management practices.

Novel V ibrio infections

The term vibriosis is generally used to define V ibrio anguillarum infection, the mostcommon bacterial condition affecting marine fish (Ghittino et al., 1994). Recently,other V ibrio species – V. alginolyticus and V. vulnificus – were found to be involved indisease outbreaks in seabream and seabass. They are generally considered as opportu-nistic invaders or weak pathogens of stressed fish, responsible for skin and fin haemor-rhaging associated with dripping mortality (Austin and Austin, 1999).A great concern for V. vulnificus is now envisaged, for its capability of acting as azoonosis agent. Human cases of this infection have been reported from different partsof the world, following handling in contaminated environments (Ghittino andBozzetta, 1994). Working in infected ponds and cages, or improper manipulation ofinfected fish, may therefore represent a potential hazard for fish farmers.Classical vibriosis may be controlled either with therapy or vaccination, throughthe use of immersion and oral bacterins, while control of novel V ibrio infections isonly based on the delivery of medicated feed. To achieve good results with therapythe removal of stress factors that are first responsible for the onset of the condition isessential. Prevention is based on the application of hygienic measures, such as goodmanagement practices.

Pseudotuberculosis

Pseudotuberculosis, commonly but erroneously known as pasteurellosis, is at presentthe most severe bacterial disease in Mediterranean mariculture (Ghittino et al., 1993,1994). It is caused by Photobacterium damselae ssp. piscicida, a Vibrionaceae represen-tative pathogenic for both seabass and seabream with water temperature above 18°C(Austin and Austin, 1999). The problem is particularly important in hatchery andnursery systems, since in juveniles the disease develops with an acute course, leadingto a mortality that may reach 70–80%, if proper therapy is not immediately applied.On the contrary, the course in adult fish is generally chronic, with average losses of15–20% and the presence of typical nodules in the spleen and kidney, histologicallycorresponding to necrotic granulomata.Control of pseudotuberculosis is basically dependent on therapy. An early diagno-sis of the disease is essential, in order to feed fish with antibiotics at the beginning ofthe infection, before the anorectic condition appears. Although present on the market,

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immersion and oral vaccines are little effective to prevent the onset of pseudotubercu-losis. Duration of protection is limited to short periods and the application of thera-peutic measures is normally necessary.

Gram-positive coccal infections

Generically known as streptococcosis, Gram-positive coccal infections were intro-duced in Europe in the mid-1980s. They soon spread, becoming dominant, and arenow considered as one of the main threats to local aquaculture production (Ghittinoand Pedroni, 2001). Unlike the great majority of fish diseases, adults (fish over 50grams) are very susceptible to these infections, while juveniles are generally resistant.From a pathogenicity point of view, they can be divided into two groups: warmwaterinfections, caused by those cocci species (L actococcus garvieae, Streptococcus iniae, S.agalactiae, S. parauberis) that are pathogenic for both cultured freshwater and marinefish at water temperature above 15°C, and coldwater infections, caused by those coccispecies (Vagococcus salmoninarum) that are pathogenic exclusively for salmonid fishat water temperature below 12°C (Eldar and Ghittino, 1999).L actococcus garvieae infection, or lactococcosis, is nowadays the most serious

disease of rainbow trout in Italy. It affects large trout during summer, when watertemperature ranges from 16 to 21°C, causing average losses up to 50–60%, in spite ofmedical treatments. Signs are typical of an acute infection, with lethargy, anorexia,melanosis, marked bilateral exophthalmos, visceral serositis and haemorrhaging,severe enteritis, notably spleen enlargement. Lactococcosis is also reported as a seri-ous disease for mariculture in the Far East, but at the moment it is not affectingcultured marine fish in the Mediterranean, despite its great distribution amongSouthern European trout farms.Streptococcus iniae infection, or streptococcosis in the strict sense of the word, is at

present restricted to the Eastern Mediterranean area, affecting both large rainbowtrout and large marine fish during the warm season. Streptococcosis has a typicalsubacute course and, due to its difficult therapeutic approach, is responsible foraverage mortality rates of 50% in trout, while losses are more moderate in seabassand seabream (10%). Affected fish show a characteristic bilateral popeye with kerati-tis but, unlike for lactococcosis, no visceral haemorrhaging is usually present.Moreover, nervous signs may also be expressed. Histologically, a severe panophthal-mitis and meningitis are the common features of this disease.There is a great concern for S. iniae, since it is regarded as a zoonosis agent of fish

origin (Ghittino and Bozzetta, 1994). Cases of human infection occuring after hand-ling infected tilapias, have recently been reported in North America (Ghittino, 1999).Cultured spiny fish may therefore represent a potential risk for consumers, besidesbeing a hazard for fish farmers and fish processors.Control of L . garvieae and S. iniae infections in trout is very problematic, since

therapy with medicated feed is not very effective. The high pathogenicity of both

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causative agents, the early anorectic condition in infected fish, the availability on the

market of few efficacious antibiotics and the frequent antibiotic resistance, lead to

continuous relapses. Prolonged therapy, besides being environmentally unfriendly,

implicates the constant presence of antibiotic residues in trout, which do not allow

marketing during the entire warm season. Therapy of S. iniae infection in marine fishis not so problematic as in trout, and positive results may be achieved by using

medicated feed. The catastrophic effects of L . garvieae and S. iniae infections in troutmay now be prevented through injection/vaccination (Bercovier et al., 1997). Intra-peritoneal delivery of specific vaccines induces good protection levels against both

diseases for about 6 months, allowing completion of the production cycle.

Vagococcus salmoninarum infection, or vagococcosis, affects large trout in coldwater

environments (Ghittino and Pedroni, 2001). In Italy it is present mainly in mountain

districts, causing average losses of 15–20%. The disease has a typically chronic course,

with marked bilateral exophthalmos leading to frequent disruption of the eyeball,

pale gills, pericarditis and faint visceral haemorrhaging. Control of vagococcosis is

not so complicated as for warmwater Gram-positive coccal infections. Good results

may be achieved through the delivery of medicated feed. At present no vaccines are

available to prevent the disease.

Tuberculosis

Tuberculosis should be regarded as a disease complex caused by acid-fast bacteria

belonging to the mycobacteria (Austin and Austin, 1999; Ghittino, 2002). By far the

most common species isolated from marine fish is Mycobacterium marinum, which isalso a zoonosis agent (Ghittino et al., 1994). Its prevalence is higher in ornamental

fish, but food fish may also be affected. Cultured seabass is particularly susceptible to

M. marinum infection, and recent cases were reported from the EasternMediterraneanarea. The course of the disease is typically chronic, with an average mortality of

5–10%, and its transmission is probably related to the procedure of feeding fish with

infected trash fish. Diseased seabass are cachectic and show swelling of the abdomen,

with presence of miliary tubercles in the liver, spleen and kidney, histologically corre-

sponding to granulomata.

Human cases of M. marinum infection have been reported from different parts ofthe world (Ghittino and Bozzetta, 1994). Handling in infected marine aquaria is the

main source of infection for man, especially aquarists, but working in infected ponds

and cages, or manipulating infected fish, may also represent a potential hazard for

fish farmers.

No therapy or medical prophylaxis is available to control fish tuberculosis.

Prevention is based on the application of hygienic measures, such as good manage-

ment practices. Feeding fish with trash fish is not recommended.

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Systemic parasitical diseases

Proliferative kidney disease (PKD)

PKD is the second most important summer pathology of rainbow trout in Italy(Ghittino et al., 1988; Marin et al., 1992). It is a systemic infection caused byT etracapsuloides bryosalmonae, a malacosporean myxozoan whose taxonomic posi-tion has only lately been established (Canning et al., 2002). PKD outbreaks appearfrom May to September, when water temperature is above 15°C. Both juvenile andadult trout may be affected, with an average mortality reaching 50–60% in fingerlingsand 20–30% in larger fish. Diseased trout show abdominal distension, anaemia,spleen enlargement and a pathognomonic swollen hind kidney. Histological examina-tion indicates a notable hyperplasia of the interstitial renal tissue, with presence ofparasitic cells.The life cycle of T . bryosalmonae is not fully known, but recent findings havedemonstrated that some invertebrates of fouling, freshwater bryozoans, are involved(Okamura and Wood, 2002). According to recent studies, bryozoans act as alternatehosts for the parasite, while salmonids are probably accidental hosts. Spores releasedfrom bryozoans infect rainbow trout, producing PKD when the appropriate temper-ature conditions are present.Therapy to control PKD is not applicable and no vaccines are available on themarket. A prophylactic measure that can be adopted in case of long production cyclesis represented by natural immunization, obtained by exposing fingerlings in infectedenvironments at the end of summer, when water temperature decreases spontane-ously. Mortality is low and a solid immunity develops, capable of protecting fish fromthe pathogen for the rest of its economic life. In case of outbreak of PKD, hygienicmeasures include good water oxygenation, in order to face anaemia, drastic reductionof feeding and avoid handling and transport of fish.

Enteromyxidiosis

This enteric infection is responsible for important economic losses in cultured sparidfish, particularly seabream (Pattono et al., 1997). The etiological agent, Enteromyxumleei, a recently renamed myxosporean myxozoan, is a true histozoic parasite(Palanzuela et al., 2002). Proliferative stages affect the digestive epithelium, causingsevere enteritis. Spores, that can be found free in the intestinal lumen, are implicatedin a direct fish-to-fish transmission of the disease. The infection contributes to acachectic and emaciated condition that ends with fish death. Mortality is generallychronic and evident in large fish with water temperature above 18°C. No therapy ormedical prophylaxis is available to control enteromyxidiosis. For limiting losses andfurther spread of the pathogen, hygienic measures are normally applied. This includegood management practices, as providing an adequate water supply and oxygenation,

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avoiding an extreme overcrowding, applying a correct feeding programme and keep-ing ponds or cages properly clean. Routine disinfections, performed weekly, are usefulto reduce the impact of secondary infections.

DISCUSSION

Dominant infectious diseases often represent a limiting factor for the development ofaquaculture (Ghittino, 1985; Roberts, 2001). The recent introduction and spreadingin Europe of warmwater Gram-positive coccal infections, as a consequence of tradingand global warming, is a clear example of negative impact on fish yield. In recentyears, lactococcosis was the cause of a 20% reduction in Italian rainbow trout pro-duction, and a spread of the disease to cultured marine fish is much feared (Ghittinoand Pedroni, 2001). Furthermore, exotic pathogens such as Streptococcus iniae arepresent in neighbouring countries and could also infect trout, seabass and seabreamin Italy, inducing extra damage (Eldar and Ghittino, 1999). To avoid or to restrictlosses due to such pathologies, correct sanitary management of fish farms should beadopted. Priority must be given to aspects of fish health and welfare, through theapplication of vaccination programmes, appropriate therapeutic cycles and goodhusbandry, and regarding the aspect of habitat preservation, by reducing environ-mental impact caused by fish farming activities. In order to face zoonoses of fishorigin, special emphasis must also be given to the aspect of labour protection, byminimizing risks for fish industry workers, and to the aspect of consumer protection,through adequate food safety (Ghittino and Bozzetta, 1994). Transmission of zoono-sis agents from fish to man may occur through the consumption of uncooked fishproducts (food zoonoses) and through the manipulation of fish or handling in infectedenvironments (professional zoonoses). Food zoonoses may be a potential problem,particularly for fisheries, while professional zoonoses are more related to aquaculture.Handling of infected aquaculture products may lead to the transmission of zoonosisagents, such as Streptococcus iniae, V ibrio vulnificus andMycobacterium marinum, bymeans of skin injuries caused by unintentional stinging from fish spines. Professionalcategories at risk include fish farmers, fish processors and people preparing fish meals.In Italy, transmission of zoonosis agents through consumption of cultured trout,seabass and seabream is very remote, since these fish products are usually well cookedbefore being consumed.

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