Weight loss and reduced body temperature determine humane endpoints in a mouse model of ocular herpesvirus infection.

Herpes simplex virus (HSV) is an important human pathogen, with the potential to cause mucocutaneous lesions, encephalitis, and complications with keratitis and blindness.18 HSV1 forms latent infections in neurons of the peripheral nervous system, particularly in the trigeminal and sacral ganglia, and less than 40% of treated human patients return to functional (preinfection) health status after infection.30 Although human HSV has been studied in the well-established mouse model of peripheral nervous system infection, a major obstacle to HSV research is the limited quantity of viral material that can be recovered from the immune-privileged neural tissue in latently infected animals. Specific research goals of the current study were to generate latently infected mice for continued assessment of HSV pathogenesis and for determination of the mechanism of HSV latency and reactivation at a molecular level. Latency is defined as the absence of any detectable replicating virus, proteins of the lytic viral replication cycle, or viral transcripts. During HSV infection of mice by the ocular (corneal) route, infectious virus can be recovered from the trigeminal ganglia (TG) of mice until days 7 to 10 after infection. To discern that latency has been accomplished, mice must be maintained beyond day 30 after infection. Estimates of the percentage of latently infected neurons are in the range of 0.1% to 1%, based on reactivation from TG cell preparations.16,44 For this HSV mouse model, prolonged periods of survival after infection are necessary to ensure viral latency.10,11 Clinical veterinary challenges for this model included the inability to predict which infected mice would develop severe acute disease with death as an outcome. IACUC protocols for a variety of infectious disease studies often indicate that the work precludes the use of supportive treatments, opioids, or other analgesics known to modify immune responses and affect data outcomes; therefore, clinical investigations of modest supportive care measures and developing methods to avoid spontaneous death are invaluable for improved laboratory animal welfare. The IACUC-approved protocol for this HSV model estimated that as many as 50% of HSV-infected mice would die, without the ability to predict which mice would become severely ill and which would survive. Furthermore, because of potential effects on neurochemical, respiratory, and vascular systems, suitable interventions for pain relief were not feasible for administration to infected mice. Subsequently, the current study was undertaken to determine whether, in fact, certain mice could predictably be identified for removal from study prior to eventual death and, furthermore, whether modest supportive care measures could be instated. To our knowledge, none of these points have been studied previously in this model. Humane interventions and endpoints continue to be evaluated in a variety of rodent biomedical models by using objective scoring systems to quantify rankings of criteria for exclusion of animals from further study.1,9,21,22,25,28,33, 34,40,41,43 Nontransient decreases in whole-body temperature, as an early predictor of impending death, have been demonstrated in mice with specific experimental LD50, bacterial, viral, and fungal infections.2,26,34,36, 40,41,43,45,46 Microchip transponder systems that remotely read subcutaneous or intraperitoneal body temperatures have been tested and shown to be sufficiently similar in readings to traditional rectal thermometry6,19 and intraabdominal telemetry.43 Infrared surface thermometers are another method of temperature acquisition considered to be entirely noninvasive and to provide rapid and repeatable temperature readouts (within seconds). These thermometers are considerably less expensive than are other methods and are readily portable between rooms and facilities; we therefore were quite interested to determine their applicability to the HSV mouse model. When aimed at a consistent location on the animal's ventral aspect, infrared thermometry has been shown to be a reliable form of temperature measurement for laboratory mice.2,45 On the basis of thermometry studies in other infectious disease models, we hypothesized that a nontransient decrease in body temperature (as measured both by microchip and infrared thermometry), in conjunction with diminished overall wellbeing and loss of body weight, would be predictive of alternative nondeath humane endpoints for this ocular mouse model of HSV latency.