Virulence variation among epidemic and non-epidemic strains of Saint Louis encephalitis virus circulating in Argentina

Saint Louis encephalitis virus caused an outbreak of febrile illness and encephalitiscases in Córdoba, Argentina, in 2005. During this outbreak, the strain CbaAr-4005 wasisolated from Culex quinquefasciatus mosquitoes. We hypothesisedthat this epidemic variant would be more virulent in a mouse model than two othernon-epidemic strains (78V-6507 and CorAn-9275) isolated under differentepidemiological conditions. To test this hypothesis, we performed a biologicalcharacterisation in a murine model, including mortality, morbidity and infectionpercentages and lethal infection indices using the three strains. Mice were separatedinto age groups (7, 10 and 21-day-old mice) and analysed after infection. The strainCbaAr-4005 was the most infective and lethal of the three variants, whereas the othertwo strains exhibited a decreasing mortality percentage with increasing animal age.The strain CbaAr-4005 produced the highest morbidity percentages and no significantdifferences among age groups were observed. The epidemic strain caused signs ofillness in all inoculated animals and showed narrower ranges from the onset ofsymptoms than the other strains. CbaAr-4005 was the most virulent for Swiss albinomice. Our results highlight the importance of performing biological characterisationsof arbovirus strains likely to be responsible for emerging or reemerging humandiseases.