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 pe
rform 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),