Rodent-borne viral infectious disease
Medical condition
Lymphocytic choriomeningitis (LCM) is a rodent-borne viral infectious disease that presents as aseptic meningitis, encephalitis or meningoencephalitis. Its causative agent is "lymphocytic choriomeningitis mammarenavirus" (LCMV), a member of the family "Arenaviridae". The name was coined by Charles Armstrong in 1934.
Lymphocytic choriomeningitis (LCM) is "a viral infection of the membranes surrounding the brain and spinal cord and of the cerebrospinal fluid". The name is based on the tendency of an individual to have abnormally high levels of lymphocytes during infection. Choriomeningitis is "cerebral meningitis in which there is marked cellular infiltration of the meninges, often with a lymphocytic infiltration of the choroid plexuses".
 ;templatestyles src="Template:TOC limit/styles.css" / ;
Signs and symptoms.
LCMV infection manifests itself in a wide range of clinical symptoms, and may even be asymptomatic for immunocompetent individuals. Onset typically occurs between one or two weeks after exposure to the virus and is followed by a biphasic febrile illness. During the initial or prodromal phase, which may last up to a week, common symptoms include fever, lack of appetite, headache, muscle aches, malaise, nausea, and/or vomiting. Less frequent symptoms include a sore throat and cough, as well as joint, chest, and parotid pain. The onset of the second phase occurs several days after recovery, and consists of symptoms of meningitis or encephalitis. Pathological findings during the first stage consist of leukopenia and thrombocytopenia. During the second phase, typical findings include elevated protein levels, increased leukocyte count, or a decrease in glucose levels of the cerebrospinal fluid).
Occasionally, a patient improves for a few days, then relapses with aseptic meningitis, or very rarely, meningoencephalitis.
Patients with meningitis may have a stiff neck, fever, headache, myalgia, nausea and malaise. In some occasions, meningitis occurs without a prodromal syndrome. Meningoencephalitis is characterized by more profound neurological signs such as confusion, drowsiness, sensory abnormalities and motor signs. Underreported complications include myelitis, Guillain–Barré-type syndrome, cranial nerve palsies, transient or permanent hydrocephalus, sensorineural hearing loss, orchitis, arthritis and parotitis. LCMV infections have also been associated with pancreatitis, pneumonitis, myocarditis and pericarditis. The entire illness usually lasts 1 to 3 weeks; nonetheless, temporary or permanent neurological damage is possible in all central nervous system infections, especially in cases of meningoencephalitis. Chronic infections have not been reported in humans and deaths rarely occur.
Cause.
Virus biology.
There are several strains of LCM virus, among which the most widely used are LCMV Armstrong and LCMV Clone 13. Armstrong is the original virus strain which was isolated from the brain by Charles Armstrong in 1934. It triggers a vigorous cytotoxic T lymphocytes (CTL) response and thus, it is cleared rapidly by the host. This is referred to as acute (Armstrong) LCMV infection. On the other hand, Clone 13 is a variant of the Armstrong viral strain, isolated from the spleen and is consequently tropic for visceral organs. It was first isolated from mice which sustained a persistent LCMV infection from birth. This variant potentiates a less vigorous CTL response in the immune system, and thus can ultimately persist in the host organism indefinitely. The latter is referred to as chronic (Clone 13) LCMV infection.
LCMV is a spherical enveloped virus with a diameter between 60 and 300 nm. The helical nucleocapsid contains an RNA genome consisting of two negative single-stranded RNA segments. The negative RNA strand, complementary to the necessary mRNA strand, indicates that it must first be transcribed into a positive mRNA strand before it can be translated into the required proteins. The L strand is ambisense RNA, encoding multiple proteins in opposite directions, separated by an intergenic region. It is approximately 7.2 kb in size and encodes a high-molecular-mass protein (L; 200 kDa) and an 11 kDa small polypeptide Z with unknown function and a ring finger motif. The viral RNA-dependent RNA polymerase is encoded by the L protein which contains conserved characteristic motifs throughout all the RNA-dependent, RNA-polymerases. The S strand is ambisense and approximately 3.4 kb in size. It encodes the two main structural proteins: nucleo-protein (NP) (63 kDa) and glycoprotein precursor complex (GPC) (75 kDa). The latter undergoes posttranslational cleavage at two separate steps: first in the endoplasmic reticulum, where following translation on the membrane, the stable signal peptide region is cleaved by signal peptidase (SPase); notably, this signal peptide is retained, in contrast to other viruses. The second cleavage occurs at the Golgi apparatus, where GP1-GP2, having been trafficked to the Golgi, is cleaved into separate mature viral glycoproteins, GP1 and GP2 (with cleavage mediated by SK1-/PS1 or MBTPS1). The spikes present on the virion envelope are dictated by tetramer formation of GP-1 and GP-2.
When LCMV attacks a cell, the process of replication starts by attachment of the virus to host receptors through its surface glycoproteins. It is then endocytosed into a vesicle inside the host cell and creates a fusion of the virus and vesicle membranes. The ribonucleocapsid is then released in the cytoplasm. The RNA-dependent, RNA-polymerase brought along with the virus initially binds to a promoter on the L and S segments and begins transcription from negative-stranded to a positive-stranded mRNA. The formation of a strong hairpin sequence at the end of each gene terminates transcription. The mRNA strands are capped by the RNA-dependent, RNA-polymerase in the cytoplasm and are then subsequently translated into the four proteins essential for LCMV assembly. The ribonucleocapsid interacts with the Z matrix protein and buds on the cell membrane, releasing the virion out from the infected cell.
The first arenavirus, "Lymphocytic choriomeningitis mammarenavirus" (LCMV), was isolated in 1933 by Charles Armstrong during a study of an epidemic in St. Louis. Although not the cause of the outbreak, LCMV was found to be a cause of nonbacterial or aseptic meningitis.
In 1996, Peter Doherty and Rolf Zinkernagel shared the Nobel Prize in Medicine and Physiology, for their work with LCMV which led to a fundamental understanding of the adaptive immune response, MHC restriction. In the 1970s, studies concerning the importance of MHC locus were done exclusively in transplantation and tumor rejection. Taking this into consideration, Doherty and Zinkernagel were working on the response of mice to virus infections. They observed that T-cell receptors must recognise a complex of foreign antigen and an MHC antigen in order to destroy infected cells. Their key experiment involved harvesting of splenocytes containing LCMV-specific cytotoxic T lymphocytes (CTL) from an infected mouse strain A. These were then mixed "in vitro" with virus infected fibroblasts or macrophages from two different mouse strains, A and B. By the method of Cr-release cytotoxicity assay, thereby tagging the CTL with chromium-51 (Cr-51), the CTL destruction of infected cells was quantified by release of Cr-51. The results showed that CTL killed only the infected cells from strain A, and they did not lyse uninfected cells or infected cells from strain B. They concluded that these virus specific CTLs only lyse cells carrying the same molecules of the major histocompatibility site (MHC) as the CTLs themselves. Thus, the virus-specific CTLs are "MHC-restricted". This discovery lead to a greater understanding of an important aspect of the immune system.
Spread.
LCMV is naturally spread by the common house mouse, "Mus musculus". Once infected, these mice can become chronically infected by maintaining virus in their blood or persistently shedding virus in their urine.
Chronically infected female mice usually transmit infection to their offspring (vertical transmission), which in turn become chronically infected. Other modes of mouse-to-mouse transmission include nasal secretions, milk from infected dams, bites, and during social grooming within mouse communities. Airborne transmission also occurs.
The virus seems to be relatively resistant to drying and therefore humans can become infected by inhaling infectious aerosolized particles of rodent urine, feces, or saliva, by ingesting food contaminated with virus, by contamination of mucous membranes with infected body fluids, or by directly exposing cuts or other open wounds to virus-infected blood. The only documented cases of transmission from animals have occurred between humans and mice or hamsters.
Cases of lymphocytic choriomeningitis have been reported in North and South America, Europe, Australia, and Japan, particularly during the 1900s. However, infection may occur wherever an infected rodent host population exists. LCMV occurs worldwide and its natural host, the rodent, has become established on all continents, except Antarctica.
Seroprevalence is approximately 5% (0.7–4.7%) of the US population. It tends to be more common among lower socio-economic groupings, probably reflecting more frequent and direct contacts with mice. However, obtaining an accurate sense of prevalence by geographic region is difficult due to underreporting.
Congenital infection.
Lymphocytic choriomeningitis is a particular concern in obstetrics, as vertical transmission is known to occur. For immunocompetent mothers, there is no significant threat, but the virus has damaging effects upon the fetus. If infection occurs during the first trimester, LCMV results in an increased risk of spontaneous abortion. Later congenital infection may lead to malformations such as intracranial calcifications, hydrocephalus, microcephaly or macrocephaly, intellectual disabilities, and seizures. Other findings include chorioretinal scars, and optic atrophy. Chorioretinitis, which is followed by chorioretinal scarring, is the most common ocular lesion. Mortality among infants is approximately 30%. Among the survivors, two-thirds have lasting neurologic abnormalities.
Other ocular defects including optic atrophy, microphthalmia, vitreitis, leukokoria and cataracts can also be seen. Most of the infants in one case series were of normal birth weight, although 30% were underweight. Aspiration pneumonia can be a fatal complication. Infants who survive may have severe neurological defects including epilepsy, impaired coordination, visual loss or blindness, spastic diplegia or quadriparesis/quadriplegia, delayed development and intellectual disability. Less severe cases with isolated cerebellar hypoplasia and symptoms of ataxia and jitteriness have been reported occasionally. There have also been rare cases with evidence of chorioretinitis but without neurological signs. Systemic signs seem to be rare, but hepatosplenomegaly, thrombocytopenia and hyperbilirubinemia have been documented in a few cases, and skin blisters were reported in one infant.
If a woman has come into contact with a rodent during pregnancy and LCM symptoms are manifested, a blood test is available to determine previous or current infection. A history of infection does not pose a risk for future pregnancies.
Organ donation.
Patients infected in solid organ transplants have developed a severe fatal illness, starting within weeks of the transplant. In all reported cases, the initial symptoms included fever, lethargy, anorexia and leukopenia, and quickly progressed to multisystem organ failure, hepatic insufficiency or severe hepatitis, dysfunction of the transplanted organ, coagulopathy, hypoxia, multiple bacteremias and shock. Localized rash and diarrhea were also seen in some patients. Nearly all cases have been fatal.
In May 2005, four solid-organ transplant recipients contracted an illness that was later diagnosed as lymphocytic choriomeningitis. All received organs from a common donor, and within a month of transplantation, three of the four recipients had died as a result of the viral infection. Epidemiologic investigation traced the source to a pet hamster that the organ donor had recently purchased from a Rhode Island pet store. Similar cases occurred in Australia in 2006 and Massachusetts in 2008. There is not a LCMV infection test that is approved by the Food and Drug Administration for organ donor screening. The "Morbidity and Mortality Weekly Report" advises health-care providers to "consider LCMV infection in patients with aseptic meningitis and encephalitis and in organ transplant recipients with unexplained fever, hepatitis, or multisystem organ failure."
Diagnosis.
Current or previous infection can be detected through a blood test. However, some authors note that such complement-fixation tests are insensitive and should not be used for diagnosis. Dr. Clare A. Dykewicz, "et al." state,
 Infection with LCMV should be considered in the differential diagnosis of any compatible, severe viral infection or aseptic meningitis, especially when there is a history of occupational exposure to laboratory rodents. Timeliness of diagnosis is important not only in expediting treatment of infected persons, but also in preventing further LCMV transmission to other workers and animals.
Clinical diagnosis of LCM can be made by the history of prodrome symptoms and by considering the period of time before the onset of meningitis symptoms, typically 15–21 days for LCM.
Pathological diagnosis of congenital infection is performed using either an immunofluorescent antibody (IFA) test or an enzyme immunoassay to detect specific antibody in blood or cerebrospinal fluid. A PCR assay has been recently developed which may be used in the future for prenatal diagnosis; however, the virus is not always present in the blood or CSF when the affected child is born." Diagnoses is subject to methodological shortcomings in regard to specificity and sensitivity of assays used. For this reason, LCMV may be