Monitoring broiler breeder flocks for Mycoplasma gallisepticum Infection after vaccination with ts-11

Abstract

Mycoplasma control in any poultry company requires an integrated approach involving diligent biosecurity, animal husbandry and disease surveillance. Mycoplasma gallisepticum (MG) infection is very costly to the broiler industry as it predisposes birds to a variety of primary and secondary respiratory diseases thus reducing production efficiency and profitability. Since the primary disease is rather insidious, relatively difficult to confirm (especially in vaccinated flocks), prone to becoming quiescent and vertically transmitted, control measures must begin at the breeder level and involve conscientious flock surveillance. While disease eradication is the best approach to MG control the economic pressures of modern broiler production often preclude such drastic measures. Vaccination programmes are often introduced to reduce the economic impact of the disease in breeder flocks and to minimize, or hopefully prevent vertical transmission. To prevent vertical transmission and lateral spread of field strain MG infection, early diagnosis is critical. Flock testing needs to be done every 2-4 weeks (depending on prevailing risk), on 90 birds (for an average flock of 7 000 birds) to satisfy statistical requirements for the detection of a 5% infection-rate with 99% confidence. The RSPA test provides an inexpensive, reliable and rapid means of evaluating the serological response to ts-11 strain vaccination during the rearing phase. A floor pen trial confirmed that, three to ten weeks after ts-11 vaccination at 10 weeks of age, the RSPA test reactor rate is between 30% and 60%. PCR was used to confirm the absence of field challenge. A retrospective analysis of 4 years of RSPA test data from broiler breeder flocks immunized with the live ts-11 strain MG vaccine indicated that traditional RSPA test monitoring protocols were unreliable as a means of differentiating ts-11 vaccination from field strain MG infection. Non-infected (PCR negative) vaccinated flocks reached sero-positive agglutination rates of 100% making the differentiation of vaccine response and field infection impossible during the lay cycle. RSPA monitoring of broiler breeders during the pullet rearing stage (0-20 weeks) was in contrast still very effective. While previously reported trials indicated that the introduction and subsequent serological monitoring of in-contact non-vaccinated sentinels may enhance the efficacy of the RSPA monitoring procedure this trial indicated that it does not. The ts-11 strain MG spread to in-contact sentinels so rapidly under field conditions that the serum agglutination pattern of these birds mimics that of the vaccinated pullets. The potential for ts-11 strain MG to spread from bird to bird is a reality and even spread from pen to pen (within the same house) may be possible if biosecurity is inadequate. The decision to vaccinate should include consideration as to the consequence of ts-11 strain MG spread to surrounding susceptible flocks. The use of molecular diagnostic techniques on pooled tracheal swabs taken from representative flock birds is a potentially cost effective and reliable means of differentiating ts-11 vaccine strain from field strain MG. PCR amplification of DNA from tracheal swab samples and strain identification based on amplicon size was shown to be a reliable and sensitive means of detecting ts-11 strain following vaccination. The proprietary PCR primer used in this trial was specifically designed to identify the ts-11 strain by amplifying a 229 bp fragment that is characteristic and distinguishable from all other MG field strain isolates based on amplicon size. This technique provides the opportunity to differentiate field strain infection from vaccine strain MG, provided strain specific PCR primers are available. It is recommended that the RSPA assay is used to differentiate effective vaccination from field exposure during pullet rearing and PCR assay is used to monitor broiler breeder flocks for MG challenge during the laying cycle and confirm that point-of-lay broiler breeder pullets are free of field strain MG infection. Where possible flocks with a confirmed field strain challenge should be eliminated and all hatching eggs removed from the hatchery and destroyed.