Epidemiological investigations in the research area revealed predisposing and
stress factors such as rapid changes in water temperature, high turbidity levels and
low oxygen concentrations in the water (Steinbronn 2009); handling stress, malnutrition (Tuan et al. 2007), pollutants such as pesticides (Steinbronn et al. 2005) and
the presence of detergents, all of which tend to suppress the immune systems of the
fish and facilitate bacterial infections.
Virological investigations were performed using different fibroblastoid and
epithelial cell lines, but no specific viruses related to grass carp mortality were
found. The same results were also obtained by testing the samples against fish
viruses related to Red Spot Disease, such as Grass Carp Hemorrhagic Virus
(GCHV) (Qiya et al. 2003) and Rhabdovirus (Ahne 1975) utilizing universal and
specific primers with polymerase chain reaction assays.
Furthermore, the presence of Aphanomyces invadans, an oomycete that is known
to produce hemorrhagic changes in the skin and high losses in several fish species
(OIE 2009), was not detected by Polymerase chain reaction (PCR) in any of the
sampled fish.
From the results here it can be concluded that the grass carp disease in Yen Chau
district in the north of Vietnam is a multifactorial disease which seems to be caused
primarily by bacterial agents. It is known that most of the opportunistic bacteria,
like that isolated from the fish samples, are able to affect fish only under poor
environmental conditions such as high temperatures, a lack of oxygen, water
turbidity and other factors that cause stress. Commonly used pesticides washed
into the water can depress the immune system of the fish, leading to a decreased
ability to fight disease (Shea and Berry 1984). In the research area, Anyusheva et al.
(2012) measured the levels of pesticides in the water of ponds and rice fields
considered toxic for common carp and Daphnia magna. It was found that pesticides
were often transported from paddies into fish ponds shortly after pesticides had
been applied on the rice paddies, due to high flow rates specific to the research area.
Due to the fact that isolated bacteria are ubiquitous opportunistic pathogens,
it is recommended that the environmental conditions the fish live in should be
improved, because stressors predispose fish to suffer from bacterial borne diseases
(Snieszko 1974; Ahmed and Rab 1995).
As a control measure, the conditions under which most fish are cultured and
reared in the research area must be improved, and on this some general advice can
be given in order to reduce the mortality rates found among grass carp. The regular
exchange of water, as well as fish between the ponds, must be reduced or stopped
completely to avoid the possibility of carrying over pathogens either via the water
flow or via the fish. Stopping the water inflow will also reduce the turbidity of the
water, and in cases where ponds are surrounded by steep slopes, it is advised that
ditches should be dug around them to avoid sediments, pesticides and fertilizers
being washed in after heavy rains. Removing mud from the bottom of the ponds,
which contains large amounts of decomposing organic material and bacteria that
contribute to oxygen depletion, would also be beneficial – increasing oxygen
308
J. Pucher et al.
stress factors such as rapid changes in water temperature, high turbidity levels and
low oxygen concentrations in the water (Steinbronn 2009); handling stress, malnutrition (Tuan et al. 2007), pollutants such as pesticides (Steinbronn et al. 2005) and
the presence of detergents, all of which tend to suppress the immune systems of the
fish and facilitate bacterial infections.
Virological investigations were performed using different fibroblastoid and
epithelial cell lines, but no specific viruses related to grass carp mortality were
found. The same results were also obtained by testing the samples against fish
viruses related to Red Spot Disease, such as Grass Carp Hemorrhagic Virus
(GCHV) (Qiya et al. 2003) and Rhabdovirus (Ahne 1975) utilizing universal and
specific primers with polymerase chain reaction assays.
Furthermore, the presence of Aphanomyces invadans, an oomycete that is known
to produce hemorrhagic changes in the skin and high losses in several fish species
(OIE 2009), was not detected by Polymerase chain reaction (PCR) in any of the
sampled fish.
From the results here it can be concluded that the grass carp disease in Yen Chau
district in the north of Vietnam is a multifactorial disease which seems to be caused
primarily by bacterial agents. It is known that most of the opportunistic bacteria,
like that isolated from the fish samples, are able to affect fish only under poor
environmental conditions such as high temperatures, a lack of oxygen, water
turbidity and other factors that cause stress. Commonly used pesticides washed
into the water can depress the immune system of the fish, leading to a decreased
ability to fight disease (Shea and Berry 1984). In the research area, Anyusheva et al.
(2012) measured the levels of pesticides in the water of ponds and rice fields
considered toxic for common carp and Daphnia magna. It was found that pesticides
were often transported from paddies into fish ponds shortly after pesticides had
been applied on the rice paddies, due to high flow rates specific to the research area.
Due to the fact that isolated bacteria are ubiquitous opportunistic pathogens,
it is recommended that the environmental conditions the fish live in should be
improved, because stressors predispose fish to suffer from bacterial borne diseases
(Snieszko 1974; Ahmed and Rab 1995).
As a control measure, the conditions under which most fish are cultured and
reared in the research area must be improved, and on this some general advice can
be given in order to reduce the mortality rates found among grass carp. The regular
exchange of water, as well as fish between the ponds, must be reduced or stopped
completely to avoid the possibility of carrying over pathogens either via the water
flow or via the fish. Stopping the water inflow will also reduce the turbidity of the
water, and in cases where ponds are surrounded by steep slopes, it is advised that
ditches should be dug around them to avoid sediments, pesticides and fertilizers
being washed in after heavy rains. Removing mud from the bottom of the ponds,
which contains large amounts of decomposing organic material and bacteria that
contribute to oxygen depletion, would also be beneficial – increasing oxygen
308
J. Pucher et al.