December 01, 2007
2 min read

Zika virus outbreak affects 43 people on Yap Island in Micronesia

The recent outbreak is the largest on record for Zika virus infection in humans.

You've successfully added to your alerts. You will receive an email when new content is published.

Click Here to Manage Email Alerts

We were unable to process your request. Please try again later. If you continue to have this issue please contact

PHILADELPHIA – Forty-three patients developed Zika virus infection on Yap Island in the Federated States of Micronesia between April and July of this year. The 43 patients represent the largest-ever documented outbreak of Zika virus in humans.

The Yap Island incident is the first ever documented outbreak of Zika virus in humans. “Zika virus was first isolated from a febrile rhesus monkey in the Zika forest near Entebbe, Uganda,” Mark Duffy, MD, epidemic intelligence service officer for the CDC, told Infectious Disease News. “Since that time, serologic evidence of previous Zika virus infection has been found in people in Africa and Asia, including the Philippine Islands. However, fewer than 15 human clinical cases have been described in the literature. Zika virus had not been previously recognized in Micronesia.”

A report about the Zika virus outbreak on Yap Island and its effect on patients was presented by Duffy and his colleagues at the 56th Annual Meeting of the American Society of Tropical Medicine and Hygiene, held here in November.


Zika virus is a mosquito-borne flavivirus. Symptoms of Zika virus infection include acute onset of generalized macular or papular rash, arthritis or arthralgia, and/or nonpurulent conjunctivitis. “Many patients also had a low-grade fever,” Duffy said. “Overall, the disease was self-limited and the symptoms typically resolved in about five days.”

The presence of Zika virus in symptomatic patients was confirmed by plaque reduction neutralization testing and enzyme-linked immunosorbent assay or by RNA polymerase chain reaction tests. When patients could be located for interview, demographics and clinical signs and symptoms were documented.

All cases of Zika virus infection in the Federated States of Micronesia occurred on Yap Island. The population of Yap Island is only 7,391. Thus, the attack rate among residents was about 5.8 per 100 residents. The median age of infected patients was 34 years; patients ranged in age from newborns to 76 years.

According to Duffy, symptomatic illness was mild but widespread and occurred primarily among adults. No deaths or hospitalizations were attributed to Zika virus infection.

Spread of Zika virus

More research is still needed to fully understand how the Zika virus outbreak in Micronesia may have begun. “Zika virus could have been present and unrecognized on Yap for some time,” Duffy said. “If this outbreak was the result of a recent introduction, the virus most likely was introduced either by a traveler to Yap Island who carried the virus in his or her bloodstream and was then bitten by a mosquito on Yap Island, thus starting the outbreak. Or, a mosquito carrying the virus could have been transported to Yap Island.”

At this point, the Zika virus outbreak has not spread beyond Yap Island. But public health officials continue to monitor the outbreak and experts said the virus could potentially be transported to other regions.

Duffy said preventive measures should be taken if another outbreak of Zika virus is reported. “The most effective prevention recommendations will be tailored to the biology and behavior of the vector mosquito species, he said. “In general, to minimize mosquito exposure, people should use mosquito repellent, wear long sleeve shirts and pants and avoid outdoor activities in morning and evening hours when some mosquito species are more active. Proper screening of sleeping quarters and sleeping under mosquito netting may help prevent mosquito-borne infections. In addition, cleaning up refuse and other containers that hold water will decrease the density of mosquito species requiring water-filled containers to breed.” – by Jay Lewis

For more information:
  • Duffy M, Chen T, Hancock WT, et al. Zika virus outbreak on Yap Island, Federated States of Micronesia, April-July 2007. Presented at: The 56th Annual Meeting of the American Society of Tropical Medicine and Hygiene; Nov. 4-8, 2007; Philadelphia.