Sunday, March 27, 2016

Yellow Fever In Angola & The Risk Of International Spread - ECDC

# 11,201

After watching the incidence of Dengue explode more than 50-fold around the globe since the early 1970s, the unexpected arrival and spread of WNV in North America beginning in the late 1990s, and the sudden and rapid diaspora of both Chikungunya and Zika from Africa over the past dozen years, one can't help but wonder what arbovirus will next threaten to bolt onto the global stage.

While there are a number of candidates (Usutu Virus, Rift Valley Fever,  Sindbis Virus (SINV), Japanese Ecephalitis, etc.), one that has seen considerable success in the past is Yellow Fever. 

While endemic in Africa's tropical rain forests, this year Angola is seeing their first urban epidemic in decades (see WHO Angola grapples with worst yellow fever outbreak in 30 years). And because of our increasingly global society, exported cases have already shown up in China, Kenya, Mauritania, and the DRC.

Incubation is typically 3 to 6 days, and infected individuals become their most viremic just before, and during the first few days after onset of fever (cite CDC's Yellow Book) , and then are most capable of spreading the virus to others via shared mosquito bites.

On Friday the ECDC released a Rapid Risk Assessment on Angola's Yellow Fever Outbreak, which cites at least 1,132 suspected and confirmed cases and 168 deaths, and which included the following statement on the risk of international spread:

Risk of international spread
The evolution of the situation in Angola is of concern. The number of suspected cases may be underestimated as surveillance is reportedly not optimal, especially in areas along the border with adjacent countries.

Viraemic patients travelling to areas where suitable vectors and susceptible human populations are present pose a risk for establishment of local transmission. Such areas exist in most of the inter-tropical zones of Africa, the Americas and Asia. Yellow fever cases have been reported recently in travellers from Angola to China, Kenya, Mauritania and the Democratic Republic of Congo. Therefore, the risk of international spread within Africa and beyond is currently significant.

As yellow fever and dengue fever share the same mosquito vector, Aedes aegypti, any area where dengue has been transmitted could be suitable for establishment of local transmission of yellow fever if the virus is introduced by a viraemic traveller. This could be the case in southern China, where dengue virus transmission occurs during the warmer mosquito vector season, leading to local outbreaks in these areas.

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A 2010 Eurosurveillance Journal devoted an entire issue to The Threat Of Vector Borne Diseases, including making the case for the reintroduction of  Yellow fever and dengue: a threat to Europe? by P Reiter.

While in 2013, Peter Hotez - Dean of the National School of Tropical Medicine at Baylor College of Medicine - along with Kristy Murray, an associate professor of pediatrics at Baylor, posed a similar question in Could Yellow Fever Return to the United States?

In both cases, the answer is `yes'.  It's possible.

As we've seen with West Nile Virus's debut in New York City in 1999, Chikungunya's arrival to the French Part of St. Martins in late 2013, and the introduction of Zika to Brazil in 2013-2014, it may only take one viremic traveler arriving to a region with a competent vector and where the right conditions exist, to start an epidemic.

Sometimes, as with Dengue in Key West in 2010, we get lucky.  While a couple of dozen local cases were detected (see MMWR: Dengue Fever In Key West), that outbreak was stopped by good mosquito control.

But in Florida alone, since the start of 2016 (cite Florida Arbovirus Surveillance Week 11: March 13-19, 2016), the State Department of Health has identified 17 travel-associated cases of Dengue, 2 travel-associated cases of Chikungunya, and 71 travel-associated cases of Zika. 

Although no local transmission of these three arboviruses have been reported 2016, mosquito season is just around the corner, and the number of viremic visitors will only increase over time.

Whether it's Yellow Fever, Usutu, Rift Valley Fever, or some other obscure virus, bacteria, or parasite - given our increasingly mobile society - the one thing you can pretty much count on is that Zika won't be the last exotic tropical disease to emerge on the world stage in the years to come.