l Using vaccination to control diseases l
isolation is unsuccessful, this assay
can sometimes indicate the serotype
(Mass., Conn., Ark. or Del.) closest to
the challenge strain.
Mycoplasmosis due to
Mycoplasma gallisepticum (MG)
The Disease—Classic MG infection,
often referred to as chronic respiratory
disease (CRD), is characterized by nasal
discharge and respiratory rales. Affected
chickens often show extensive airsac-culitis on post mortem examination although subclinical MG is a more commonly encountered in flocks.
Infected chickens may show no obvious signs of CRD but undergo prolonged
and severe reactions to IB/ND vaccines
administered by the spray route, record
sub-optimal egg production, demonstrate severe respiratory signs following
challenge with field strains of IB and
elevated mortality from E.coli airsaccu-
litis and septicemia. All of these effects
result in a financial loss to producers.
Vaccination—Two approaches to
vaccination are using either live or
killed products. Three types of live
vaccines are available.
Ts- 11 is frozen, stored in canisters
under liquid nitrogen, and administered
by eye drop. The 6/85 and F-strain-derived vaccines are freeze-dried and are
applied by spray. Flocks are usually
vaccinated between eight and 12 weeks
of age. Ideally, other respiratory vaccines (ND/IB/ILT) and especially antibiotics should be withheld for two
weeks before and after vaccination
with a live MG product.
Each live vaccine differs by the extent that it colonizes the respiratory
tract but they all function by stimulating local immunity. Killed MG vaccines (bacterins), provide high levels
of both circulating and tissue antibod-
ies during the periods preceding and
subsequent to peak egg production.
Monitoring—Serology can be used
to measure the efficacy of vaccination
using Ts- 11, F-strain-derived vaccines
and bacterins. Maximum serum plate
agglutination (SPA) reactions are evident within four weeks of vaccination.
Serology is also useful in determining
the timing of field challenge. Generally, vaccine-induced plate reactions
start to decline with the onset of egg
A subsequent rise in plate reactions
is inevitable and helps determine when
➤ Field data confirms the
positive benefit of pro-
tecting flocks using the
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the flock has been exposed to the field
challenge. Figure 2 is a theoretical
example of how vaccination and field
challenge jointly determine serocon-version. If the vaccination program
is adequate to the level of challenge,
there should be no drop in production
concurrent with a rise in titer.
E. coli Infection
The Disease—Avian Pathogenic
E. coli, or APEC, often results in elevated mortality from peritonitis, an
extensive inflammation of the lining of
the body cavity and tissues surrounding the intestine and liver. Previously
E.coli strains were regarded as causing
secondary disease only following a primary viral infection.
More recently APECs appear to be
capable of causing disease as a primary
respiratory infection—usually following inhalation of contaminated dust
particles in the house. The most common presentation of E. coli peritonitis
is a sudden rise in mortality as flocks
near peak egg production in either the
first or second cycles. It is not uncommon for flocks in high-challenge areas
to suffer 5% to 10% losses.
Mortality often persists in the flock
after the acute phase and weekly values