Contagious agalactia monitoring in caprine herds through regular bulk tank milk sampling

Abstract

Surveillance and control of Mycoplasma spp. responsible for contagious agalactia (CA) in caprine herds are important challenges in countries with a large smallruminant dairy industry. In the absence of any clinical signs, being able to determine the potential circulation of mycoplasmas within a herd could help to prevent biosecurity issues during animal exchanges between farms and improve health management practices. The objective of this study was to determine whether regular sampling of bulk tank milk was suitable for such surveillance. Twenty farms were sampled once a month for 2 yr and CA-responsible mycoplasmas were detected by real-time PCR on DNA extracted from milk, using 3 different DNA extraction methods. The pattern of mycoplasma excretion in bulk tank milk was assessed over time and several herd characteristics were recorded together with any event occurring within the herds. In general, the results obtained with the different detection methods were comparable and mainly agreed with the culture results. Several patterns of excretion were observed but were not related to herd characteristics (size, breed, and so on). Recurrence of the same (sub)species and same pulsed-field gel electrophoresis subtype during the 2-yr period is indicative of the considerable persistence of mycoplasmas. This persistence was associated with intermittent excretion. In conclusion, bulk tank milk sampling could be valuable for controlling CA in caprine herds provided it is repeated several times, yet to be defined, per year and analyzed using an appropriate methodology and the right cut-off for interpretation.

Validation of Loop-Mediated Isothermal Amplification (LAMP) Field Tool for Rapid and Sensitive Diagnosis of Contagious Agalactia in Small Ruminants

“…[14,15,23,24]. However, it is known that performances of molecular diagnostic methods highly depend on the DNA extraction methods used and their efficiency to remove the natural inhibitors, which are present in the milk samples [25,26].…”

<p>Contagious Agalactia In Sheep And Goats: Current Perspectives</p>

“…There are also extensive reports of asymptomatic mycoplasmal circulation in goat herds, evidenced mainly in bulk tank milk or ear canal samples, that can persist for years without manifesting any clinical sign, see for instance, Refs. 47,81–85 It can result from past history of the disease in the herd and lead to new clinical CA outbreaks, as carriage strains are as pathogenic as those isolated from acute CA cases 82,86…”

“…The problem is compounded by the fact that physiological monitoring as a herd health management tool does not apply for CA. Indeed, in active outbreaks, somatic cell counts (SCC) may increase in correlation with mammary signs80,102,103 but remain in normal ranges in chronically infected herds 47,85,104,105. Other milk quality traits are affected similarly by mastitis, whatever the etiological agent 106…”

“…Milk (individual/pooled/tank), joint fluids, and eye swabs are relevant samples in diseased herds, and it is recommended practice to sample several animals in the same herd, due to inter-individual variability in shedding 1,107. Excretion in milk is higher in the clinical phase and might become intermittent with time,49,83,85,108 necessitating repeated samplings. Eye swabs and joint fluids tend to contain less mycoplasma (especially if lesions are not recent) and do not make the best samples in extensive surveys as they require animal containment or post-mortem sampling 43,51,84.…”

“…However, to date, there is still no one-step technique that can individually identify the four subspecies. A multiplex real-time-PCR method targeting all CA agents ( M. agalactiae on one hand and the M. mycoides cluster on the other hand) has recently been developed and brought to market but it may prove expensive, especially for large-scale screening studies 85,113. Performances of all-round PCR diagnosis remain highly dependent on the DNA extraction methods used 85,108.…”