Merits of Sheep Antisera for Antivenom Manufacture

The first immunotherapeutic product was of ovine origin but horses were used from the outset for antivenom production and continue to be the species of choice. Nonetheless, sheep offer some advantages. The practicalities of raising antisera in sheep are reviewed and a schedule for immunising, sampling and bleeding sheep is presented. Among factors of critical importance are the choice of adjuvant, the quality and amount of venom injected, the care taken in ensuring a stable venom:adjuvant emulsion, the frequency of immunisation and the site and number of injections given. The main advantage of using sheep lies in the excellence of their humoral immune response which probably depends, in part, on being able to employ Freund's adjuvant routinely. Virtually 100% of sheep attain specific antibody levels of more than 6 g/l and maintain their response for as long as immunisation is continued. Each sheep provides 4 to 5 litres of antisera annually for some 5 to 8 years. Sheep are relatively inexpensive to purchase, house and feed and are easy to handle, immunise and bleed. They provide a viable alternative to horses and the eventual choice of species may well depend upon the relative costs.