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A Comparative Study of the Infection Thresholds of Thogoto Virus in Rhipicephalus appendiculatus and Amblyomma variegatum

Infection thresholds of Thogoto virus in 2 ixodid tick species, Rhipicephalus appendiculatus and Amblyomma variegatum, were compared. Thogoto virus has been isolated from naturally infected R. appendiculatus and A. variegatum in Central Africa, where both commonly parasitize the same hosts. No significant difference was found between the infection thresholds of Thogoto virus in the 2 species. The percentage of nymphs of both species infected by feeding on viremic hamsters was directly correlated with the time between host inoculation and tick engorgement. The blood titers in infected hamsters increased each day during the 3–4 day viremia until the hamsters died. The percentage of nymphs infected correlated with the viremic titer on the final day of engorgement (the time of maximum blood uptake). The 5% infection threshold for nymphs of R. appendiculatus and A. variegatum was estimated as 10^2.8 and 10^2.7 plaque forming units (PFU)/ml viremia, respectively. The prevalence of infection approached 100% for blood titers > 10^6.3 PFU/ml and > 10⁷ PFU/ml, respectively. The apparent bloodmeal size of the 2 species differed by 8-fold and suggested that, in terms of the number of PFU ingested, R. appendiculatus was more susceptible than A. variegatum to per os infection by Thogoto virus.