When nothing happens: could Theileria parva-like endemic stability explain Rift Valley fever's cryptic persistence during the inter-epidemic periods?

The epidemiological pattern of Rift Valley fever (RVF) is marked by explosive epidemics followed by prolonged inter-epidemic periods (IEPs), during which the virus circulates at low levels without apparent outbreaks. This technical note proposes a conceptual framework comparing RVF maintenance during IEPs to Theileria parva endemic stability in cattle. Despite differences in pathogen and vector biology, both diseases display similar dynamics that sustain a balanced host-pathogen relationship. We identify three key features supporting this comparison: continuous low-level exposure through persistent vector activity and vertical transmission in mosquitoes; low disease incidence due to early-life exposure and immunity; and stable population immunity maintained through ongoing virus circulation. Serological evidence of coexisting IgG and IgM antibodies in clinically healthy animals during IEPs-pooled prevalences of 7.8% and 9.3%, respectively-supports this hypothesis. Like T. parva, RVF "endemic stability" varies geographically and may be disrupted by insecticides, demographic shifts, or climatic factors affecting vector populations. The herd immunity threshold (HIT) offers a quantitative lens to assess the proportion of immune individuals required to prevent transmission. Applying HIT principles to RVF may guide context-specific surveillance and vaccination strategies that sustain population immunity, mitigate outbreak risk, and enhance our understanding of RVF virus persistence during IEPs.