Because the intestine represents one of the largest body surfaces it is a frequent site of multiplication for many commensal and pathogenic microorganisms. Traditionally, most murine intestinal infections were considered to be acute, producing a transient gastroenteritis and a strong protective local immune response. Histopathology was clearly evident and the pathogenic organism was readily isolated or cultured. Historically, only viruses which caused clinical disease were included in serologic screening panels and viruses were only isolated from ill mice. While intestinal contents from both ill and well mice were routinely screened for bacteria and parasites, only those organisms known to cause clinical disease were reported. With the advent of molecular methods, many more intestinal infectious agents have been identified. Many of these agents cause persistent, subclinical infections. Early in the infection during the acute phase, virus and bacterial loads are high. Later in the infection, duringthe persistent/chronic phase, viral and bacterial loads are reduced. Persistence is defined as the failure of the host immune system to effectively clear the agent. Most viruses and bacteria which produce persistent infections result in the chronic production of infectious particles throughout the infection. Many infectious agents which normally cause acute infections will persist in severely immunocompromised mice producing high levels of the agent for weeks to months. The transmissibility of an agentduring persistent infection is related to the organ infected and its ability to excrete the organism into the environment. Intestinal agents such as murine norovirus (MNV) and Helicobacter species are excreted in feces for months and are readily transmitted via exposure to soiled bedding. The numbers of immunocompromised mice used in most research institutions has increased dramatically over the last decade. Genetically engineered mice with deficiencies in only a single immune function often respond toinfectious agents in unexpected ways. Unexpected pathology observed in genetically-engineered mice has lead to the discovery of several new infectious agents, including MNV. The reminder of this manuscript will focus on the persistence of mouse parvovirus (MPV), MNV and Helicobacter species in the intestine and associated gastrointestinal tissues.
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