The Americas set a gloomy record in 2019: the most dengue cases ever reported. More than 2.7 million cases of the mosquito-borne disease have struck the region, largely in Brazil, the Pan American Health Organization reported on November 13.

Dengue is one of the top 10
threats to global health, according to the World Health Organization, with
cases of the viral disease climbing rapidly around the world in recent decades.
An estimated 390 million dengue infections occur each year, which can be mild
or cause flulike symptoms and headaches. Less commonly, dengue can lead to a severe,
life-threatening illness. South Asian countries including Pakistan, Bangladesh and
Nepal have
also been slammed with large dengue outbreaks this year (SN: 10/7/19).

The last record-breaking
year for the Americas was 2015, when there were more than 2.4 million cases.
After that, cases dropped slightly in 2016 and then precipitously in 2017 and
2018, coming in below 600,000 each of those years. “Dengue is endemic in the
Americas, with cycles of the epidemics that are repeated every three to five years,”
says Jose Luis San Martin, an advisor on dengue and other mosquito-borne
diseases for PAHO in Washington D.C. “During those two years there was an
accumulation of a large number of people susceptible to the disease.”

That may seem like too short
a time for so many people to be vulnerable again to dengue, transmitted by the
bite of an Aedes aegypti mosquito. But dengue outbreaks are complicated by the fact
that there are four different types of the virus. Infection with one type provokes
the development of antibodies that provide lifelong immunity to that type. Those
antibodies can initially protect against other types, but that effect is
temporary, lasting roughly one to three years.

All four types of dengue are
regularly found in the Americas, and all four have been circulating this year
in Brazil, Mexico and Guatemala. Brazil has had the most cases in the Americas
so far this year, with more than 2 million, followed by Mexico, where there are
close to 214,000 cases.

A population’s susceptibility
to dengue depends upon its past history of epidemics — which dengue viruses were circulating, and how
intense the epidemics were. And cases don’t always hit the same places in a region
with each epidemic, so the herd immunity —
the threshold at which enough people are exposed that others who aren’t are
still protected — varies depending on the area.

The Latin American Zika outbreak of 2015 to 2016 (SN:
10/30/17) may have impacted the region’s dengue cases in recent years, says
Albert Ko, an infectious disease physician and epidemiologist at the Yale
School of Public Health. In February, Ko and his colleagues reported that
having dengue first may have protected some people from Zika in the northeast Brazilian city of Salvador (SN: 2/7/19). Perhaps that works the
other way around as well, Ko says, noting that Salvador experienced a large decrease
in cases of dengue after the Zika epidemic, which hit northeastern Brazil
especially hard. It’s possible that the Zika epidemic “gave some protection,
and now that’s wearing off.”

Ko has had dengue three
times. “It’s not a pleasant thing,” he says. “It can be pretty debilitating for
a period.” He had bad headaches and felt “washed out” afterwards.

Having all four types of
dengue circulating can also lead to severe cases. If a person is infected with
dengue type 1, for example, a subsequent infection with any other type can be much
worse, because of a phenomenon called antibody dependent enhancement (SN: 11/8/17).
There have been almost twice as many severe cases of dengue in the Americas so
far in 2019 compared with 2015; 22,127 rather than 12,495. But there have also been
fewer deaths, PAHO reports: 1,206 as of November 13, compared with 1,355 for
all of 2015.