Human Surveillance

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Passive Surveillance and Case Investigation

Arboviral diseases are nationally notifiable conditions and many are explicitly reportable U.S. states and territories. Most disease cases are reported to public health authorities from public health or commercial laboratories; healthcare providers can also directly submit reports of suspected cases. State and local health departments are responsible for ensuring that reported human disease cases meet the national case definitions. The most recent case definitions for arboviral diseases can be found on the CDC Nationally Notifiable Disease Surveillance System website ( For some arboviruses (e.g., West Nile and Zika viruses) presumptive viremic donors are identified through universal screening of the blood supply; case definitions and reporting practices for viremic donors vary by jurisdiction and blood services agency.

All identified human disease cases and presumptive viremic blood donors should be investigated promptly. Jurisdictions may choose to interview the patient’s healthcare provider, the patient, or both depending on information needs and resources. Whenever possible, the following information should be gathered:

  • Basic demographic information (age, sex, race/ethnicity, state and county of residence)
  • Clinical syndrome (e.g., asymptomatic blood donor, uncomplicated fever, meningitis, encephalitis, acute flaccid paralysis)
  • Illness onset date and/or date of blood donation
  • If the patient was hospitalized and if he/she survived or died
  • Travel history in the four weeks prior to onset
  • If the patient was an organ donor or a transplant recipient in the 4 weeks prior to onset
  • If the patient was a blood donor or blood transfusion recipient in the 4 weeks prior to onset
  • If the patient was pregnant at illness onset
  • If the patient is an infant, was he/she breastfed before illness onset

If the patient donated blood, tissues or organs in the 4 weeks prior to illness onset, immediately inform the blood or tissue bank and appropriate public health authorities. Similarly, any infections temporally associated with blood transfusion or organ transplantation should be reported. Prompt reporting of these cases will facilitate the identification and quarantine of any remaining infected products and the identification of any other exposed recipients so they may be managed appropriately.

Passive surveillance systems are dependent on clinicians considering the diagnosis of an arboviral disease and obtaining the appropriate diagnostic test and reporting of laboratory-confirmed cases to public health authorities. Because of incomplete diagnosis and reporting, the incidence of arboviral diseases is underestimated. Where applicable, reported neuroinvasive disease cases are considered the most accurate indicator of activity in humans because of the substantial associated morbidity. In contrast, reported cases of non-neuroinvasive disease are more likely to be affected by disease awareness and healthcare-seeking behavior in different communities and by the availability and specificity of laboratory tests performed.

Enhanced Surveillance Activities

Enhanced surveillance for human arboviral disease cases should be considered when environmental or human surveillance suggests that an outbreak is suspected or anticipated. Educating healthcare providers and infection control nurses about the need for arbovirus testing and reporting of all suspected cases could increase the sensitivity of the surveillance system. This might be accomplished by distributing print materials, participating in local hospital meetings and grand rounds, and providing lectures/seminars. Public health agencies should also work to establish guidelines and protocols with local blood collection agencies for reporting viremic blood donors. At the end of the year, an active review of medical records and laboratory results from local hospitals and associated commercial laboratories should be conducted to identify any previously unreported cases. In addition, an active review of appropriate records from blood collection agencies could be conducted to identify any positive donors that were not reported.