Rapid diagnosis of LaCrosse encephalitis: detection of specific immunoglobulin M in cerebrospinal fluid

Evaluation of widely used diagnostic tests to detect West Nile virus infections in horses previously infected with St. Louis encephalitis virus or dengue virus type 2

Primary West Nile virus (WNV) infections can be diagnosed using a number of tests that detect infectious particles, nucleic acid, and specific IgM and/or IgG antibodies. However, serological identification of the infecting agent in secondary or subsequent flavivirus infections is problematic due to the extensive cross-reactivity of flavivirus antibodies. This is particularly difficult in the tropical Americas where multiple flaviviruses cocirculate. A study of sequential flavivirus infection in horses was undertaken using three medically important flaviviruses and five widely utilized diagnostic assays to determine if WNV infection in horses that had a previous St. Louis encephalitis virus (SLEV) or dengue virus type 2 (DENV-2) infection could be diagnosed. Following the primary inoculation, 25% (3/12) and 75% (3/4) of the horses mounted antibody responses against SLEV and DENV-2, respectively. Eighty-eight percent of horses subsequently inoculated with WNV had a WNV-specific antibody response that could be detected with one of these assays. The plaque reduction neutralization test (PRNT) was sensitive in detection but lacked specificity, especially following repeated flavivirus exposure. The WNV-specific IgM enzyme-linked immunosorbent assay (IgM ELISA) was able to detect an IgM antibody response and was not cross-reactive in a primary SLEV or DENV response. The WNV-specific blocking ELISA was specific, showing positives only following a WNV injection. Of great importance, we demonstrated that timing of sample collection and the need for multiple samples are important, as the infecting etiology could be misdiagnosed if only a single sample is tested.

La Crosse and other forms of California encephalitis

The California serogroup viruses are mosquito viruses that cause human infections on five continents. They are maintained and amplified in nature by a wide variety of mosquito vectors and mammalian hosts; they thrive in a remarkably wide variety of microclimates (eg, tropical, coastal temperate marshland, lowland river valleys, alpine valleys and highlands, high boreal deserts, and arctic steppes). In 1993, California serogroup viruses caused 71% of all cases of arboviral illness in the United States, principally La Crosse encephalitis. The 30 to 180 annual cases of La Crosse encephalitis represent 8% to 30% of all cases of encephalitis, rendering this illness the most common and important endemic mosquito-borne illness in the USA. Subclinical or mild infections are much more common. Methods and results acquired from intense study of California serogroup viruses have been applied, with benefit, to the study of the ecology and pathogenesis of many more serious human arboviral illnesses. The evolutionary potential of viruses, with particular reference to the development of more virulent strains, has been studied more closely in the California serogroup viruses than in almost any other agent of human disease.

Reverse transcription-PCR detection of LaCrosse virus in mosquitoes and comparison with enzyme immunoassay and virus isolation

A reverse transcription-PCR (RT-PCR) assay was developed and compared with enzyme immunoassay (EIA) and virus isolation for detecting LaCrosse virus (LAC) in mosquito pools. All three techniques were able to detect a single LAC-infected mosquito in a pool of 99 negative mosquitoes. Virus isolation was the most sensitive of the three techniques; it was possible to isolate virus immediately following intrathoracic inoculation of mosquitoes. RT-PCR was second in sensitivity; LAC RNA was detected 1 day postinfection. EIA detected LAC antigen 2 days postinfection. Additionally, RT-PCR and EIA were able to detect LAC RNA and protein, respectively, from mosquito samples which were subjected to seven freeze-thaw cycles, and RT-PCR was able to detect LAC RNA from mosquito samples which remained at room temperature for up to 7 days.

Intrathecal immune response and virus-specific immunoglobulin M antibodies in laboratory diagnosis of acute poliomyelitis

The intrathecal immune response in 114 patients with clinically diagnosed acute poliomyelitis was studied by measuring poliovirus-specific immunoglobulin M (IgM) antibodies in cerebrospinal fluid (CSF) by a mu-capture immunoassay and by assessing the ratio between levels of poliovirus-neutralizing antibodies in serum and CSF. Fecal specimens were used for attempts to isolate the causative agents. Eighty-five percent of CSF specimens collected during the first 15 days of disease contained virus-specific IgM antibodies. Forty-five of 48 tested children (94%) also showed virus-specific IgM responses in their sera. Later on, the antibody levels decreased, and positive results after 30 days of onset of paralytic symptoms were rare. If the presence of poliovirus-specific IgM antibodies in the CSF was considered diagnostic, more cases were confirmed by this test than by virus isolation. A relative increase in poliovirus-neutralizing antibodies in the CSF was observed in about one-third of the cases; in all but three cases the increase was observed together with the presence of virus-specific IgM antibodies. A systemic virus-specific response can be seen and poliovirus can be isolated from a subclinically infected individual suffering from a concomitant poliomyelitis-like disease, while positive results by the two methods demonstrating an intrathecal immune response are likely to indicate a true causal relationship between infection and disease. Demonstration of poliovirus-specific IgM antibodies in the CSF thus appears to be a sensitive and specific method for laboratory confirmation of clinically diagnosed poliomyelitis.