has to be made to reach out to all farmers. If poor farmers are to gain from these
innovations, new institutional arrangements such as ‘share-cropping’ or ‘producerassociations’ have to be developed, adopted and directed towards more concerted
action in aquaculture.
8.7 Disease Management
For more than 10 years, grass carp populations in almost all the northern provinces
of Vietnam have suffered from high mortality rates of up to 100 %, due to unknown
pathogens and/or exogenic factors. In Vietnam, the diseases show a seasonal
pattern, occurring mainly at the beginning (March to April) and end (October to
November) of the wet season, the worst being Red Spot Disease (RSD) which leads
to hemorrhagic changes and the appearance of ulcers on the skin of the diseased fish
(Van et al. 2002).
Between 2008 and 2011, the Uplands Program research area in Yen Chau district
was visited six times as part of an epidemiological study and to collect samples.
In total, 197 fish were sampled, including 76 grass carp with clinical signs,
50 control grass carp without symptoms, plus 37 diseased, as well as 34 apparently
healthy control fish from other species.
Beside the main symptom of hemorrhagic changes and ulcers on the skin, other
clinical symptoms such as a darkening or bleaching of the skin, a loss of scales,
hemorrhagic intestines, necrosis of the gills, exophthalmia or enophthalmia,
and erratic swimming behavior, were found in some of the diseased fish without
red spots.
Parasitological examinations employed during the dissection of the sampled
fish revealed moderate infestations of ectoparasites. Monogenean trematodes
like Dactylogyrus sp. and Gyrodactylus sp., ciliata including Trichodina sp. and
Ichthyophthirius sp., flagellate such as Ichthyobodo sp. and Lernaea sp., and a
copepod crustacean were detected. Most of the parasites found on the grass carp do
not kill healthy fish under good environmental conditions, though Ichthyophthirius
sp. can lead to high mortality rates among grass carp and other fish species
(Uzbu ¨lek and Yildiz 2002; Elser 1955; Wurtsbaugh and Alfaro 1988), but this
was detected in only three sampled grass carp at a moderate level of infestation.
Therefore, the high mortality rates among grass carp in the research area were not
primarily attributable to parasites; however, the presence of external parasites could
be considered a predisposing factor for the entry of secondary invading bacteria due
to micro lesions or injuries to the skin and gills (Liu and Lu 2004; Xu et al. 2007;
Busch et al. 2003; Berry et al. 1991).
Different bacterial species were isolated from the diseased grass carp using
conventional bacteriological techniques and were identified morphologically and
biochemically as motile Aeromonads, Flavobacterium columnare, Vibrio sp. and
Pseudomonas sp. (Mayrhofer et al. 2011). These bacteria are well known facultative fish pathogens, able to cause the characteristic RSD symptoms.
8 Improved Sustainable Aquaculture Systems for Small-Scale Farmers . . .
307
innovations, new institutional arrangements such as ‘share-cropping’ or ‘producerassociations’ have to be developed, adopted and directed towards more concerted
action in aquaculture.
8.7 Disease Management
For more than 10 years, grass carp populations in almost all the northern provinces
of Vietnam have suffered from high mortality rates of up to 100 %, due to unknown
pathogens and/or exogenic factors. In Vietnam, the diseases show a seasonal
pattern, occurring mainly at the beginning (March to April) and end (October to
November) of the wet season, the worst being Red Spot Disease (RSD) which leads
to hemorrhagic changes and the appearance of ulcers on the skin of the diseased fish
(Van et al. 2002).
Between 2008 and 2011, the Uplands Program research area in Yen Chau district
was visited six times as part of an epidemiological study and to collect samples.
In total, 197 fish were sampled, including 76 grass carp with clinical signs,
50 control grass carp without symptoms, plus 37 diseased, as well as 34 apparently
healthy control fish from other species.
Beside the main symptom of hemorrhagic changes and ulcers on the skin, other
clinical symptoms such as a darkening or bleaching of the skin, a loss of scales,
hemorrhagic intestines, necrosis of the gills, exophthalmia or enophthalmia,
and erratic swimming behavior, were found in some of the diseased fish without
red spots.
Parasitological examinations employed during the dissection of the sampled
fish revealed moderate infestations of ectoparasites. Monogenean trematodes
like Dactylogyrus sp. and Gyrodactylus sp., ciliata including Trichodina sp. and
Ichthyophthirius sp., flagellate such as Ichthyobodo sp. and Lernaea sp., and a
copepod crustacean were detected. Most of the parasites found on the grass carp do
not kill healthy fish under good environmental conditions, though Ichthyophthirius
sp. can lead to high mortality rates among grass carp and other fish species
(Uzbu ¨lek and Yildiz 2002; Elser 1955; Wurtsbaugh and Alfaro 1988), but this
was detected in only three sampled grass carp at a moderate level of infestation.
Therefore, the high mortality rates among grass carp in the research area were not
primarily attributable to parasites; however, the presence of external parasites could
be considered a predisposing factor for the entry of secondary invading bacteria due
to micro lesions or injuries to the skin and gills (Liu and Lu 2004; Xu et al. 2007;
Busch et al. 2003; Berry et al. 1991).
Different bacterial species were isolated from the diseased grass carp using
conventional bacteriological techniques and were identified morphologically and
biochemically as motile Aeromonads, Flavobacterium columnare, Vibrio sp. and
Pseudomonas sp. (Mayrhofer et al. 2011). These bacteria are well known facultative fish pathogens, able to cause the characteristic RSD symptoms.
8 Improved Sustainable Aquaculture Systems for Small-Scale Farmers . . .
307