Myxosporidean infections in Adriatic cage- reared fish

Myxosporidian parasite infestation was monitered in Adriatic cage reared fish. Fish were collected once every three months from offshore netpens in the eastern Adriatic, over a nine month period. Fish samples comprised from 9 to 15 each of sea bass (Dicentrarchus labrax), sea bream (Sparus aurata), sharpsnout bream (Diplodus puntazzo) and black sea bream (Pagellus bogaraveo), all aged 1+. Only four myxosporean species were identified from these cage-reared fish in the Adriatic Sea. There were differing dynamics in their prevalence and intensity. Introduction The fast and progressive development of mariculture along the eastern Adriatic coast coupled with the trend of introduction of new fish species in intensive rearing systems, has lead to increasing incidence of a range of myxosporidian infections. These are becoming hard to eradicate and are often chronic problems. Although large scale mortalities caused by these pathogens are still rare, damage resulting in reduction of the immunocompetency of infected individuals may make the population susceptible to other pathogens and reduce the collective resistance of the population to disease such that occasionally very high mortalities may occur (Rigos et al., 1997). Slow growth rate and poor food conversion are other side effects that decrease final economical viability of the production. Additionally, there may be negative effect on fertilty and reproductive success (SitjàBobadilla and Alvarez-Pellitero, 1990; AlvarezPellitero and Sitjà-Bobadilla, 1993). The subclinical and chronic impact of myxosporean infestation, mostly neglected in past and regarded as parasites of wild, has emerged as a complex, hard to eradicate and persistent problem in mariculture. With the continuous introduction of new species into rearing systems, myxosporean parasites are also introduced to the new environment and easily adapt to fish species already present which were completely allopathric for the parasite in the past (Diamant, 1997). Once kept under highly intensive rearing conditions with their host, myxosporean populations can expand rapidly causing occasional high mortalities and serious epizootic problems. About 1100 myxosporean species have been described from fish, and they are recognized as the largest metazoan group infecting marine organisms (Kudo, 1966). In addition there are unrecognized synonyms of distinct species described under a single name and many have been described without species names (Lom and Arthur, 1989). The aim of this study was to observe the myxosporidian fauna from divers facilities on Adriatic coast to determine information on their prevalence, abundance and their proliferation season. Such information is required to enable intervention at key times during the production cycle to help prevent or at least attenuate the potential damage to the stock. Bull. Eur. Ass. Fish Pathol., 23(3) 2003, 114 Materials and Methods Seven Adriatic fish facilities were monitored from June of 2001 to March 2002 for the presence of myxosporean parasites (figure 1). Fish were collected once every three months from off shore netpens, always from the same cage, so the same population of fish was followed over a nine-month period. Fish sample size comprised from 9 to 15 each of sea bass (Dicentrarchus labrax), sea bream (Sparus aurata), sharpsnout bream (Diplodus puntazzo) and black sea bream (Pagellus bogaraveo), all aged 1+. In total 654 individuals were examined; 296 sea bass (weighting 165.11 ± 59.61 g), 251 sea bream (255.69 ± 126.02 g), 72 sharpsnout bream (247.60 ± 35.97 g) and 35 black sea bream (198.66 ± 45.70 g). During fish sampling procedure, abiotic parameters (temperature, salinity, nutrient salts and oxygen) were measured. Fish were put on ice and brought in laboratory between two to twelve hours after collection, depending on facility location. They were autopsied and biometrical measures were recorded. Fresh smears were taken from gills, skin and fins, from three different parts of alimentary duct (pyloric area, middle intestine and rectal part), spleen, liver, gonads and kidney. If positive, smears were stained by MayGrünwald Giemsa. The number of myxozoans was counted semiquantitatively per microscope field at 400 x magnification: * y t i l i c a f . 1 0 ' y l u J . 1 0 ' r e b m e t p e S . 1 0 ' r e b m e c e D . 2 0 ' h c r a M C ° ‰ C ° ‰ C ° ‰ C ° ‰ I 3 . 3 2 7 3 6 . 0 2 6 3 0 0 1 . 5 1 5 . 7 3 I I 7 . 0 2 7 3 8 . 0 2 7 3 2 . 3 1 5 3 8 . 3 1 6 3 I I I 2 . 3 2 7 3 1 . 0 2 5 . 6 3 4 . 5 1 6 3 1 . 5 1 8 3 V I 9 . 3 2 5 . 4 3 9 . 0 2 8 3 0 0 4 . 4 1 5 . 6 3 V 5 . 3 2 0 4 6 . 0 2 9 3 0 0 4 . 4 1 9 3 I V 8 1 8 3 7 . 9 1 5 . 5 3 0 0 8 . 4 1 6 3 I I V 1 . 9 1 6 3 3 . 0 2 0 4 0 0 5 1 5 . 5 3 Table 1: Temperature (°C) and salinity (‰) of sampled localities in nine-months period. Figure 1: Localities of seven sampled facilities in Adriatic Sea. Bull. Eur. Ass. Fish Pathol., 23(3) 2003, 115 (10 individuals), ** (20), *** (30), **** (40) and ***** (50) (Alvarez-Pellitero et al., 1995). Myxosporean were measured and identified based upon the descriptions of Lom and Arthur, 1989. In case of positive smears, tissue sections were prepared from paraffin-embedded blocks and stained by haematoxylin-eosin method. For every fish species from a particular facility, prevalence and intensity were calculated according to Bush et al. (1997). Results Only four myxosporean species were found and identified from cage-reared fish in the Adriatic Sea during the nine-month survey period. Temperature and salinity are in table 1. Polysporoplasma sparis The spores were round, pumpkin-like, with the width marginally wider than the length. The length measured 13.25 μm ± 0.83, the width 13.5 μm ± 0.52, and the polar capsule length was 4.34 μm ± 0.3, width 3.41 μm ± 0.29. The polar capsules were round, of equal size and with 6 coils of the polar filament. In relation of polar capsule to spore length, the capsules were 3 times smaller than the spore. Anterior ends of polar capsule were set apart one from the other. The suture was prominent on the anterior end, with two thicker bulges on the posterior. In the spring time, very high intensity of pansporoblastic forms was detected from sharpsnout bream, with the presence of old granuloma in the glomerulo-tubular tissue, without MMC aggregation. The parasite was found in the sea bream kidney only once during the spring period. It was found also in sharpsnout bream kept in cages neighbouring the infected sea bream, at low prevalence. The prevalence and intensity are in table 2. As a histozoic and sporogenic, P. sparis in kidney was mostly surrounded by melanomacrophage centres (MMC). Spores were observed in disporoblast incorporated in glomerular tissue, sometimes masked by the high MMC agglomeration. Except for MMC, no significant histopathology was observed. Glomerular capillaries were not degenerated or infarciated by the spores, and the vascular lamina intima was continuous. Epithelial cells did not show hyperplasia or degeneration. In old spores, the fibrous capsule was feebly visible indicating the age of the process. y t i l i c a f . 1 0 ' y l u J . 1 0 ' r e b m e t p e S . 1 0 ' r e b m e c e D . 2 0 ' h c r a M t s o h ) % ( p a t s o h ) % ( p a t s o h ) % ( p a t s o h ) % ( p a I b 7 6 . 6 6 * * 0 0 0 0 0 0 b 0 3 * *