Page 11 - World Aquaculture Magazine - March 2011

World Aquaculture 9
Epidemiology
Aquaculture epide-
miological information
has been routinely sup-
ported by a combina-
tion of molecular bi-
ology, bioinformatics
and taxonomy to iden-
tify specific names and
biological properties
of the new and emerg-
ing infectious agents or
strains. For example,
retrospective molecular
sequence analysis of
the evolutionary story
of etiological agents
corroborated suspected
transboundery routes
of disease transmission
and the characteriza-
tion of emerging circu-
lating strains in aquaculture operations
around the world (McBeath et al. 2009,
Sabry et al. 2009, Wertheim et al. 2009,
Muller et al. 2010).
Surveillance has become more im-
portant since the formation of the
World Trade Organization and subse-
quent implementation of various mul-
tilateral agreements on trade aimed
at reducing the risk of international
spread of important aquatic animal
diseases, early warning of disease out-
breaks, planning and monitoring of
disease control programs, provision of
sound aquatic animal health advice to
farms, certification of exports, as well
as international reporting and veri-
fication of freedom from particular
diseases. Sites and facilities at greatest
risk of exposure to a target pathogen/
disease, should be monitoring rou-
tinely to determine the cause of an
outbreak of clinical disease (general
surveillance) and/or to screen popula-
tions for specific diseases for develop-
ment and maintenance of domesticat-
ed stocks (target surveillance). Both
target and general surveillance are key
components that have been performed
in modern biosecurity/epidemiologi-
cal control programs. Additional rec-
ommended reading may be found on
this topic in Baldock et al. (2006), OIE
(2009a) and Lightner et al. (2009).
Geographic information systems
Fig. 4. Suggested scheme of the quarantine rearing and testing flow for
the development of SPF lines and genetically improved stocks of shrimp till
stocking in the commodity farms (T.P.D. Andrade 2005, Andrade et al. 2006).
based on remote sensing and mapping
have also emerged as a powerful ana-
lytic and decision making technology
to assist epidemiologists in govern-
ment, industry and reference labora-
tories to minimize the likelihood of
rapid spread of disease in aquaculture
operations (Smith and Jordan1993,
Bayot et al. 2004, 2008, Kapetsky and
Aguilar-Manjarrez 2007).
Disease Exclusion
In the early years, aquaculture was
plagued by misdiagnosed diseases in
wild broodstock and larvae or juve-
niles. Presently, a variety of improve-
ments have been made in applying bio-
security principles, best management
practices and disinfection for control
of pathogens. This has been facilitated
by 1) the use of more reliable accurate
diagnostic methods, 2) educational ap-
proaches for training, 3) better low wa-
ter exchange management systems that
reduce opportunities for pathogen in-
troduction, 4) improved feed formula-
tions and increased routine sanitation.
A number of fish vaccines have also
been used for slowing diseases progres-
sion mostly in Rhabdovirus infected
fish, but they may or may not exclude
pathogens from the disease site.
In addition, there are no vaccines
available for many fish bacterial and
viral pathogens (Kurath 2008). Mol-
luscs and crustaceans
do not produce anti-
bodies as a response
to infection, and the
delivery of viral-inhib-
itors, such as RNA in
reference, is not well es-
tablished and available
to industry. Thus, over
the past two decades
strategies have been re-
fined and adopted by
many aquaculture op-
erations based on use
of a combination of 1)
early detection of spe-
cific pathogens over the
time, 2) development
of infrastructure for
commercial supplies of
specific pathogen-free
(SPF) stocks, 3) im-
provement of stocks for
desirable performance traits, including
disease tolerance, growth rate, feed
conversion efficiency and, 4) develop-
ment of consistent documented history
for a particular stock assuring freedom
of specific listed pathogens over time.
Figure 4 shows a scheme for introduc-
tion of new genetic lines of shrimp,
quarantine and testing for the devel-
opment of SPF lines and genetically
improved stocks for eventual stocking
in commodity farms (Andrade 2005,
Andrade et al. 2006). Although, only
a few efficient shrimp breeding pro-
grams are internationally in operation,
the benefits of the use of domesticated
SPF stocks by shrimp aquaculture op-
erations have been shown in some re-
gions, such as Brazil and Asia. In some
cases, the use of assured stocks has led
to less disease and improved survival
in places previously dominated by un-
improved and undomesticated native
species. More detailed reviews on this
topic can be found in Lightner et al.
(2009) and Benzie (2009).
Conclusions
The rapid expansion of aquacul-
ture has provided opportunities for
increased pathogenecity of existing
infections and additional exposure to
emerging disease etiologies. Although
future success in realizing effective di-
agnostic or exclusion technologies for