Performance assessment of SARS-CoV-2 IgM & IgG ELISAs in comparison with plaque reduction neutralization test

Background & objectives

Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) continues to be a devastating pandemic. This study was aimed at performance assessment of SARS-CoV-2 IgM and IgG ELISAs, and investigation of their utility for patient diagnosis and sero-epidemiologic investigations.

Methods

Serum/plasma samples from COVID-19 patients or asymptomatic contacts (n=180) and healthy donors (n=90) were tested in parallel using two commercial IgM ELISAs (Erbalisa and Inbios), and four IgG ELISAs (Kavach, Euroimmun, Erbalisa and Inbios) along with an indigenous β-propiolactone inactivated virus-based ELISA (IRSHA-IgG-ELISA). Plaque reduction neutralization test (PRNT) was used as reference test.

Results

Among 180 COVID-19 patients, 125 tested positive by PRNT. Inbios-IgM-ELISA showed sensitivity (Se)/specificity (Sp)/positive predictive value (PPV)/negative predictive value (NPV) of 93.6/97.8/98.4/94.4 per cent in relation to PRNT, and performed better than Erbalisa-IgM-ELISA (Se: 48%, Sp: 95.6%, PPV: 95.2%, NPV: 65.2%). During the first week of disease, only 47.4 per cent of the COVID-19 patients tested IgM positive by Inbios-IgM-ELISA, detection improving at two weeks and beyond (~86-100%). Among IgG tests, Inbios-IgG-ELISA ranked first in terms of sensitivity (83.2%), followed by IRSHA (64.8%), Euroimmun (64%), Erbalisa (57.6%) and Kavach (56%) tests. For all IgG tests, sensitivity improved during the third (73.9-95.7%) and fourth week (100%) of illness. The specificity (96.7-100%) and PPV (96.2-100%) of all IgG tests were high; NPV ranged between 71.9 and 87.1 per cent with Inbios-IgG-ELISA scoring highest.

Interpretation & conclusions

Our results show that IgM detection by the current, most sensitive ELISAs cannot replace molecular diagnosis, but may aid as a supplement test. The available IgG tests are suitable for serosurveys for the assessment of previous virus exposure.

Study of Kyasanur forest disease viremia, antibody kinetics, and virus infection in target organs of Macaca radiata

The present manuscript deals with experimental infections of bonnet macaques (Macaca radiata) to study disease progression for better insights into the Kyasanur Forest Disease (KFD) pathogenesis and transmission. Experimentally, 10 monkeys were inoculated with KFD virus (KFDV) (high or low dose) and were regularly monitored and sampled for various body fluids and tissues at preset time points. We found that only 2 out of the 10 animals showed marked clinical signs becoming moribund, both in the low dose group, even though viremia, virus shedding in the secretions and excretions were evident in all inoculated monkeys. Anti-KFDV immunoglobulin (Ig)M antibody response was observed around a week after inoculation and anti-KFDV IgG antibody response after two weeks. Anaemia, leucopenia, thrombocytopenia, monocytosis, increase in average clotting time, and reduction in the serum protein levels were evident. The virus could be re-isolated from the skin during the viremic period. The persistence of viral RNA in the gastrointestinal tract and lymph nodes was seen up to 53 and 81 days respectively. Neuro-invasion was observed only in moribund macaques. Re-challenge with the virus after 21 days of initial inoculation in a monkey did not result in virus shedding or immune response boosting.

Growth Kinetics of Kyasanur Forest Disease Virus in Mammalian Cell Lines and Development of Plaque Reduction Neutralization Test

Kyasanur forest disease virus (KFDV) is a tick-borne flavivirus identified in 1957 in the Karnataka state of India causing fatalities in monkeys and humans. Even after the introduction of a vaccine in the endemic areas, hundreds of cases are reported every year. Being a high-risk category pathogen, the studies on this virus in India were limited till the past decade. The growth characteristics of this virus in various mammalian cell lines have not yet been studied. In this study, we have demonstrated the growth pattern of virus in BHK-21, Vero E6, Vero CCL81, rhabdomyosarcoma, porcine stable kidney, and Pipistrellus ceylonicus bat embryo cell lines, and found BHK-21 to be the best. We have developed KFDV plaque reduction neutralization test for the first time.

Zika virus Pathogenesis in Infant Mice after Natural Transmission by the Bite of Infected Mosquitoes

OBJECTIVES

The objective of this study was to understand natural disease progression in infant CD1 mice after the bite of Aedes aegypti mosquitoes infected by the Zika virus (ZIKV, MR-766 strain).

METHODS

A. aegypti mosquitoes were experimentally infected with ZIKV MR-766 strain via the oral feeding route. Infected mosquitoes were allowed to feed on infant CD1 mice. Sick mice were euthanized, and their organs were collected and subjected to real-time RT-PCR, histo-pathology, and immunohistochemistry.

RESULTS

Clinical symptoms appeared in mice after 4-5 days of being bitten by mosquitoes, following which they were euthanized. Real-time RT-PCR analysis showed the presence of viral RNA in various organs such as the brain, liver, kidney, spleen, lungs, and intestines of the mice. The brain tissue specimens showed higher viral loads as determined by threshold values (Ct value) in the real-time RT-PCR assay. Histopathological and immunohistochemistry studies also revealed the presence of the virus and associated lesions in the brain, indicating that ZIKV shows tropism for neuronal tissue.

CONCLUSIONS

This study demonstrates ZIKV pathogenesis in infant CD1 mice and that these mice are highly susceptible to natural infection with this ZIKV strain.

Development of single step RT-PCR for detection of Kyasanur forest disease virus from clinical samples

Background

Kyasanur Forest Disease (KFD), a tick borne flavivirus, which was earlier endemic to Karnataka state, India, has been confirmed and detected from neighboring states of Tamil Nadu, Maharashtra, Goa and Kerala states in India. Increased human and vector surveillance therefore becomes essential for the identification of KFD affected regions and control of further spread of the disease. Currently, available KFD detection assays include realtime RT-PCR and nested RT-PCR assays. Here we describe the development of a sensitive single step RT-PCR assay for the detection of KFD viral RNA. This can be easily used in any BSL-2 laboratory for screening of KFD suspected cases or for differential diagnosis of viral hemorrhagic fever panel.

Method

Three primer sets were designed and checked for sensitivity using known dilutions of KFD viral RNA (Ranging from 10⁶ copies to 10 copies). The primer set (2) was found to be most sensitive was selected and tested for specificity for Kyasanur forest disease virus (KFDV) by testing against zika, dengue, chikungunya, crimean congo hemorrhagic fever (CCHF), yellow fever, japanese encephalitis (JE) and west nile viruses. A total of 104 samples (human, monkey and tick positive and negative samples) were tested using this assay.

Result

No false positive or false negative results were seen for human, monkey or tick samples. The assay was specific for KFD and could detect upto 100 copies of KFD viral RNA.

Discussion and conclusion

The previously published sensitive real time RT-PCR assay requires higher cost in terms of reagents and machine setup and technical expertise has been the primary reason for development of this assay. A single step RT-PCR is relatively easy to perform and more cost effective than real time RT-PCR in smaller setups in the absence of Biosafety Level-3 facility. This study reports the development and optimization of single step RT-PCR assay which is more sensitive and less time-consuming than nested RT-PCR and cost effective for rapid diagnosis of KFD viral RNA.