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 production.

A subsequent rise in plate reactions is inevitable and helps determine when

➤ Field data confirms the
positive benefit of pro-
tecting flocks using the
live vaccine.

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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.