Evaluation of the performance of cellular and serological diagnostic tests for the diagnosis of tuberculosis in an alpaca (Vicugna pacos) herd naturally infected with Mycobacterium bovis

Abstract

Tuberculosis (TB) in llamas and alpacas has gained importance in recent years since they are imported into the European Union mainly for serving as pets and for production of natural fibre. The intradermal tuberculin test has been widely used for diagnosis of TB in these species showing lack of sensitivity (Se) although little information has been previously reported evaluating the effect on its performance of different PPD inoculation sites and time of readings. Moreover, different cost-effective serological assays have been developed in the recent years for TB diagnosis in camelids obtaining a variety of results and, for this reason, new assays still being developed. The main objectives of this study were: (1) to evaluate the performance of the intradermal tuberculin test using different inoculation sites (axillary, prescapular and cervical) and times of reading (72 and 120 h) and (2) to test a novel serological assay based on MPB83 antigen in a Mycobacterium bovis naturally infected alpaca herd in Spain. In regards to skin test, single intradermal tuberculin (SIT) test at the prescapular site and reading at 72 h showed the highest proportion of test-positive-culture positive animals among all culture positive animals (T+/C+), ranging from 53.8% (95% CI, 37.2-69.9) to 80% (95% CI, 44.4-97.5) using a more stringent interpretation than typically prescribed although, in general, low T+/C+ was achieved using both SIT and single comparative intradermal tuberculin (SCIT) tests alone. T+/C+ of the serological assay increased using samples collected 15-30 days after PPD injection [76.9% (95% CI, 60.7-88.9) - 100% (95% CI, 69.2-100)]. The best results of T+/C+ were obtained applying in parallel the most sensitive SIT test and serology using samples collected 15-30 days after PPD inoculation [90% (95% CI, 55.5-99.7)-100% (95% CI, 69.2-100)]. Therefore implementation of serology in parallel with the most sensitive skin test could maximize the detection of infected animals.

Keywords: Alpaca; Diagnosis; Intradermal test; Serology; Tuberculosis.