C. E. Kirkpatrick,¹ D. A. Leiby,^1,* D. Abraham,^1,** C. H. Duffy<sup>1

1</sup>Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, 3800 Spruce S

^*Present address: Department of Zoology, Ohio State University, Columbus, Ohio 43210, USA.
^**Present address: Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin 53706, USA.