Английский язык: Сборник текстов и упражнений. Дьякова Н.П. - 81 стр.

   scenario                           сценарий
   adhesive                           клейкий
   fry                                малек
   gravel bottom                      дно из гравия
   to replicate                       повторять, копировать
   to induce                          вызывать



                                  Техt 28
                         DISEASES AND PARASITES

    Like other organisms, aquaculture species are susceptible to a broad array of
diseases and parasites such as viruses, bacteria, parasitic protozoans, helminths
(worms), copepods and others. When a disease or parasite outbreak occurs in a
population of aquatic organisms, it is known not as an epidemic (a term used for
the same problem in human populations) but as an epizootic. In most instances,
epizootics only result after a population has been stressed.
    Stress can occur in a number of ways. If water quality deteriorates, even for a
short period, the animals exposed to that water will undergo stress. Handling is
another cause of stress in aquaculture species, as are overfeeding or sustained un-
derfeeding. Following exposure to a stressful situation, an epizootic may occur
within as little as 24 hours or as long as 2 weeks. The period between the stress
episode and the onset of disease or parasite infestations depends on the time in-
volved in building the numbers of disease and parasitic organisms to a high
enough level that signs of the disease are observable. Disease and parasite organ-
isms are almost always present in the culture environment, but at very low levels.
They only produce disease signs when they are promoted by a lack of resistance
in the aquaculture species. An analogy can be drawn in humans. Students in a
classroom may be exposed to people with the flu, but not everyone is infected.
The immune status of the various individuals plays an important role in who will
ultimately show flue symptoms. Students who are stressed (for example, by get-
ting too little sleep) are often much more susceptible than those who are not.
    The number of chemicals that can be used to treat disease and parasite prob-
lems in aquatic animals is small. Only about 10 compounds have been approved
for species that are being reared for direct human consumption. Among them, few
are effective at controlling bacteria, some work on a few parasites but not others,
and many are themselves toxic if given in improper doses. Some treatment chem-
icals are effective when added to the water, but some must be ingested by the aq-
uaculture organisms. Animals that are experiencing disease or parasite problems
often refuse to eat, making treatment difficult.
    Good overall management of the culture system is perhaps the best way to
avoid disease and parasite problems. However, even the best managers experience
epizootics on occasion. Treatment chemicals of various kinds should be available,
and the culturist should know how to use those chemicals. Chemicals such as an-
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