Dengue outbreak overlapped COVID-19 crisis in Key Largo, Florida in 2020: Call for enhanced disease testing protocols

By Pooja Toshniwal PahariaReviewed by Benedette Cuffari, M.Sc.Jul 5 2023

In a recent study published in Emerging Infectious Diseases, researchers reported a dengue epidemic among Key Largo residents between February and August 2020. This outbreak coincided with the early months of the coronavirus disease 2019 (COVID-19), which affected the dengue epidemic's investigations and the need to raise physician understanding of dengue virus (DENV) testing protocols.

Study: Florida Department of Health Dengue Investigation Team. Dengue outbreak response during COVID-19 pandemic, Key Largo, Florida, USA, 2020. Image Credit: anatoliy_gleb / Shutterstock.com

Background

Dengue is an arboviral illness caused by DENV1 to 4, the causative organism of which is the Aedes aegypti mosquito. Florida, where DENV has been endemic since 1935, annually experiences at least one regionally transmitted DENV infection. Thus, surveillance is essential to identify viral introduction, as the state is susceptible to the establishment of Aedes Aegypti-vectored viral pathogens such as the Zika virus, chikungunya virus, and DENV.

Health authorities informed the Florida Department of Health (FDOH) of a probable locally transmitted case of DENV infection in Key Largo in February 2020. On March 9, the FDOH published a countywide population health mosquito-borne disease advisory in Monroe County to increase the availability of non-essential services and businesses. Additional regionally transmitted dengue cases were not detected till June 16, after which numerous Florida residents reported possible dengue illness to the FDOH.

About the study

In the present study, researchers describe the response to an outbreak of dengue in Florida in early 2020, during which determining the source of other febrile diseases was difficult, and individuals were hesitant to seek healthcare.

This study was part of standard public health outbreak surveillance and response involving the Florida Keys Mosquito Control District (FKMCD) and the United States Centers for Disease Control and Prevention (CDC).

During the two-week incubation period through the prospective one-week viremic phase following the onset of symptoms, FDOH informed FKMCD of possible locations of Aedes Aegypti mosquito exposure for probable cases. The FKMCD increased vehicular and aerial spraying, surveyed neighborhoods, performed vector monitoring, eliminated or treated mosquito larvae sites, and educated residents regarding mosquito control. 

FDOH answered helpline calls from citizens suffering from dengue-like sickness, questioned probable case patients, inspected sites, offered regular medical care and outreach to the community, obtained sera for DENV testing, and timely updated the local press and FKMCD. Individuals with probable dengue fever were requested to submit contact details for additional individuals who might be susceptible to mosquito bites, such as family members and others at outdoor gatherings and workplaces. If the person indicated a recent inexplicable febrile sickness, the FDOH tested for DENV.

In addition, the FDOH performed syndromic monitoring on the primary complaint and discharge diagnostic records at nearby hospitals containing fever or dengue, as well as any possible combination of arthralgia, rash, or thrombocytopenia. If no other diagnosis was previously established and no dengue testing was requested previously, FDOH offered to examine individuals who were suspected of DENV infection.

Following CDC recommendations, FDOH used immunoglobulin M (IgM) testing, and DENV reverse transcription-polymerase chain reaction (RT-PCR) on acute samples obtained within a week of symptomatic onset. Convalescent sera were tested for antibodies a week after the onset of symptoms. The CDC helped with serological verification, serotyping RT-PCR-confirmed DENV-positive specimens, and performing RT-PCR tests for the FKMCD mosquito populations.

Study findings

In Key Largo, 72 locally transmitted cases of DENV infection were identified, primarily among non-Hispanics and females at 83% and 51%, respectively. About 61% of the initial cases were self-reported, followed by laboratory documentation and syndromic monitoring at 22% and 7%, respectively.

In total, 31 individuals tested positive for RT-PCR, and 41 tested positive for IgM. DENV-1 was identified among all individuals with RT-PCR-confirmed DENV infections. In January, igM-positive individuals with symptomatic onset were detected through retrospective case identification and testing.

Sequence information from 96 Aedes aegypti mosquito populations obtained between June 18 and September 21 was obtained through a phylogenetic study of the DENV-1 subtype V clustering from the Caribbean strain. Sequencing results were uploaded to GenBank, revealing clear distinctions between the Caribbean DENV strain and the Central African strain detected in Key West between 2009 and 2010.

During the acute phase, 60% of DEBV-infected individuals visited healthcare facilities one week after the onset of symptoms. COVID-19 was evaluated as a viable diagnosis by providers, including 88% of acute DENV infections receiving COVID-19 testing orders.

Dengue was deemed a possible diagnosis in just 16 acute instances, with 75% of patients requiring testing for anti-DENV antibodies. The DENV non-structural protein-type 1 (NS-1) assay was not performed on any acute samples, 26 of which were subjected to IgM and RT-PCR tests in the CDC or FDOH reference laboratories.

Conclusions

Between January and August 2020, Key Largo witnessed a continuous dengue epidemic, with 1,692 COVID-19 cases documented in only Monroe County. According to the CDC, COVID-19 impacted DENV monitoring due to a lack of willingness to visit healthcare facilities, conflicting demands upon medical professionals, and comparable clinical manifestations between dengue and COVID-19.

COVID-19 tests were most commonly prescribed by clinicians for patients seeking treatment for acute DENV infections. For samples collected during the acute stage of dengue fever, the CDC advocates the use of commercial DENV NS1 or RT-PCR tests in conjunction with serological testing. Improving CDC guidelines awareness among physicians could enhance case identification in non-endemic regions where DENV is more likely to spread.