Epidemiology of Chikungunya in the Americas
Epidemiology of Chikungunya in the Americas
Sergio Yactayo 2
J. Erin Staples 1
Véronique Millot 2
Laurence Cibrelus 2
Pilar Ramon-Pardo 0
0 Department of International Health Regulations, Pan American Health Organization/World Health Organization , Washington D.C
1 Division of Vector-Borne Diseases, National Center for Emerging and Zoonotic Infectious Diseases, Centers for Disease Control and Prevention , Fort Collins, Colorado
2 Department of Pandemic and Epidemic Diseases, World Health Organization , Geneva , Switzerland
Chikungunya virus (CHIKV) emerged in the Americas in late 2013 to cause substantial acute and chronic morbidity. About 1.1 million cases of chikungunya were reported within a year, including severe cases and deaths. The burden of chikungunya is unclear owing to inadequate disease surveillance and underdiagnosis. Virus evolution, globalization, and climate change may further CHIKV spread. No approved vaccine or antiviral therapeutics exist. Early detection and appropriate management could reduce the burden of severe atypical and chronic arthritic disease. Improved surveillance and risk assessment are needed to mitigate the impact of chikungunya.
Chikungunya virus (CHIKV) is an arthropod-borne alphavirus
primarily transmitted by Aedes mosquitoes, which are endemic
in tropical areas of Africa and Asia. It is also capable of causing
explosive outbreaks of human disease in areas with no prior
immunity, such as those in Europe and the Americas. Studies of
prior outbreaks have determined that 10%–70% of persons in
an affected area become infected [1, 2]. This potentially high
infection rate is coupled with a high symptomatic attack rate
among infected people, with 50%–97% developing clinical
disease with fever and polyarthralgia [2–4], and yields large
outbreaks of disease that often tax existing healthcare systems
and public health infrastructure.
Serosurveys conducted during postepidemic phases detected
immunoglobulin M/immunoglobulin G in 38.2% of surveyed
populations in the Indian Ocean island of La Reunion ,
75% in the Kenyan island of Lamu, 63% in Grande Comoro
Island , and 90.4% in Suriname . These findings suggest
that the CHIKV was broadly transmitted during the outbreak.
A total of 3%–22% of infected patients are asymptomatic or
pauci-symptomatic [2, 4].
The recent expansion of CHIKV in the Americas highlights
several of the features of this reemerging virus. It spreads
rapidly, affecting populated areas with no prior immunity. Areas with
outbreaks of other arboviruses, such as dengue virus and yellow
fever virus, are at substantial risk of large CHIKV outbreaks
since these viruses share similar ecology and vector
transmission cycles with CHIKV. Although chikungunya is often
described as a self-limited illness, rare but serious disease and
death associated with CHIKV have been observed. In La
Reunion, 51% of infected children had a global
neurodevelopmental delay, compared with 15% of uninfected children .
Nevertheless the neurotropism of chikungunya has not been
completely defined, and animal studies show some
inconsistencies in the capacity for CHIKV to invade the brain parenchyma
. In addition, outbreaks from other regions, such as La
Reunion, highlighted that CHIKV is capable of undergoing
mutation that increases its infectivity for Aedes albopictus, which
allows the virus to spread into areas where there is minimal
population immunity. Nevertheless, there is no evidence that
human clinical presentation in the Americas differs from that
in La Reunion .
CHIKUNGUNYA IN THE PAST
Before December 2013, local CHIKV transmission had not
been identified in the Americas. The potential first reports of
a chikungunya-like illness were recorded in 1823, in Zanzibar,
where it was called kidinga pepo, a Swahili term meaning a
disease characterized by a sudden cramp-like seizure. However, it
was only years later that an epidemic of this disease was
described in Zanzibar by Christie, in an 1872 issue of the British
Medical Journal . In 1827 and 1828, a chikungunya-like
disease was described in St. Thomas Island in the Caribbean and
later affected New Orleans, Louisiana, and part of South
Carolina in the United States .
In 1928, Dumaresq, who observed a “denga” epidemic in
New Orleans, explained how the chikungunya-like disease
was imported into Havana, Cuba, by slaves coming from Africa.
As recently recalled by Halstead, Dumaresq provided a precise
clinical description of the disease as “[a] person on the
disappearance of this fever would attempt to rise from bed,
feeling not much loss of strength, and a consciousness of being able
to move about and attend to a little to business; but how
egregiously would he be mistaken when he assumed the upright
posture! The joints felt as if fettered or anchylosed, and the
advance of one foot or leg beyond the other, would cost more pain
and effort than the purpose for which it may have been
advanced was worth, —aye,—a thousand times told!” [9, 10].
The word chikungunya appeared for the first time in
Tanzania in the 20th century, where in Makonde language it means
“to walk bent over” and refers to the stooped posture of patients
experiencing the joint pains that characterize this dengue-like
infection . Following the first identification of CHIKV in
Tanzania, in 1952, the virus was then identified to cause
sporadic cases of disease and localized outbreaks in Africa and parts
of Asia. In 2004, the virus seemed to reemerge, initially in
Kenya, before spreading through countries in and around the
Indian Ocean, resulting in millions of disease cases and the first
description of severe disease and death related to this virus.
Age-related clinical presentation was reported in Suriname .
These large-scale outbreaks eventually led to the introduction
of CHIKV into more-temperate locations, with Italy reporting a
local outbreak of CHIKV infection in 2007. Aggressive vector
control activities combined with cooler temperatures eventually
resulted in cessation of CHIKV transmission in Italy. The
largescale outbreaks also led to an increase of travel-related cases in
the Americas. From 1995 to 2005, only 3 cases of chikungunya
were identified in US travelers. From 2006 through 2013, 28
cases of CHIKV infection per year were reported among
travelers in the United States, with most of the infections acquired in
areas of Asia where there were large-scale outbreaks. Between
2013 and 2015, at least 3467 imported cases were reported in
the United States .
CURRENT CHIKUNGUNYA SITUATION IN THE
The emergence (or, most likely, reemergence) of CHIKV in the
Americas was announced in December 2013, when the French
National Reference Center for Arboviruses diagnosed the first
local chikungunya cases in Saint Martin [9–11]. At that time,
this area was facing a concomitant dengue virus outbreak and
the first chikungunya cases were initially identified as
denguelike fever. Chikungunya and dengue have a similar clinical
presentation, which makes the clinical diagnosis difficult. Patients
with either disease can present with fever, myalgia, headache,
arthralgia, and rash. The different symptoms’ frequency and
description of the clinical presentation may help, especially when
several cases occur in identical locations and where scarce
laboratory capacities limit diagnostic options .
Since it was first reported in Saint Martin, chikungunya has
spread to 45 countries and territories in North, Central, and
South America (Figure 1), causing >2.9 million suspected and
confirmed cases and 296 deaths as of late July 2016 . As
in many other vector-borne diseases, outbreaks were mainly
reported during the rainy season, when the density of mosquitoes
is maximal. High disease attack rates were reported in areas
such as in the Dominican Republic, (41%) or Suriname
(90.4%), and the transmission peak was reached within 3
months [13, 14]. Although comparable to the attack rates
noted in Malaysia (55.6%) and India (37.5%), these are higher
than the attack rate observed in La Reunion (16.5%) [1, 2, 4].
Differences in attack rates are expected and may be explained
by factors such as surveillance practices; season of CHIKV
introduction into a country or a region, which has a direct effect
on vector density and activity; vector control measures; and
lifestyle differences (eg, use of air conditioning; Table 1).
These factors may result in underestimation or overestimation
of attack rates during an epidemic.
The illness caused by CHIKV is typically not fatal, with <1%
of individuals with chikungunya in Colombia and Venezuela
dying . However, the real proportion of CHIKV-associated
deaths is unknown and remains under investigation. The disease
has also been associated with significant acute and long-term
morbidity. According to a recent model based on the number
of patients with acute chikungunya reported in 2014 in the
Americas (n = 855 890 ), an estimated 385 835–429 058
persons (48%; 95% confidence interval, 45%–50%) will develop
postchikungunya chronic inflammatory rheumatism in a
median time of 20 months . Also, the economic cost of the
disease, including lost wages, in the Americas is expected to be
overwhelming. A study in Venezuela has estimated the cost of
chikungunya in that country to be >1 billion dollars .
It is currently unclear to what degree CHIKV will persist and
continue to circulate in the Americas. Most of the disease that
has been reported to date was likely a result of
human-to-mosquito transmission. An enzootic cycle can be established among
the nonhuman primates (NHPs) in Africa and the Americas,
depending on a species’ susceptibility to the development of
chikungunya viremia. However, no important clinical
manifestations of the disease have been observed in NHPs so far,
compared with yellow fever [25, 26]. Studies are necessary to assess
the role of NHPs but also of other potential reservoirs (such as
rodents, birds, or other vertebrates yet to be identified) in
maintaining CHIKV in the environment.
CLINICAL CASE DEFINITION
Newer chikungunya case definitions were recently proposed for
the Americas. The definitions include 4 categories of cases: (1)
acute clinical cases, characterized by fever (temperature, >38.5°C
[101.3°F]) and joint pain (usually incapacitating) with acute
onset and/or epidemiological and laboratory criteria; (2) atypical
cases, characterized by clinical cases of laboratory-confirmed
CHIKV accompanied by other manifestations (ie, neurological,
cardiovascular, dermatological, ophthalmological, hepatic, renal,
Table 1. Epidemiological Indicators for Chikungunya Outbreaks in La Reunion and Asia and in the Americas
Mother-to-child vertical transmission rate, %
La Reunion and Asia
respiratory, or hematological conditions); (3) severe acute cases,
characterized by clinical cases of laboratory-confirmed CHIKV
presenting with dysfunction of at least 1 organ or system that
threatens life and requires hospitalization; and (4) suspected/
confirmed chronic cases, characterized by previous clinical
diagnosis of chikungunya 12 weeks after symptom onset and
presentation with at least 1 rheumatologic (joint) manifestation (ie,
pain, rigidity, or edema) that is continuous or recurrent .
ECOLOGICAL AND ENVIRONMENTAL FACTORS
Disease spread cannot be entirely determined by risk factors for
chikungunya, but some of them may have strong influences on
human ( population density, activity, and migration) and vector
(type, density, and infectivity) populations.
Ecological and environmental factors provide
complementary information for risk assessment. Access to data and
information on the environmental and demographic conditions in the
area of interest, both historically and at the time of the study,
may support the interpretation and analyses of CHIKV activity.
The interpretation of human serosurvey findings and
entomological assessments is helped by using ecological and
environmental indicators to explain how and why these indicators
influence the changes in vector and human populations.
Some ecological and environment indicators can potentially
impact CHIKV activity , such as (i) temperature, which has
an impact on human migration and the range of the mosquito
habitat; (ii) elevation, which limits the reach of mosquito habitat
to altitudes of <2300 m; (iii) rainfall and vegetation, which can
change the range of mosquito habitat; (iv) land use and
industry, which can influence human demographic changes and their
accompanying effects on CHIKV; and (v) population
movements due to migration, tourism, and trade within and across
THE FUTURE OF CHIKUNGUNYA IN THE AMERICAS
As of late 2015, CHIKV continues to circulate and cause disease
in many of the countries in the Americas. As noted above, it is
unclear whether CHIKV will establish an enzootic cycle in the
Americas that would facilitate continual occurrence of CHIKV
outbreaks as immunologically naive humans encroach on this
cycle and become infected. If CHIKV develops an enzootic
cycle like that of yellow fever virus, it may be possible for
early detection to rely not only on traditional human
surveillance, but also on ecologic surveillance, such as epizootic and
entomological surveillance for vector density and infectivity
. Early risk detection using selected ecological and
environmental indicators such as rain, temperature, altitude, vegetation,
and land use could identify new at-risk areas where CHIKV
circulation could be confirmed by local serosurveys.
The continued geographical spread of CHIKV, through travel
and trade, to temperate areas with favorable
virus-vector-vertebrate and environmental conditions is conceivable. Since
isolated cases and outbreaks have already been reported in the United
States and Europe [2, 9, 10], it is important to focus future work
on identifying potential factors that might help limit the spread
of CHIKV, exploring therapeutic options for both acute and
chronic disease, and developing a global strategy to combat
the progression of the disease.
We have gained more understanding of the epidemiology and
clinical spectrum of the disease, owing to the recent outbreaks
in Africa and Asia. These lessons learned will help the Americas
not only to better understand the acute and chronic phases of
the disease, but also to better follow up at-risk population
Acknowledgments. We thank Patricia Najera from the Pan American
Health Organization/World Health Organization (WHO), for designing
the map of chikungunya cases reported in the Americas, and Erika Garcia,
for critical reading of the manuscript.
Disclaimer. The authors alone are responsible for the views expressed in
this publication, which do not necessarily represent the decisions, policy or
views of the WHO (applies to S. Y., V. M., L. C., and P. R.-P.) or the Centers
for Disease Control and Prevention (applies to J. E. S.).
Financial support. This work was supported by the National Institute
of Allergy and Infectious Diseases and the WHO.
Potential conflicts of interest. All authors: No reported conflicts. All
authors have submitted the ICMJE Form for Disclosure of Potential
Conflicts of Interest. Conflicts that the editors consider relevant to the content
of the manuscript have been disclosed.
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