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Enteric Redmouth (ERM) Vaccination

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Ever wonder whether the ERM vaccine used on your fingerling fish
has helped to protect them against an outbreak of redmouth??
Chances are good that it has if properly administered in the first
place. However, there are limitations to the effects of proper
vaccination. The following are several factors which could affect
the vaccination.

Factor 1: Temperature. The most common problem
encountered in administering the ERM bacterin (vaccine) is a
tendency to attempt the process when the water is too cold. The
following table illustrates the time necessary for rainbow trout to
develop maximum immune resistance against ERM at varying


Days to Maximum Protection


40 42 to over 60
45 24
50 15
60 7
65 5

Too often, the tendency is to rush to get the fingerlings
outside in the spring, potentially exposing them to the live
bacteria before the vaccine has had a chance to work. Even if an
acute outbreak of the disease does not occur immediately, the
result may the development of “carrier” fish that harbor the live
bacteria only to reinfect the group and others at a later time. A
good technique to practice is to vaccinate after the water
temperature is at least 50 OF, then hold
the fish inside or away from possible exposure to the ERM bacteria
for at least two weeks.

Factor 2: Fish Size. Just remember that the smaller the
fish when vaccinating (up to between 100 – 200/pound), the shorter
the duration of protection. Typically, between 85% and 100% of
trout vaccinated will survive a challenge with live bacteria thirty
days after vaccination. To decline to a 50% level of protection
(50% survival to challenge) takes approximately 180 days if the
fish were smaller than 500/pound, 250 days if the fish were
400/pound, about 300 days at 300/pound, and over a year if they
were larger than 200/pound. Obviously, a 50% mortality is less than
desirable, so a booster application might be advisable if the fish
are going to be kept for longer than 300 days, the point at which
the immune response from vaccination begins to decline rapidly.

Factor 3: Concentration and Application Method. Remember
the saying “when all else fails, read the directions”?!? To obtain
a good immune response from the ERM vaccine, it is essential to
expose the fish to an adequate quantity of the bacterin for the
minimum exposure time. The manufacturer has provided instructions
with the vaccine based upon the concentration of killed bacteria in
the vaccine. If you choose to administer the vaccine by an
alternate method, it very likely will not work. Diluting the
vaccine by trying to “stretch” the weight of fish exposed to a
single batch, or by adding the vaccine to too large a volume of
water (such as in a hauling tank) is not advisable for the initial
vaccination process. The simple fact is that the fish will not take
up an adequate concentration of the bacterin into their system. A
dilute exposure is suggested as a method of “boosting” the effect
several months later, if required. Request information on
appropriate booster concentrations from the vaccine supplier.

Factor 4: Take an Active Fish Health Management Approach.
Consider the length of time the fish are protected by the vaccine,
then consider what your target market size is for the fish and work
backward. If it takes 350 days for your fish to reach market size,
and the vaccine provides maximum protection for only 300 days, then
try to hold them for the first 50 days in an area free of ERM, then
vaccinate just prior (but at least two weeks) to moving them onto
your main production area. An alternative for persons who have
chronic ERM problems on their farms would be to purchase fish at a
large enough size to be able to complete production within 300
days. And make sure they were properly vaccinated two weeks prior
to stocking. If the fish are to be grown through more than one
growing season, a booster vaccine is advisable.

Finally, remember that no vaccine is a substitute for good
management of your farm. Always provide the best water quality
possible, use proper feeding methods, remove mortalities ASAP, and
avoid crowding and any unnecessary stress of the fish.

Written by:Dr. Jeffrey M. Hinshaw
North Carolina State University – Department of Zoology
Extension Fisheries Specialist

Vaccinating Fingerlings

Vaccinating rainbow trout fingerlings against Enteric Redmouth
(ERM) is effective if done correctly. Applying the vaccine without
following the recommendations and understanding the limitations
wastes effort and money. The following are some facts and
procedures to keep in mind.

  • Rainbow trout fingerlings should be vaccinated at least 2
    weeks at 50 OF before stocking in ponds
    or raceways with prior disease history. This allows the trout’s
    immune system to effectively respond to the vaccine.
  • Vaccinate fingerlings at 100 – 200 per pound (2.3 – 2.9 inches)
    when possible. The larger the fingerling, the longer the duration
    of protection.
  • Follow the directions on the label!! Dilute the vaccine with the
    appropriate amount of water and apply to the proper weight of fish.
    Vaccinating during transportation is not an effective method for
    two reasons: 1) the fish are not exposed to an adequate amount of
    the vaccine and 2) the trout are not allowed at least two weeks to
    respond to the vaccine.
  • Consider the timing of the vaccine. The “redmouth season” is
    typically from late March or early April until September or
    October. When possible, fingerlings should be vaccinated in late
    February or early March to extend the duration of the vaccine
    through the “redmouth season”.
  • Vaccinating trout for redmouth is inexpensive insurance which
    adds about 1/3 of a penny to the cost of production of the
    fingerling at 100/lb. If your fingerling supplier refuses to
    vaccinate, offer to buy the vaccine yourself. Suggesting the trout
    be vaccinated is not an insult to the fingerling producer. It is an
    intelligent management decision for production facilities.
  • Using the vaccine as a booster for trout that have previously
    been properly vaccinated
    appears to be effective. One
    evaluation of a booster applied for 1 hour during transportation of
    trout showed an immune level three times greater 20 days after the

There is only one licensed supplier of ERM vaccine in the

Injection vaccination equipment can be obtained from:

North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, religion, creed, national origin, sex, age, disability, or veteran’s status. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.