Biosecurity – Fighting Disease Transmission
By
Randy Peach
Reprinted Courtesy of
Hatchery Magazine, October 2003
A fish's life-long battle against infectious diseases often starts while it
is still an egg, as a fight against pathogenic fungi in heavily infested
incubation trays. The struggle continues during the production process where
factors such as other fish, equipment, and predators, threaten to introduce
pathogenic organisms. At every stage fish are challenged with disease of one
type or another, caused by the transmission of fungal (e.g. Saprolegnia
saprolegnia), bacterial (e.g. Enteric Redmouth Disease; ERM), or even
viral (e.g. Infectious Pancreatic Necrosis; IPN) infections.
Infection and Disease
Although the details of a specific disease may be quite complex, the basic
concept of disease transmission is quite simple. It almost plays out like an
episode from "Mission Impossible". The disease organism frantically seeks
out a host fish, where in which it maneuvers through the host to an
unsuspecting cell or group of cells. Once the target is found, the pathogen
replicates, often killing cells and tissues in the process. When all
available resources have been utilized, the organism must leave in search of
another host. Unfortunately, by that time, the total number of infectious
organisms has increased to unthinkable numbers (Table 1), creating an army
of disease-causing agents. The best way to avoid such an invasion is to
build barriers that will interrupt the disease transmission steps and reduce
the likelihood of allowing any pathogen to become established or be
transmitted onward.
Table 1: Aeromonas salmonicida shedding rates from infected Atlantic
salmon
|
Group |
No. Fish |
Mean fish weight (g) |
Bacterial shedding rate* (cfu/fish/h) |
|
Sea water | | | |
|
Bath challenge (105 cfu/ml) dead |
6 |
25.8 |
1.7 x 106
(1.7 x 105 – 1.1 x 107) |
|
i.m. challenge (103 cfu) live |
4 |
23.3 |
1.3 X 107
(5.7 x 105 – 2.1 x 107) |
|
i.m. challenge (105 cfu) dead |
2 |
1200 |
5.4 X 107
(9.0 x 106 – 6.4 x 108) |
|
Fresh water | | | |
|
Bath challenge (105 cfu/ml) dead |
3 |
6.9 |
4.1 X 104
(1.7 x 104 – 7.0 x 104) |
*median values, range in brackets.
Reference: Rose AS, Ellis AE, Munro LS. Journal of Fish Disease.
1989;12:573-578.
Biosecurity Battles
Hatchery managers and their staff are on the front lines of the battle
against disease transmission. Superior fish health programs include,
vaccination, optimum husbandry, biosecurity, and treatment protocols.
Although this article focuses primarily on biosecurity, other aspects of
successful fish health programs are very important and should not be
ignored.
Advanced biosecurity programs include detailed protocols for isolation,
disinfection, monitoring and if necessary, destruction of the fish;
activities that work best if they are specifically outlined in Standard
Operating Procedures (SOP’s).
Isolation
The first step in any well thought out biosecurity battle plan is to
quarantine or isolate susceptible individuals. All fish should be isolated
from other fish, people, and predators. Any vectors that can potentially
transmit disease should be identified and dealt with on a case-by-case
basis. Clear divisions should be made that separate fish by species, strain,
year class, and health status, or any other division that may seem
appropriate.
The movement of fish between areas of the hatchery or nursery should be kept
at a minimum in order to isolate fish, or maintain secured zones. When it is
necessary to move fish between zones, they should be moved only from high to
low biosecurity priority. For example, older fish may be have been exposed
to more pathogenic organisms than their younger cohorts. Therefore, to avoid
disease transmission to younger fish, fish should only move from the
incubation room to a first feeding room or a nursery, never the other way.
Another way to isolate fish is to restrict the movements of people and
equipment between zones. Hatchery staff should have designated working areas
and whenever possible, each area should have designated equipment that
remains in that area at all times.
If movement of people and equipment between zones is absolutely necessary,
procedures, usually involving disinfection, should be in place to ensure
that there is no cross contamination. Separate battle gear (raingear and
boots) in each area can reduce the risk of transferring opportunistic
diseases. If that is not possible, disinfection protocols, such as the use
of footbaths and handwash stations, must be implemented.
Footbaths are a good way to ensure that biosecurity zones remain isolated.
However, organic matter on the boots can severely reduce the efficacy of
disinfectant solutions. For this reason, footbaths work best if they are
laid out in a two-stage process. The first step is to remove any organic
mater, such as mucus, mud, feces or otherwise. This may be done by simply
brushing it away with a scrub brush, or by first rinsing the boot in water.
The second step is to immerse the boot or shoe in disinfectant solution. A
two-stage process will help ensure adequate disinfection and maintain
isolation. Table 2 demonstrates how removing organic matter through brushing
can increase the efficacy of footbaths.
Table 2: Post-treatment aerobic bacterial counts in cultures from 75-mm2
(0.12-sq in) of boot sole.
|
Treatment |
Mean bacterial count
n=5 |
Standard deviation |
|
No boot bath |
2.78 x 108 a |
6.77 x 107 |
|
Step through Virkon S |
1.76 x 108 a |
6.06 x 107 |
|
Stand in Virkon S (2 min.) |
2.59 x 107 a |
1.01 x 107 |
|
Scrub in water (30 sec.) |
1.04 x 105 b |
7.09 x 104 |
|
Scrub in water (30 sec.) and then step through Virkon S |
120 c |
268 |
|
Scrub in Virkon S (30 sec) |
20 c |
45 |
abc Counts with different
superscripts are different (P<0.0001).
Boot baths were rubber tubs containing 7.6 L of freshly prepared 1% Virkon S
solution, or 7.58 L of water.
Brushes used for scrubbing boots were dedicated to either water or Virkon.
Reference: Amass SF, Ragland D, Spicer P, Evaluation of the efficacy of a
peroxygen compound,
Virkon S as a boot bath disinfectant. J. Swine Health Prod. 2001;
9(3):121-123.
Handwash stations can also help isolate fish from outside invaders. A simple
alcohol based hand disinfectant, like Purell’s instant hand sanitizers, will
easily remove 99.9% of most unwanted germs, and ensure that quarantine or
isolation is not breached.
Disinfection
The second phase of the biosecurity battle plan should be disinfection. When
total isolation is not possible, the only way to avoid introducing diseases
is through cleaning and disinfection. In order to avoid cross contamination
among different groups of fish, it is important that all incoming equipment
is disinfected, and that each area, including, floors, walls, and tanks is
disinfected on a routine basis. Hard-to-reach areas must not be overlooked
or neglected.
Selection of effective disinfection products is important. Chlorine products
such as bleach (sodium hypochlorite), while very cheap, are adversely
affected by organic matter, sunlight and evaporation. Chlorhexidene
gluconates such as Novasan, Virosan, and Hibistat are relatively non-toxic
but tend to be less stable and their effectiveness is lowered by organics.
Gluteraldehydes like Sterol, Wavecide, and Sporicide on the other hand are
extremely stable but may cause eye, skin, and respiratory irritations among
the staff. Iodine solutions such as Ovadine, Povidone, and Wescodyne are
relatively safe and effective to use but can discolour surfaces (walls
tables, etc) and are unstable in light. Finally Quaternary Ammonium
Compounds like Roccal, Parvosol, and Merquat, are not as susceptible to
organics, however, they are not always effective against bacterial spores,
Pseudomonas, fungi or mycobacteria, and hydrophilic viruses. In
addition, they are tough to rinse away and may leave residues.
Newer families of peroxygen compound disinfectants such as Virkon S (Virkon
in Canada) and hydrogen peroxide compound disinfectants such as PeroxiGard
and Hyperox are user-friendly, fast acting, and tough on invading pathogens.
Products like Virkon (Table 3) and Hyperox are specially formulated and are
extremely effective against a wide range of fish pathogens. In addition they
contain surfactants that help deal with organic matter.
Table 3: Virkon S: effective dilutions for use against specific pathogens
|
Infectious Organism |
Fish Disease |
Effective Dilution |
|
Bacteria | | |
|
Aeromonas hydrophila |
Generally secondary invader |
1:200 |
|
Aeromonas salmonicida |
Furunculosis |
1:1000 – 1:200 |
|
Yersinia ruckerii |
Enteric Redmouth Disease (ERM) |
1:50 |
|
Pseudomonas aeruginosa | |
1:100 |
|
Pseudomonas anguilliseptica | |
1:100 |
|
Renibacterium salmoninarum |
Bacterial Kidney Disease (BKD) |
1:100 |
|
Vibrio anguillarum |
Vibriosis |
1:100 |
|
Vibrio |
Vibriosis in shrimp/prawns |
1:500,000 |
|
Viruses | | |
|
ISAV |
Infectious Salmon Anemia |
1:200 – 1:100 |
|
Rhabdovirus |
Infectious Hematopoietic Necrosis, Viral Haemorrhagic Septicaemia, Spring
Viraemia of carp |
1:500 |
|
Birnavirus |
Infectious Pancreatic Necrosis (IPN) |
1:100 |
|
White spotted red baculovirus |
White spot syndrome in shrimp/prawns |
1:111,111 |
|
Yellow headed virus | |
1:333,333 |
Monitoring
When the biosecurity battle plan is established and isolation and
disinfection protocols are in place, the next phase in the war against
pathogens is routine surveillance or monitoring. Biosecurity protocols
require taking culture samples on a routine basis. This will establish a
baseline level of pathogen loading and show how the battle is going. It will
also bring attention to any invaders that may have passed the front lines,
indicating a breach in biosecurity.
The biosecurity plan itself may need auditing on a regular basis. Local
staff can audit daily activities and make note of any obvious shortcomings.
Fish health technicians can make routine visits and alert the local staff to
any changes that may be needed. Finally, an outside fish health professional
or veterinarian can do an independent third party check of the program. This
three step monitoring system should ensure that nothing is overlooked, and
that any needed change is implemented.
Destruction
If all else fails, and disease-causing organisms penetrate the biosecurity
shield, the last option is to destroy the stock. Destruction is a last-ditch
effort and must only be performed in the most serious of cases. When
infectious disease threatens an entire industry, eradication procedures must
be enforced. Eradication often requires the slaughter of all animals that
are infected or have been in contact with infected animals. Once the
infected animals are removed and properly disposed of, the site must be
disinfected and/or fallowed to ensure there is no re-infection.
Conclusion
The battle against infectious diseases is complex and involves many factors
from vaccination to fish husbandry to medication. Biosecurity is an
important and sometimes overlooked factor in this fight. It involves:
isolation
from vectors that may transport disease, disinfection of anything, or
anybody, that may enter the unit, and routine monitoring of the
system to ensure that pathogens are not getting through. The threat of
disease is present at all stages of fish production. For this reason,
biosecurity must be woven into hatchery procedures and become company
policy. By lowering our guard we open doors to opportunistic organisms,
starting as early as the ones that can infect our incubation trays.
Randy Peach is a Biosecurity expert with Syndel International Inc.,
Vancouver, British Columbia. For more information or to contact him call
1-800-663-2282 or email randy@syndel.com.
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