The increasing prevalence of PDD over the last decade has contributed significantly to world wide economic losses in the dairy industry. In response to this problem, Hygieia Biological Laboratories has developed a Serpens Species Bacterin vaccine to aid in the control of PDD.

The Organism

Serpens species bacteria are pleiomorphic gram negative rods. Although they can exist as functional anaerobes, Serpens spp. are primarily aerobic bacteria. These organisms are capable of assuming spiral morphologies and of exhibiting spiral motility. Oligonucleotide cataloging indicates that Serpens may be closely related to members of the genus Pseudomonas. Serpens spp. are exceptionally invasive and can travel up to five times as fast as spirochetes under experimental conditions.

The 1977 isolation of the Serpens type strain from pond water suggests an environmental reservoir for this organism. Hygieia Biological Laboratories has obtained positive cultures for Serpens species from four of seven fecal samples from cows on PDD affected dairies. This indicates that Serpens spp. can survive in and be shed from the G.I. tract of dairy cows.

Serpens species have been isolated from 80-90% of the PDD lesions analyzed by Hygieia. These organisms exhibit both spiral and rod morphologies. The proportion of these forms changes with the stage of the lesion; the rod form is more common in later stage lesions. Motility patterns in wet mount preparations and electron microscopy both confirm the presence of flagella on these organisms. Possession of flagella precludes these bacteria from being spirochetes. In addition, electron microscopy of these bacteria has shown no evidence of the axial filaments necessary for classification of an organism as a spirochete. Silver stained tissue sections of PDD lesions show slender, spiral-shaped organisms deep within the dermis. These organisms cluster around the capillaries in areas of high oxygen tension.

Pathogenesis

PDD is characterized by initial ulceration followed by first a granulomatous then a proliferative response. The lesions are typically superficial, and most animals apparently are unable to generate a complete immune response to natural challenge. In a naive herd, the disease affects all age groups with high morbidity. In chronically affected herds, a low morbidity infection cycles through the herd, primarily impacting younger animals.

The disease appears to be readily transmissible. On one dairy, heifers from a PDD free pasture were kept in a non-isolated holding pen for four to six weeks and then were returned to the original pasture. These heifers likely were responsible for the subsequent introduction of PDD to that pasture.

Clinical and serological responses to vaccination with the Serpens Species Bacterin provide strong evidence of a role for this bacteria in PDD pathogenesis. In addition, a PDD lesion has been produced in a dairy cow challenged with a pure culture of Serpens spp. bacteria.

Control

Prior to the advent of the Serpens Species Bacterin, the most efficacious treatments for PDD involved debridement, antibiotic bandaging, various footbaths, and topical sprays. These methods have been used with varying success in alleviating existing lesions.

These treatments have potential drawbacks. Repeated debridement leads to lameness resulting from excess scar tissue formation. In addition, reports exist of loss of antibiotic efficacy, suggesting that the agent of PDD may be capable of developing antibiotic resistance. Also, many of the chemicals used in footbaths pose potential safety hazards. Finally, with the exception of a few new topical products, the majority of these treatments represent extra-label drug use and carry the potential for introducing antibiotic residues.