Simple Summary Alpacas have a quite recent history in Europe. Only in the 1990's were a considerable number of alpacas imported into the UK and then into continental Europe, where the number of animals increased significantly. This development has necessitated a more thorough knowledge of infectious diseases in alpaca husbandry. One of the most serious infectious diseases in alpacas is caseous lymphadenitis (CLA), which is caused by Corynebacterium (C.) pseudotuberculosis. As shown by the post-mortem examinations in this study, CLA is characterized by the formation of multiple, visible abscesses, particularly affecting the lungs and liver as the pathogen spreads throughout the body. While post-mortem examinations and isolation of the pathogen are pivotal for a proven diagnosis, serological examinations are the basis of epidemiological and monitoring studies. We examined 232 alpacas living in three independent herds for this current study. C. pseudotuberculosis was able to be isolated in several alpacas. All of the alpacas were tested serologically using a commercially available ELISA, an in-lab ELISA, and, in select animals, immunoblot. The immunoblot showed the highest sensitivity in the early phase of the infection in alpacas proven to be infected with C. pseudotuberculosis. Testing for antimicrobial susceptibility based on minimal inhibitory concentration using the broth microdilution method revealed uniform susceptibility to aminopenicillins, cephalosporines, macrolides, enrofloxacin, florfenicol, tetracycline, sulfonamid/trimethoprime, tiamulin, gentamicin, neomycin, spectinomycin, and vancomycin, but resistance to colistin, nitrofurantoin, and oxacillin. The present study shows that C. pseudotuberculosis poses an imminent health risk for alpaca husbandry. Laboratory diagnostics based on post-mortem, bacteriological and serological examinations are valuable tools for implementing specific measures as a basis for tackling this infectious disease in alpaca herds. Alpacas are the major camelid species in Europe held for hobbies, animal-aided therapy, and commercial reasons. As a result, health-related issues associated with alpacas are of growing significance. This especially holds true for one of the most serious infectious diseases, caseous lymphadenitis, which is caused by the bacterial pathogen Corynebacterium (C.) pseudotuberculosis. Our study focuses on post-mortem examinations, the laboratory diagnostic tool ELISA, and the immunoblot technique for the detection of specific antibodies against C. pseudotuberculosis and detection of the causative pathogen in alpaca herds. We examined a total of 232 alpacas living in three herds. Four of these alpacas were submitted for post-mortem examination, revealing abscesses, apostematous and fibrinous inflammation in inner organs, pleura, and peritoneum. Serological investigation using a commercial ELISA based on phospholipase D (PLD) as antigen and an in-lab ELISA based on whole cell antigens (WCA) revealed an overall seroprevalence of 56% and 61.2%, respectively. A total of 247 alpaca sera originating from 232 animals were tested comparatively using the in-lab and the commercial ELISA and showed a substantial degree of agreement, of 89.5% (Cohen's kappa coefficient of 0.784), for both tests. Further comparative serological studies using the two ELISAs and the immunoblot technique were carried out on selected sera originating from 12 breeding stallions and six breeding mares for which epidemiological data and partial C. pseudotuberculosis isolates were available. The results showed the immunoblot to have a sensitivity that was superior to both ELISAs. In this context, it should be emphasized that evaluation of these investigations and the epidemiological data suggest an incubation period of one to two months. Antibiotic susceptibility testing of 13 C. pseudotuberculosis isolates based on the determination of minimal inhibitory concentrations using the broth microdilution method revealed uniform susceptibility to aminopenicillins, cephalosporines, macrolides, enrofloxacin, florfenicol, tetracycline, sulfonamid/trimethoprime, tiamulin, gentamicin, neomycin, spectinomycin, and vancomycin, but resistance to colistin, nitrofurantoin, and oxacillin.
