Objectives of Surveillance

Arboviral surveillance consists of two distinct, but complementary activities. Epidemiological surveillance measures human disease to quantify disease burden, detect early signs of an outbreak and identify information needed for timely responses, including seasonal, geographic, and demographic patterns in human morbidity and mortality. Environmental surveillance monitors local mosquito populations, virus activity in vectors and non-human vertebrate hosts, and other relevant environmental parameters to predict human risk and prevent outbreaks of arboviral disease in humans.

In addition to monitoring disease burden and distribution, epidemiological surveillance has been instrumental in characterizing clinical disease presentation and disease outcome, as well as identifying high-risk populations and factors associated with serious disease. Epidemiological surveillance has also detected and quantified alternative routes of transmission to humans, such as contaminated blood donations and organ transplantation.

Epidemiological and environmental surveillance for arboviruses is facilitated by ArboNET, the national arbovirus surveillance system. ArboNET was developed in 2000 as a comprehensive surveillance data capture platform to monitor West Nile virus (WNV) infections in humans, mosquitoes, birds, and other animals. This comprehensive approach was essential to tracking the progression of WNV as it spread and became established across the United States, and it remains a significant source of data on the epidemiology and ecology of WNV. Since 2003, ArboNET has also collected data on other domestic and exotic arboviruses of public health significance.

In the absence of effective human vaccines for most domestic arboviruses, preventing arboviral disease in humans primarily depends on measures to keep infected vectors from biting people. A principal objective of environmental surveillance is to quantify the intensity of virus transmission in a region and provide a predictive index of human infection risk. This risk prediction, along with information about the local conditions and habitats that impact vector abundance and infection, can be used to inform an integrated vector management program and decisions about implementing interventions to control mosquitoes and prevent disease.

Though epidemiological surveillance is essential for understanding arboviral disease burden, utilizing human case surveillance by itself is insufficient for predicting outbreaks. Outbreaks can develop quickly, with most human cases occurring over a few weeks during the peak of transmission. The time from human infection to onset of symptoms to diagnosis and reporting can be several weeks or longer. As a result, human case reports typically lag well behind the transmission from vectors that initiated the infection. By monitoring infection prevalence in vectors and incidence in non-human vertebrate hosts and comparing these indices to historical environmental and epidemiological surveillance data, conditions associated with increasing human risk can be detected 2-4 weeks in advance of human disease onset. This provides additional lead time for critical vector control interventions and public education programs to be put in place. The following sections describe the elements of epidemiological and environmental arboviral surveillance and how they may be used to monitor and predict risk and to trigger interventions.