Abstract. Fifteen bovine herpesvirus-1 (BHV-1)-negative calves were vaccinated intramuscularly with 10 7.4 plaque-forming units of a double-deletion BHV-1 mutant (IBRV(NG)dltkdlgIII), and 6 remained as nonvaccinated controls. Thirty days after vaccination, the animals were challenged by nasal instillation of 10 8.2 CCID 50 of a virulent BHV-1 strain (Cooper). The vaccinated calves were protected against wildtype virus challenge as demonstrated by clinical evaluation. Most of the vaccinates developed only a mild rhinitis (lasting an average of 6.5 days) with almost no systemic symptoms, whereas the controls developed a serious illness characterized by rhinitis (X = 11.5 days), conjunctivitis, hyperthermia, apathy, loss of appetite, and dyspnea. The vaccinates also shed significantly less virus and for a shorter period of time (X = 5.5 days) than the controls (X = 9 days). Thirty days after vaccination, the vaccinates were negative in an anti-gIII specific blocking enzyme-linked immunosorbent assay (ELISA), despite the fact that most of them had developed neutralizing antibodies (serum neutralization titers ranging from 1:2 to 1: 16). Seroconversion to gIII was detected as early as 7 days postinfection (dpi). Fourteen days after the challenge, all the animals exposed to wildtype BHV-1 had developed anti-gIII antibodies and were positive in this differential serologic test. Six controls plus 8 vaccinates kept in isolation were still positive to gIII when tested at 75 dpi. The use of the IBRV(NG)dltkdlgIII strain in conjunction with an anti-gIII specific blocking ELISA kit represents a powerful tool for BHV-1 control/eradication programs.Bovine herpesvirus type 1 (BHV-1), also known as However, these traditional vaccines induce an aninfectious bovine rhinotracheitis virus (IBRV), is the tibody response that is indistinguishable from that proetiologic agent of important diseases of cattle, such as duced in response to natural infection. Thus, all seinfectious bovine rhinotracheitis (IBR), vulvovaginitis ropositive animals tend to be considered potential (IPV), balanopostitis (IBP), abortion, encephalitis in carriers of the virus, complicating the establishment calves, and fatal multisystemic infection in new-of control measures regarding cattle movement, trade, borns 5,15 Bovine herpesvirus-1, like other alphaher-. import/export, and certification of BHV-1 -free herds. pesviruses, can establish latency in neural sensory ganglia and can be reactivated either by stressful conditions A few years ago, a new temperature-resistant, genetically engineered BHV-1 strain was described as a or by corticosteroid treatment. 1,19 potential vaccine candidate.8 This strain, denominated Bovine herpesvirus-1 infection constitutes one of the IBRV(NG)dltk, was derived from the parental BHV-1 most important infectious diseases of cattle and causes significant economic losses to livestock around the "Los Angeles" strain and contained a 343-bp deletion in the thymidine kinase (tk) gene as the principal basis world. Because of its sanitary ...