The Revenge of the Aedes

With almost 1400 cases and 250 deaths, Brazil is currently experiencing the largest outbreak of yellow fever since the 1940s. Here, we look at the potential risks of the further expansion and development of this outbreak, given the changing global health landscape of the moment.

In December 2016, in the Brazilian state of Minas Gerais, a stretch of forest grew eerily silent as howler monkeys started dying of yellow fever. While we in the US typically consider yellow fever as one of those ‘strange’ diseases that we only read about in history books, it is well and alive in tropical areas of the world, maintained in a sylvatic cycle between forest-dwelling primates and the (Aedes or Haemagogus) mosquitoes that bite them. Occasionally, infected mosquitoes bite humans who venture into these forests, typically in the Amazon, either as part of their profession or as tourists, and we do have very effective vaccines to protect against such exposures. However, this particular outbreak started hundreds of miles to the south from the Amazon, and did not seem to stop.

By January 24, 2017, 404 cases were reported in Minas Gerais, of which 84 died, as well as cases in Espirito Santo and Sao Paulo. In addition, Bahia state reported 6 yellow fever human cases. The state of Espirito Santo had its first locally transmitted case since 1940. Additionally, a large number of epizootics were reported in the States of Sao Paulo and Espirito Santo. The Brazilian Ministry of Health has distributed 4.2 million doses of yellow fever vaccine across these states, and initiated vector control activities. Unfortunately, the number of yellow fever cases as well as the area of the outbreak has expanded during both February and March. As of March 6, 2017, Brazil has reported 1337 yellow fever cases, including 233 deaths, in seven states, including Rio Grande do Norte. At this point, the outbreak is the largest in Brazil since the 1940s, when vaccination campaigns were started. The Ministry of Health has expanded the mass vaccination campaign, distributing 14.85 million extra doses of yellow fever vaccine.

The biggest concern with this outbreak is the possibility of its extension into urban areas, where the abundant Aedes aegypti could transmit it rapidly in a high density population. In particular at risk are large urban cities such as Sao Paulo, Brazil, as well as Rio de Janeiro. Two cases of yellow fever have been confirmed in rural areas of Rio de Janeiro state on March 16, but there hasn’t been any evidence of Aedes aegypti related transmission yet. In order to avoid such an urban outbreak, the state is planning to vaccinate its entire population, requiring 12 million doses.

In the meantime, yellow fever cases have also been reported in other countries in South America, namely in Colombia, Ecuador, Peru, the Plurinational State of Bolivia, as well as Suriname. These seem to be sporadic cases, so far, unrelated to the ongoing outbreak in Brazil, perhaps indicating favorable ecological conditions for yellow fever transmission over large areas. However, there is a concern of the spread of yellow fever across state borders as epizootics have been reported in States of Brazil (such as Mato Grosso de Sul, Santa Catarina, Rio Grande do Sul, Rondonia, Para and Parana) that border other countries. In addition, if the outbreak proceeds uncontrolled, the risk of international spread will increase as well. The US CDC has elevated the yellow fever status in Brazil to Level 2 (Alert: Practice Enhanced Precautions), and suggests that anyone 9 month or older should receive a yellow fever vaccine if travelling to the affected areas within Brazil. Because of the shortage of the yellow fever vaccine, this might be difficult on short notice. In addition, they suggest preventing mosquito bites, including permethrin-treated clothing.   The European CDC similarly suggest that travelers should be vaccinated against yellow fever, and is monitoring the outbreak. Already, several EU travelers returned from South America after getting infected with yellow fever. While the risk of onward transmission in Europe currently is low, Aedes aegypti is present on the island of Madiera, and Aedes albopictus, another potential vector, is active in large parts of Southern Europe.

The current outbreak comes at a particularly dangerous time for global health. The US CDC is poised to lose significant amounts of funding both through President Trump’s proposed budget allocations as well through the repeal and replacement of the Affordable Care Act. In addition, the President’s budget proposes a number of cuts that would severely limit or eliminate several programs that currently provide international aid, training and support to strengthen disease control efforts in other countries, such as USAID and the Fogarty International Center at NIH. The changing global health landscape, with severely diminished role of the United States, as well as the United Kingdom, risks letting disease outbreaks develop uncontrolled in remote locations until they become difficult and costly to mitigate (such as the Ebola outbreak in West Africa in 2014). The decreased role and funding of the US CDC then further limits the ability to detect imported cases and local transmission when outbreaks are small and localized. While yellow fever is certainly visible due its high mortality rate, cases can still be mild and even asymptomatic. Let’s hope that the Brazilian authorities will bring this yellow fever outbreak swiftly under control, and it does not become a prime example of the consequences of diverting funds from disease surveillance.

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