Temporal Variability and Seasonality in Infectious Disease
With any "uptick" in pathogen results, always consider “temporal variability”- in particular, seasonal changes and how that impacts human exposure to microbial organisms- such as changes in weather, temperature, soil and water exposures, and the impact on vector-borne illnesses with diseases that are transmitted to humans through the bite or sting of infected mosquitoes, ticks, fleas, and flies etc.
Powassan Virus- Geographical Considerations, Seasonal Trends, and Tick Lifecycle
Powassan virus cases in the United States occur primarily from late spring through mid-fall, coinciding with peak tick activity in the Northeast and Great Lakes regions. The specific timing of risk depends on the life stage of the ticks carrying the virus, although infected ticks can be active any time temperatures are above freezing.
- The seasonal distribution of Powassan virus reflects the activity cycles of its tick vectors, especially the blacklegged tick (Ixodes scapularis)
- Peak risk: Most cases are reported from late spring through mid-fall, typically between May and October. This period includes two major tick-activity peaks:
◦ A major peak from March/April through August, corresponding to the active nymph (immature tick) population.
◦ A secondary peak in October and November from adult ticks. - Year-round potential: While the risk is highest in warmer months, ticks can be active whenever temperatures are above freezing, meaning human infection is possible during any season, including winter. Cases reported outside of the typical season, while rare, highlight the need for year-round vigilance.
Powassan Virus- Rising Case Numbers, Yet Underreporting
Powassan virus reported infections has increased significantly in recent years. Some researchers attribute this rise to increased surveillance, improved diagnostic testing, climate change, and more frequent human contact with tick habitats. However, underreporting of Powassan virus may still occur for several reasons, according to the CDC:
- Under-reporting is common in surveillance systems since they rely on healthcare providers to suspect the disease as a possible diagnosis, order the correct lab test, and report confirmed cases to public health authorities.
- Mild illness (non-neuroinvasive cases) is more likely to be underreported than severe illness (neuroinvasive cases). The extent of underreporting depends on disease awareness and healthcare-seeking behavior in each area. Data on non-neuroinvasive disease should not be used for comparing disease activity across regions or time periods.
- Surveillance data are recorded by the patient’s county of residence, not where the exposure actually occurred.
- Non-human surveillance (e.g., animals, vectors) varies across the country. No reported activity to CDC does not necessarily mean there is no risk.
- There is often a time lag in reporting cases to CDC. States and territories may publish their surveillance data on different schedules than CDC.
Vibrant Tickborne Panel- Methodology, Sensitivity and Specificity, and Reporting of Powassan Virus
- Vibrant’s methodology and high sensitivity and specificity may allow for earlier detection and lower concentration of Powassan virus antibodies.
- Vibrant reports all infectious disease results to the appropriate state, federal, and regulatory agencies, in compliance with applicable public health reporting requirements.