Xem mẫu
- Tilapia Diseases 101
Tilapia tolerate adverse water quality and other stressors
better than most other commercial aquaculture species.
Because stress and environmental quality play such
important roles in the disease process, tilapia are labeled as
being very "disease-resistant." This basically means that in
the presence of pathogens, tilapia are the last to break with
disease.
As a result, tilapia growers worldwide did not historically
practice clean culture methods. Moreover, they did not
generally implement the biosecurity measures that had
become standard in industries that grew less disease-
resistant fish such as trout and salmon. In other words,
there was no apparent penalty for being careless - or so it
seemed.
Ten years ago, it was generally believed that there were
very few commercially significant diseases in aquaculture.
This is no longer true. There are now several significant
diseases in tilapia. Some are very new, while others are old
foes that have come on with a new vengeance. This
- resurgence of disease in tilapia is most likely related to the
intensification of fish culture methods globally. Tilapia are
being reared at higher densities than ever before and more
tilapia are being reared in recirculating systems every year.
Although tilapia perform exceptionally well in recirculation
systems, so do pathogens.
Once a pathogen is introduced into a recirculating system,
it is nearly impossible to eradicate. Eradication of a
pathogen generally involves depopulating, sterilizing, and
repopulating the facility. Of course, the farmers lose all the
money they had invested in the fish themselves, and even
after the "sterilization," they never quite knows if they
destroyed all the pathogens. And, if by some miracle the
farmers survive the next seven months without revenues,
while pouring money into new fish, they've lost their
faithful customers. Bleak picture? - just ask any tilapia
grower that suffers from Streptococcus, Trichodina,
systemic Columnaris, or Aeromonas.
In order to avoid disease, one needs to consider how the
pathogens reach a facility, and once there, how they
- overwhelm the disease resistance of tilapia. The most
common means of introducing disease to a clean facility is
by introducing contaminated fish. Before AmeriCulture
entered the industry, most commercial growers suffered
from one or more diseases because there was a dearth of
disease-free hatcheries. Pathogens would generally come
into a facility with the fingerlings purchased from
hatcheries. Once a pathogen reached the facility, it was able
to multiply at a very fast rate. The recirculating systems
provided an ideal environment for the pathogens to
multiply - warmth, nutrient-rich water, lots of places to
hide, and PLENTY OF HOSTS!
Other growers were infected when water from infected
facilities came onto their premises. Dripping live-haul
trucks that travel from one farm to another provide a wide
variety of pathogens that are just waiting to get into a
farmer's facility. The pathogens can enter a facility on the
soles of employees shoes, on a live-hauler's dipnet that was
used at another facility, or on the hands of a driver that is
allowed to feel the temperature of a farmer's tank water.
- People can’t afford to live in a bubble, but by addressing
the obvious routes of pathogen transfer (fish, water,
employees' hands and shoes), a farmer can dramatically
reduce the risk of becoming infected.
The best way to avoid disease is to buy clean fish in the
first place. A farmer can further reduce his or her risk of
disease by implementing the following simple methods:
maintain good fish nutrition
avoid over-crowding
maintain good personal hygiene
hand-washing with antibacterial soaps
disinfectant foot baths
live-haul truck disinfection
limit visitors
Specific Pathogens
The clinically significant tilapia pathogens fall into the
general categories of viruses, bacteria, and protozoa.
Mycotic (fungal) diseases are only significant if the tilapia
- are under constant stress. In certain systems, metazoan
ectoparasites and endoparasites cause problems, but do not
significantly impact the tilapia industry.
Streptococcus
One of the most significant diseases in tilapia culture
worldwide, and particularly in indoor systems, is caused by
Streptococcus. The primary strain of strep infecting
aquaculture facilities is suspected to be Streptococcus iniae,
although other strains have been implicated and are in the
process of being identified. This disease results in the
clinical signs of generalized hemorrhagic septicemia such
as:
Lethargy, weakness, loss of appetite, red discoloration
at the anus and base of fins, hemorrhagic eyes, gills,
internal organs, and muscle, blood tinged abdominal fluid,
and swollen kidney, spleen, and liver.
Streptococcus has additional clinical signs including
an erratic spiral swimming motion, a curved body, corneal
opacity in one or more eyes, exopthalmia (protruding eyes),
- and abdominal distention.
Antibiotic Therapy
Streptococcal infections respond to antibiotic therapy, but
since the withdrawal period for all effective antibiotics is
longer than it takes for the streptococcal infection to return,
the disease cannot be legally controlled with antibiotics all
the way to market. Furthermore, it is only a matter of time
before strep develops resistance to the antibiotics now used.
Streptococcal strains at several facilities have already
developed resistance to some antibiotics.
Vaccines
Injectable vaccines are being developed in earnest, and
initial results seem promising. However, it is not confirmed
that vaccinated fish in infected facilities perform as well as
unvaccinated fish in uninfected facilities. Currently, the
vaccines have to be custom-developed from the strain of
strep at each facility.
Vaccines are also expensive. It costs approximately 5¢ to
vaccinate a tilapia. That’s over 60% of what it costs to buy
- the fingerling in the first place. And since the fingerlings
can’t be vaccinated until they reach 20 grams, they are still
vulnerable to strep for their first month on the farm.
Additionally, each fish has to be individually vaccinated by
hand.
The cost of not vaccinating fingerlings in a Strep infected
facility is even greater. Mortalities of up to 75% have been
observed on some farms although the highest mortality rate
that we have heard of in a large commercial operation is
40%. One operation was reportedly losing 4,000 market-
sized animals per day during a severe outbreak.
Growth Effect
Strep also severely reduces the appetite of the fish, thereby
significantly reducing their growth rates. It is not
uncommon for a 7-8 month growout in a clean facility to
stretch to 10-12 months in an infected facility - and the end
products of the two are like apples and oranges. Fish from
infected facilities that make it to harvest without coming
down with strep don’t tolerate live haul as well as healthy
fish and have markedly reduced shelf life once they’ve
- reached the market.
Market Effect
The physical appearance of infected and uninfected fish in
the market place can be vastly different. It is not
uncommon for infected fish being held in live tanks at
Asian stores to be missing one or both eyes, be covered
with patches of fungus, and have hemorrhages all over their
bodies. Infected fish don't last well, sell well, or market
well. A farmer's objective is to grow fish from fry to adult
as fast and efficiently as possible. Streptococcus can single-
handedly alter a farmer's ability to control his or her own
commercial destiny. It's not worth the risk.
Aeromonas
Another bacterial disease that has significantly impacted
production at some farms is the disease aeromonad
septicemia ("Aeromonas"). This disease is caused by the
bacteria Aeromonas hydrophila. Much like Streptococcus,
Aeromonas results in the clinical signs of generalized
hemorrhagic septicemia such as lethargy, weakness, loss of
appetite, red discoloration at the anus and base of fins,
- hemorrhagic eyes, gills, internal organs, and muscle, blood
tinged abdominal fluid, and swollen kidney, spleen, and
liver. Aeromonas generally affects systems that have
systemic poor water quality or over-crowding. In other
words, a farmer really has to be abusing the fish, or have
another nasty pathogen in his or her system, to break with
Aeromonas. Aeromonas temporarily responds to antibiotic
therapy, but if a farm has Aeromonas, they really need to
either change their source of fingerlings or drastically
improve their husbandry, whichever is to blame. As
always, avoid getting fish from infected stocks at all costs.
Trichodina
Trichodina, or "Trich", is a protozoan parasite that has
severely impacted production at many facilities. "Trich"
can result in extremely high mortality rates, particularly in
young fish. The parasites heavily infest the gill and body
surfaces of infected fish. Infected fish display flashing
(swimming against floors of tanks to scrape parasites off),
rapid breathing, weakness, and uncoordinated swimming.
Since trich attacks the gills, the gills are less efficient in
- doing their primary job - absorbing oxygen, giving off
carbon dioxide, excreting ammonia, and maintaining
chemical balance between their body and the environment.
Trichodina populations can be temporarily controlled with
copper sulfate and salt (forget doing hydroponics) or
formalin (bye-bye biofilter). Treated fish remain carriers
even after treatment and much like Streptococcus, it is
nearly impossible to eliminate trich from a system once it
has been introduced. Any fish that come to you from
outdoor ponds should be carefully examined for trich
before letting them on your premises.
Columnaris
Columnaris is a disease caused by the myxobacteria,
Flexibacter Columnaris. In general, tilapia really have to be
significantly stressed to break with this organism. Systems
that use outdoor surface water are at particular risk.
Outbreaks generally result from temperature fluctuations,
trauma, and poor water quality. Crowding and poor
nutrition further increase the severity of the disease.
Infected fish generally show lethargy, anorexia, weak
- swimming, and mortality. Additionally, raised white
patches appear on the skin or fins. These patches may later
develop into ulcers. Certain antibiotics, copper sulfate, and
potassium permanganate are reported to be effective for
temporary treatment.
The only currently significant viral pathogen that we are
aware of is an irido-like virus that has been traced to fish
from a single fish producer in the U.S. This virus has been
credited with massive, synchronized die-offs at infected
facilities. We know very little about this pathogen, but
exercise great caution.
Disease is impacting the production of tilapia at fish farms
worldwide, particularly in indoor systems. You can
dramatically reduce the risk of introducing pathogens by
implementing simple biosecurity and management
measures. A clean facility should begin with uninfected
fish. You also have the right to insist on examining a
current health inspection that has taken place within the last
six months. The inspection should be performed by a
certified pathologist from a USDA accredited aquatic
- disease diagnostic laboratory for it to be considered valid. It
should also specifically indicate that after examining a
sample of at least 60 fish, that the fish are free of
Streptococcus, Trichodina, Columnaris, and Aeromonas.
Any heath inspection that only states that there is "no
indications of disease", or an equivalent statement, is not
credible and should not be trusted. Infected fish often do
not show clinical signs of disease. However, they are still
infected and very capable of transmitting a pathogen to
your facility. Don't be shy about specifically asking your
fingerling producer if they have ever had Streptococcus,
Trichodina, Columnaris, or Aeromonas. If they have, ask
them if and how they have gotten rid of it and what
measures have they taken to insure that they don’t give it to
you.
AmeriCulture provides disease-free fingerlings to domestic
and international commercial producers. Some customers
even leverage off their disease-free status to maintain
market position in discriminating markets. Take the time
necessary to research your fingerling supplier and install
- biosecurity measures. It is worth the effort.
nguon tai.lieu . vn
