Journal of General Virology (1993), 74, 819 828. Printed in Great Britain 819 Pathogenicity of a thymidine kinase-deficient mutant of equine herpesvirus 1 in mice and specific pathogen-free foals J. D. Slater, J. S. Gibson* and H. J. Field Department of Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 0ES, U.K. Both intranasal (i.n.) and intracerebral (i.c.) inoculation of mice with wild-type equine herpesvirus type 1 (wt EHV-1) caused clinical signs and mortality. Virus could be recovered from target organs (turbinates, lungs and blood) for several days. By contrast, the thymidine kinase (TK)-deficient deletion mutant PR1 produced markedly less clinical disease following both i.n. and i.c. inoculation, and, in particular, no mortality occurred. PR1 did, however, establish productive infections following either route of inoculation. High titres of virus were recovered from target organs although virus did not persist for as long as wt EHV-1 and no viraemia was detected. Primary i.n. infection of mice with either wt EHV-1 or PR1 protected against subsequent challenge with wt EHV-1 5 weeks later. I.n. inoculation of specific pathogen-free (EHV-free) foals with PR1 produced results similar to those observed after infection of mice. Clinical signs were milder than for wt EHV-1 and pyrexia was short-lived or absent. PR1 could be recovered from nasal mucus at high titres but it persisted for only 5 days post-infection compared to 11 days in the case of wt EHV-1. No viraemia was detected in foals infected with PR1. On challenge with wt EHV-1, foals given a primary infection with the mutant were partially protected; but a viraemia with a TK + EHV-1 was observed. These results demonstrate that our TK mutant PR1 is markedly less pathogenic than wt EHV- 1, despite being able to replicate in the host. The use of TK-deficient mutants of EHV-1 as potential vaccines in the horse is discussed. Introduction The alphaherpesvirus equine herpesvirus type 1 (EHV-1) is one of several herpesviruses known to infect horses. EHV-1 can cause abortion and neurological disease, as well as respiratory problems. It is a major pathogen of horses with considerable economic and veterinary importance (Allen & Bryans, 1986). To date, control measures have proved inadequate, and although vaccines are available, their efficacy is controversial (Burrows et al., 1984; Allen & Bryans, 1986; Burki et al., 1990). Progress towards more effective immunoprophylaxis has been hindered by incomplete understanding of the pathogenesis and epidemiology of EHV- 1. Our approach to this problem has involved the use of two animal models: colostrum-deprived specific pathogen-free (SPF), EHV-free, foals and mice. SPF foals are taken from their dams at birth and reared by hand using bovine colostrum (Chong et al., 1991). They are free from EHV infection and also from maternal antibodies. They are invaluable in the study of EHV infection, particularly the interaction between different EHVs (Gibson et al., 1992a, b). Because foal production is limited and sea- sonal, we have also developed a murine model for EHV- 1 infection whose features show remarkable similarity to infection in the natural host (for a review, see Field et al., 1992b). The study of thymidine kinase (TK)-deficient mutants of other herpesviruses has received considerable at- tention. In particular, TK is implicated in the neuro- pathogenicity of various alphaherpesviruses, including herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) and suid herpesvirus type 1 (SHV-1) (Field & Wildy, 1978; Becker et al., 1984; Kit et al., 1985). Although EHV-1 is not thought to infect neurons directly, it does cause neurological problems in the horse, probably via infection of endothelial cells in the central nervous system (CNS) and subsequent ischaemic damage of nervous tissue (Patel et al., 1982). A similar sequence of events involving infection of placental blood vessels is postulated in EHV-l-induced abortion. Previous reports of EHV-1 TK-deficient mutants suggest that they are indeed less pathogenic than the wild-type (wt) virus (Cornick et al., 1990). The behaviour of such mutants, however, has not been studied in detail. Furthermore, previous work has made use of naturally reared horses, which have the disadvantage that their EHV status is uncertain and results obtained from them are therefore difficult to interpret (Cornick et al., 1990). Here we describe the characteristics of a drug-selected 0001-1452 © 1993SGM