Vertical transmission of infectious disease in cattle (Proceedings)
The potential for several economically important infectious diseases of cattle to be vertically transmitted from dam to offspring is of understandable concern to veterinarians and producers alike. A proportion of the research into several of the major contagious, infectious diseases of cattle such as bovine leukosis virus (BLV), bovine viral diarrhea virus BVDV), and Johne's disease has been oriented towards elucidating the risks that maternal infection represents for true transplacental spread in utero to the fetus, or the immediate post-natal spread of disease from dam to calf. Included within a more complete list of infectious agents for which there has been avid discussion of the relative risk of vertical transmission are bovine viral diarrhea virus (BVDV), bovine leukosis virus (BLV), Mycobacterium paratuberculosis, blue tongue virus (BTV), Neospora caninum and bovine spongiform encephalopathy. Veterinarians and producers that deal with predominantly grade cattle of lesser individual value are necessarily concerned about the production and economic implications of vertically acquired infectious agents, whereas those who work with individually valuable animals, particularly those involved in assisted reproductive technologies such as conventional in vivo embryo transfer, oocyte recovery and in vitro fertilization have greater concerns about product quality and the pure infectious disease status of the dam and progeny produced. For several of the diseases named above international export and import concerns are prominent.This presentation will focus on the current knowledge regarding vertical transmission of several of these key diseases and the implications for veterinarians working with all types of cattle.
Bovine leukosis virus
The ability of BLV to effect both horizontal and vertical transmission has long been recognized. The proportionate importance of vertical transmission is far less than horizontal, with the majority of infectious spread being via transfer of BLV infected lymphocytes in blood, colostrum and milk. Iatrogenic transmission during cattle handling, processing and routine husbandry is a frequent means of spread that is commonly targeted by control programs. Serologic surveys within the US show wide variation in state by state infection levels but a consistent finding amongst studies is that dairy breeds have a higher seroprevalence than do beef breeds1. The possibility of transplacental infection from a BLV positive cow to her calf in utero has long since been verified but there is little recent data that adds to the literature from the 1980s and 1990s. Epidemiologic studies examining in utero spread in BLV positive dams via pre-colostral blood testing of neonates have demonstrated vertical transmission rates that vary from 0.5% to 18%1. Maternal lymphocytosis (>12,000/µL) and a high herd seroprevalence appear to be factors that may increase the vertical transmission rates in these epidemiologic studies1,2. The dam's age, parity and whether or not maternal infection takes place during pregnancy are factors that do not appear to increase the likelihood of in utero transmission1. Studies on commercial dairies in the US would suggest that 4% is a reasonable estimate for the proportion of progeny that will be born infected from seropositive dams. Semen from BLV infected bulls is not considered infective, unless there is significant leucopsermia and infected lymphocytes are transferred. Similarly, ova and embryos from BLV positive donors are not considered sources of infection for recipient animals.