by
E.S. IVERSEN
Division of Fishery Sciences
University of Miami
Miami, Florida, 33149, U.S.A.
Abstract
Several species of Microsporida are present in the musculature of commercial penaeid shrimp. Although these parasites do not harm humans they reduce the quality of processed shrimp and perhaps cause epidemics in nature. Some species of Microsporida are known to occur in shrimp but have not been identified. Also fundamental knowledge about the intensity and incidence of infection and mechanics of transmission of these parasites is lacking. If this information were obtained, possibly fluctuations in abundance of shrimp stocks could be more clearly explained, the quality of shrimp improved, and shrimp farming could be placed on a more solid footing.
LES MICROSPORIDOSES DES PENAEIDAE D'INTERET COMMERCIAL
Resumé
Plusieurs espèces de Microsporida sont présentes dans la musculature des Penaeidae d'intérêt commercial. Ces parasites ne sont pas nocifs pour l'homme, mais ils font baisser la qualité des crevettes après traitement industriel et causent peut-être des épidémies dans les stocks. On connaît l'existence d'un certain nombre d'espèces de Microsporida de la crevette, mais elles n'ont pas été identifiées. On manque également de renseignements de base sur l'intensité et l'incidence de l'infection et le mécanisme de transmission de ces parasites. Peut-être serait-il possible, si l'on parvenait à obtenir ces informations, d'expliquer plus clairement les fluctuations quantitatives des stocks de crevettes, d'améliorer la qualité de celles-ci et d'assurer une plus grande stabilité aux élevages.
MICROSPORIDIOSIS EN LOS CAMARONES PENEIDOS COMERCIALES
Extracto
En el tejido muscular de los camarones peneidos comerciales existen varias especies de microsporidios. Aunque estos parásitos no perjudican al hombre rebajan la calidad de los camarones elaborados y tal vez causan epidemias. Se sabe que en los camarones existen algunas especies de microsporidios pero éstas no han sido identificadas. Faltan igualmente conocimientos precisos sobre la intensidad e incidencia de la infección y del modo de transmitirse estos parásitos. Si pudiera obtenerse esta informacción, posiblemente se explicarían con más claridad las fluctuaciones que se registran en la abundancia de las poblaciones de camarones, se mejoraría la calidad de éstas y se podría establecer su cultivo sobre una base más sólida.
Since the early fifties shrimp has been the most valuable marine resource of the United States, surpassing tuna and salmon. In addition to domestic production, which reached over 234 million lb (106,000 t) in 1966, shrimp have been imported from foreign countries in sharply increasing quantities in recent years; in 1966 over 178 million lb (80,740 t) were imported. The pink shrimp, Penaeus duorarum Burkenroad supports the most valuable commercial fishery in Florida. During 1966 over 20 million lb (9,070 t) of pink shrimp tails, valued in excess of $8.5 million to fishermen, were landed from the Tortugas grounds alone. Even a small proportionate loss of this catch through the discard of parasitized shrimp produces a considerable economic loss because of the high unit value of the catch.
Because penaeid shrimp are farmed in many areas of the world and are being consider for farming in southeastern United States, research on diseases should keep pace with study of the other aspects of the biology of these valuable crustaceans.
Eight species of parasites are known from the pink shrimp (Eldred et al., 1961). Nearly all occur in the cephalothorax which is discarded before the shrimp is marketed. Many Microsporida occur in all organs of the body and the tail muscles. This report discusses the occurrence of microsporidian species which invade mainly tail muscles; these Microsporida cause a reduction in shrimp quality.
Several species of Microsporida cause infections in commercial shrimp. In Louisia the brown shrimp, P. aztecus, is infected with Nosema nelsoni, and the white shrimp, P. setiferus, with Thelohania penaei (Sprague, 1950). In Florida the pink shrimp, P. duorarum, and the Caribbean brown shrimp, P. brasiliensis are infected with T. duorara (Iversen and Manning, 1959; Iversen and Van Meter, 1964). Mr. J. Ewald, Instituto Venezolano de Investigacíones Cientificas, who is studying the biology of the South American white shrimp, P. Schmitti, in the Lake of Maracaibo, Venezuela, has observed discolouration of the tails of this species. In preserved whole shrimp and sections provided by Mr. Ewald, I have found spores, probably of a Microsporida. Mr. Harvey Bullis, U.S. Fish and Wildlife Service, has observed whitish or chalkish colouration in royal red shrimp, Hymenopenaeus robustus, from the Gulf of Mexico. He kindly sent me specimens of the royal red shrimp which contained spore-like bodies similar to known Miscrosporida. Microsporida have not been reported from P. schmitti or H. robustus nor described in the literature.
Microsporidian species in penaeid shrimp are not known to be harmful to man, but degradation of quality and the consequent economic loss by reason of their presence in commercial shrimp is of concern.
When an infected shrimp dies or molts the microsporidian spores are released into the water and ingested by other shrimp as they feed. It is generally believed that there are no intermediate hosts in the life cycle. The cannibalistic behaviour of shrimp would favour transmission of this parasite. Once in the digestive tract of a host, the polar filament inside the spore is released, and probably anchors the parasite to the gut wall. The sporoplasm emerges from the spore case as a small amoebulae which enters the blood stream of the shrimp and reaches the sites of infection. There, large numbers of spores are produced during sporulation. The host tissue reacts with extensive hypertrophy of the cytoplasmic body and nuclei. When the infection is heavy the musculature of the shrimp becomes whitish and opaque. This white pus-like material is formed by the liquification of muscles and the breakdown of spore-forming bodies during sporulation. Shrimp, so heavily infected that muscle fibres are completely infiltrated with the parasite, are of poor quality, and the postmortem breakdown of their tissues is rapid. This condition is familiar to fishermen who call these heavily infected shrimp “milk” or “cotton” shrimp. Fishermen discard them from the catch because their appearance is unattractive, and the muscle is soft and has poor texture. It is not known whether the quality degradation produced by this parasite impairs the keeping quality of shrimp.
There are numerous reports of adverse effects on the quality of flesh of commercial fish caused by histozoic Myxosporida which are related closely to Microsporida. The barracouta, Thyrsites atun, of commercial importance in both Australia and South Africa, is infected with a histozoic myxosporidian species, Chloromyxum thyrsites. Willis (1949) working on barracouta in Australia reports that “… from a few to twenty-four hours after capture the flesh softens and finally disintegrates into a thick, viscuous mass.” Such infected fish are called “milky barracouta”.
These accounts and many others leave little doubt that the Microsporida and Myxosporida are destructive to host tissue in many areas throughout the world and that, although proper handling procedures of fresh fish are followed in infected fish, a considerable reduction in quality begins soon after death of the fish.
There are available some estimates of the incidence of infection of pink shrimp based on heavily infected specimens which fishermen and fish dealers recognize as “cotton shrimp” (Woodburn et al., 1957; Hutton et al, 1959; Kruse, 1959). These observations suggest a light incidence of infection, but the estimates are low since no systematic, careful search has ever been made, and only the obviously infected shrimp can be detected without careful scrutiny. In northern Florida, Kruse (1959) found about 2 percent of the pink shrimp he examined microscopically to be infected with these parasites. His samples included only young shrimp. In general there is a tendency for the incidence of infection of parasites to increase with the age of the host (Noble, 1960). Because some Myxosporida exhibit seasonal changes in intensity of infection (Noble, 1957), data to determine incidence must be gathered throughout the year. Seasonal data on incidence of Microsporida have not been gathered for shrimp.
Microsporidian species have caused epidemics among marine animals and there are indications from the high intensities of infection and serious pathological changes in infected aquatic animals that epizootics could take place although undetected due to predation. For example, Viosca (1943) noted that about 90 percent of the ovaries of the white shrimp were infected with a microsporidian species in Louisiana. In another study, Haley (1953) found that about 23 percent of the smelt Osmerus mordax from the Great Bay region of New Hampshire were infected with the microsporidian species Glugea hertwigi. He concluded that the serious “pathological manifestations” of the disease caused by cysts covering the pyloric caeca, liver, gonads, and swim bladder suggest that it is a serious factor in the decline of the smelt population in that area. A myxosporidian species is known to invade “from 40 percent to 90 percent of the populations” of four species of commercial fish in South Africa (Rowan, 1956). In one species alone about 25 percent of the catch was unsuitable for the market due to this histozoic parasite.
There is no information on the average infection by Microsporida of individual pink shrimp. Light infections have been noted (Kruse, 1959) as well as shrimp completely infiltrated with this parasite. Rowan (1956) found spore counts of a myxosporidian species ranging from “tens of thousands” to “tens of millions per gram” of fish flesh in some South African fishes.
Some general external characteristics of heavily infected pink shrimp are obvious, but medium to light infections are not described. In heavy infections the shrimp becomes whitish, and may show blue-black areas of the exoskeleton. Early and light infections apparently require microscopic examination.
The means by which Microsporida are transmitted from shrimp to shrimp are unknown. Experiments involving feeding healthy pink shrimp with pieces of shrimp infected with Thelohania duorara at the Institute of Marine Sciences were unsuccessful in transmitting the disease. This failure together with other similar ones illustrates our lack of knowledge of the life cycles of Microsporida.
For many of our valuable commercial species of fishes including tunas, salmon and herring, the important parasites have been identified, and for some species the incidence and intensity of infection have been determined. The commercial value of these fish species individually is below that of the commercial penaeid shrimp, yet for the shrimp similar studies have not been made. Studies are required to provide knowledge of the mechanics of infection of Microsporida and of the importance of these parasites in causing population fluctuations. Research should be designed to produce answers to these questions:
What is the incidence of microsporidian infection of penaeid shrimp in nature at different times of the year and for different sizes of shrimp? Are these rates sufficiently high to be of concern to the shrimp industry in terms of quality loss? Is the intensity of infection sufficiently high to suggest that population fluctuations may be influenced by these parasites?
Are there certain times of the year or places on the fishing grounds where heavily infected shrimp occur, and can fishermen avoid them?
Are there any clearly recognizable external characteristics which permit culling of infected shrimp at various intensities of infection to prevent them from reaching the consumer?
At what size (age) do shrimp first become infected?
How are infections transmitted from shrimp to shrimp? Are there immediate hosts in the life cycles?
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Haley, A.J., 1953 Microsporidian parasite, Glugea hertwigi, in American smelt from the Great Bay Region, New Hampshire. Trans.Am.Fish.Soc., 83:84–90
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