Studies on parasites of ornamental fish from South America with particular reference to their pathogenicity and potential for transfaunation.

Abstract

This study, by a variety of approaches, investigated the health of wild ornamental fish species prior to export and on their arrival at the importers. It was conducted in collaboration with British importers in the UK, exporters from Venezuela, and fishermen and exporters from Brazil. The study in the UK involved a series of interviews with British importers and hobbyists and a long-term period of sampling of shipments of fish from the 4 major exporting countries of the Amazon Basin. A total of 735 specimens of tropical ornamental fish, comprising 9 families, 22 genera and 33 species, were sampled. Although around 100 species of ornamental fish are being exported from South America, the majority of the species selected for study belonged to the family Callichthyidae, predominantly the genus Corydoras Lacepede, 1803. In the 2 8 shipments of ornamental fish examined in the UK, 8 species of protozoans, 7 species of monogeneans, 1 crustacean and 13 species of digeneans (of which 6 were adults and 7 metacercariae), 13 species of nematodes (of which 10 were adult stages), 1 pentastomid nymph and 1acanthocephalan species were found. With the exception of the protozoan Piscinoodinium sp., and the mesocercariae of Strigeoidea, all major groups of parasites were generally found at low prevalence and intensity of infection. These results may be related to the exporters' procedures and/or a series of prophylactic treatments that the fish were exposed to prior to export or because heavily infected fish die prior to export. Very few importers in the UK deal with the direct import of fish from South America. Importers of wild freshwater fish often prefer to buy wild quarantined fish from the United States, Germany or the Netherlands rather than buy directly from South America because they believe that these quarantined fish carry less parasites and present fewer disease problems. The study in South America was conducted in Brazil and Venezuela during the beginning of the fishing season for ornamental species. Routine techniques for the detection of common fish diseases are not used by the ornamental fish trade in Venezuela and quarantine procedures appear to be related more to the demand of the market rather than to a concern for the health of the fish. In Brazil, 11 exporters were dealing with ornamental fish from the Amazon basin and the study was conducted in collaboration with the three major exporters from the region and selected fishermen working under their supervision. In general, no fish health programmes are reported by Brazilian exporters. A total of 456 specimens of fish, comprising 5 families, 6 genera and 15 species, were sampled at the exporters' holding facilities. The parasite fauna was composed of 9 species of protozoans, 10 species of monogeneans, 6 digeneans, (of which 5 were metacercarial stages), 6 nematodes, (of which 1 was a larval stage) and 1 acanthocephalan. Only minor differences were observed between the results obtained in Brazil and in the UK and these appeared to be related more to the exporters' procedures than to differences in the natural composition of the parasite fauna of the fish. Fifteen monogeneans were found in the species of fish studied. In the UK samples, 7 species were found in single infections on the skin, gills, and excretory system. Five species belonged to the genus Gyrodactylus Nordmann, 1832 of which 4 were parasites of the callichthyids Corydoras spp., Brochis splendens (Castelnau,1855) and Callichthyscallichthys (Linnaeus,1758) and one a parasite of a curimatid, Semaprochilodus taeniurus (Steindachner). One of these species, G. gemini Ferraz, Shinn and Sommerville, 1994 has been fully described and published. Amongst the dactylogyrids, an interesting finding was the presence of one species parasitising the excretory system of Mylossoma aureum(Spix), the silver dollar, from Colombia and Peru. This species is fully described and placed in the genus Kritskyia Kohn, 1990. This appears to constitute only the second species to be described for this genus. In the fish sampled in Brazil, 10 monogeneans, all gyrodactylids, were found, of which 8 occurred either in single or in mixed infections in Corydoras spp. and two occurred in mixed infections in B. splendens. Overall, single infections of gyrodactylids were more common than mixed infections. It is possible that some of the mixed infections were transient infections as these fish are commonly captured in large schools and kept under overcrowded conditions. These Gyrodactylus spp. were examined using recently developed SEM techniques and 8 species are fully described. Many of the problems associated with wild ornamental fish species from South America were found to be related to the stressful holding and transport conditions, and to opportunistic parasites, which take advantage of the favourable conditions to multiply and cause mortalities. Only a limited number of parasite species were associated with pathological conditions on arrival of the shipments in the UK. Among these species the most common were the protozoans Piscinoodinium sp., Chilodonella hexasticha (Kiernik, 1909) and Ichthyophthirius multifiliis (Fouquet, 1876), the mesocercariae of Strigeoidea, and the monogenean Kritskyiasp. The pathology of Piscinoodinium sp., the mesocercariae of Strigeoidea and the monogenean Kritskyia sp. have previously been given little attention and were therefore studied in detail. A unique feature of Piscinoodinium was described where individual groups were enclosed in crypts within the epithelium. The mesocercariae were also unusual in that 1 to 15 individuals were commonly found enclosed by the same host cyst. Such cysts were widespread in the body of callicthyids. Specimens not enclosed by a host cyst were only found within atresic oocytes. Minimal pathology was associated with the monogenean Kritskyia sp. in light infections. However, heavily infected fish presented an intense infiltrative cellular response. The epithelium of the urinary bladder was metaplastic and separating from the basal layer with the formation of vesicles. Several species of parasites with direct or indirect life cycles with the potential for transfaunation were found in the UK samples. Experimental studies were conducted utilising nematodes of the genus Spirocamallanus Olsen, 1952 and native and commercial species of copepods commonly used as live food for ornamental species, to evaluate their potential for transfaunation. Only early L3 were obtained from the copepods that became infected with the larvae of Spirocamallanus after 17 d.p.i. The low temperature (19-23QC) appeared to be responsible for the slow development of the larvae and its thermophilic character appears to limit the range of distribution of these nematodes to warm waters. In the UK, these stages were developed in the copepod, Cyclopsviridis (Jurine), which commonly occurs in British waters and is also commonly supplied commercially as live food for ornamental species. The life cycle of one species of Spirocamallanus was also investigated in Brazil, utilising two native species occurring naturally in the holding tanks at the exporters' holding facilities where the fish were kept. One of these species of copepods, Thermocyclops dicipiens (Kiefer, 192 9) exhibits a wide distribution in South and Central America and the tropical regions of Asia. The complete development of the larvae to the infective stage was obtained in both species in 10 days, contrasting with the results obtained in the UK. The development of this Spirocamallanus in copepods collected from ponds where the fish were held supports the hypothesis that parasites with indirect life cycles can be acquired by fish during the holding period. The development of these nematodes in T. dicipiens also illustrates the vulnerability of those countries with a large aquaculture production of freshwater aquarium fish to the introduction of parasites from other tropical regions.