Comparative study of the commonly used virulence tests for laboratory diagnosis of ovine footrot caused by Dichelobacter nodosus in Australia

Abstract

Footrot in sheep and goats is expressed as a spectrum of clinical entities ranging from benign, which is a self limiting interdigital dermatitis to highly virulent, in which severe under running of the horn of the hoof occurs. Interactions between the host, the virulence of the causative strain of Dichelobacter nodosus and environmental conditions determine the severity of the disease. Clinical diagnosis of virulent footrot, which a notifiable disease in some states of Australia, is not always straightforward. Therefore, the Gelatin Gel and Elastase tests for protease activity, and the intA PCR test for an inserted genetic element in D. nodosus are commonly used to support or to confirm a clinical diagnosis. A comparative study of these laboratory tests with a large number of samples collected from 12 flocks of sheep with clinically virulent footrot was conducted. Based on the elastase test, 64% of the isolates tested were classified as virulent compared to 91% on the gelatin gel test and 41% according to the intA test. The agreement between the elastase and the gelatin gel test was low (kappa =0.12) as were the agreements between other tests. Only about 21% of the isolates were virulent in all 3 tests. Therefore these tests on their own may not provide standard and reliable results and are likely to remain as supplementary tests for clinical diagnosis of the disease.2

Evaluation of Genotypic and Phenotypic Protease Virulence Tests for Dichelobacter nodosus Infection in Sheep

“…The elastase test, which measures temporal and quantitative variations in activity of the extracellular proteases expressed by virulent and benign D. nodosus strains (13), has been shown to correlate well with clinical diagnoses (14, 15). The gelatin gel test, which measures the differences in the thermostability of extracellular proteases between virulent and benign D.…”

“…The gelatin gel test, which measures the differences in the thermostability of extracellular proteases between virulent and benign D. nodosus strains (12), is also used by Australian regulatory authorities but can be unreliable (14–16). …”

“…Furthermore, there is evidence of discrepancies between the phenotypic virulence tests and clinical observations, with phenotypically virulent D. nodosus strains isolated from clinically benign outbreaks of footrot (13, 15, 16). These discrepancies may reflect the lack of reproducibility of the tests themselves, which are sensitive to culture conditions (12, 13), or the influences of other host, pathogen, or environmental factors on the expression of the disease.…”

Simultaneous Detection and Discrimination of Virulent and Benign Dichelobacter nodosus in Sheep of Flocks Affected by Foot Rot and in Clinically Healthy Flocks by Competitive Real-Time PCR

“…Diagnosis by cultivation and subsequent typing of this fastidious anaerobe is difficult, laborious, and not routinely done in diagnostic laboratories, and it is often not fast enough to implement the necessary measures to segregate infected animals and prevent the disease from spreading (3). Moreover, the gelatin-gel and elastase assays to test for protease activity, traditionally carried out to detect and differentiate virulent and benign D. nodosus strains, is difficult to perform and is strongly dependent on the quality of the growth medium and hence may yield inconsistent results (4). PCR-based approaches have markedly improved sensitivity and speed of detection of D. nodosus (3,(5)(6)(7)(8)(9).…”

Prevalence of ovine footrot in Brahmaputra Chars (shoals) in Mymensingh

“…Different strains of Dichelobacter nodosus cause disease of different severity, ranging from benign to virulent (Cheetham et al, 2006). Benign footrot is a self-limiting interdigital dermatitis, whereas severe underrunning of the horn of the hoof defines virulent footrot (Dhungyel et al, 2013).…”