Digital dermatitis-like disease and pastern dermatitis in heifers kept in a public breeding farm in Japan

Introduction 
Digital dermatitis (DD) is a disease producing mild to severe lameness in cattle . Animals affected with DD show a reduction in milk yield and declining fertility. Moreover, DD-affected cattles kept in free stall cowsheds tend to develop digestive diseases because the animals cannot frequently access the food container due to foot pain and cannot intake a large amount of food at a time. DD is an infectious disease caused by several bacteria, and the genus Treponema is thought to be especially important. However, the infection route and source of the treponema are not clear. We report the incidence of DD-like disease and pastern dermatitis in heifers kept in a public breeding farm in Japan. 

Materials and Methods
1.Farm examined  
We examined one public breeding farm in the Tokachi district in Hokkaido, Japan. The number of heifers kept was about 2,000. 2.Epidemiological survey  The crisis rates of DD-like disease and pastern dermatitis for the past three years on this farm were investigated.
3.Clinical examination 
The level of lameness and morphological features of lesions with both diseases were examined.
4.Imprint cytology of specimens 
We tried to detect spirochete-like bacteria with the cytology method imprint for biopsy samples from lesions with both diseases and staining with Giemsa stain.
5.Histopathological examination 
Tissue sections from both biopsy samples were stained with hematoxylin-eosin and Warthin-Starry stains. They were examined for the existence of spirochete-like bacteria. 

Results 
The ratios of DD-like disease among all the foot rot diseases were 24% in 2015, 39% in 2016, and 56% in 2017, showing an increasing trend. The number of affected animals tended to increase in summer. There was a correlation between monthly frequency of DD-like disease and pastern dermatitis. Heifers affected with DD-like disease tended to show mild lameness, and the lesion closely resembled DD lesions with papillomatous proliferation in dairy cows. On the other hand, heifers affected with pastern dermatitis tended to show milder lameness, and the lesion morphologically resembled pastern dermatitis in horses. Spirochete- like bacteria were detected with the imprinted specimen of DD-like disease samples, but not detected in pastern dermatitis samples. Histopathologically, the lesion with DD-like disease was almost the same as DD lesions of cattle on dairy farms, and spirochete-like bacteria were detected in epithelial tissue. However, there was no spirochete-like bacteria in epithelial tissue of pastern dermatitis lesion. 

Conclusions 
The clinical and histopathological findings of DD-like disease observed in heifers in this study closely resembled that of DD in cattle on Japanese dairy farms. These findings suggested that heifers infected with spirochete-like bacteria while they were kept on a breeding farm might transmit the bacteria to dairy cows later. Furthermore, because there was a correlation between the monthly incidence of DD-like disease and pastern dermatitis and the lesions of both diseases appear at same position on the foot, DD-like disease in this breeding farm might be formed by secondary infection with spirochete-like bacteria after the onset of pastern dermatitis caused by another pathogen.