Chikungunya virus has, quite unexpectedly and quickly, taken the Caribbean by storm. The first case in the region was reported in mid-December among residents of the French side of Saint Martin. It was the first time the virus was identified among non-travelers in the Western Hemisphere.
Since those first few cases in December, the case count in the Caribbean has seen a staggering increase: As of Aug. 1, the Pan American Health Organization (PAHO) reported 508,122 suspected cases and 4,732 confirmed cases of chikungunya throughout the Caribbean and Central and South America. Most of the cases have occurred in the Dominican Republic, Haiti, Martinique, Guadeloupe and the French side of Saint Martin.
“This number is, undoubtedly, an underestimate, simply because many people just don’t go to the doctor,” Lyle Petersen, MD, MPH, director of the division of vector-borne diseases at the CDC and an Infectious Disease News Editorial Board member, said in an interview. “When faced with thousands and thousands of cases, many are not reported. The bottom line is that it’s a big number and it’s growing.”
As of Aug. 5, there have been 480 cases in 41 US states, reported to ArboNET, the CDC’s national surveillance system for arthropod-borne diseases. Most of these cases have been travel-associated, but four locally transmitted cases were confirmed by the CDC. There have also been 221 cases in Puerto Rico and the US Virgin Islands, most of which were locally-transmitted.
Lyle Petersen, MD, MPH, said housing
conditions in the US help reduce the risk
of transmission of chikungunya virus.
Photo courtesy of Petersen L
However, chikungunya is not a notifiable disease, so it is impossible to know the true number of cases, according to Petersen.
Infectious Disease News spoke with several experts to discuss the Caribbean outbreak, the history of chikungunya, and whether US residents are at risk for acquiring it in their own backyards.
The word chikungunya is derived from Makonde language in Africa, specifically the root verb kungunyala, which literally translates to “that which bends up.” This definition is indicative of one of the main symptoms of chikungunya: joint pain.
“People with chikungunya infection experience joint pain that is so bad, they can’t walk,” Petersen said. “The primarily affected joints include the small joints of the hands and feet, as well as ankle and knee joints. Joint pain and fever are hallmarks of illness.”
Other potential symptoms, Petersen said, include muscle pain and headaches. Some may have gastrointestinal issues or develop a skin rash. It is a debilitating illness for which there is no treatment. Most people will start to feel better within a few days or weeks, but some may develop chronic symptoms that last weeks, months or, in some cases, even years.
The good news is that there has not been a large number of deaths. As of Aug. 1, PAHO reported only 32 deaths related to chikungunya. According to Petersen, most, if not all of these deaths were among people with an underlying medical condition, elderly people, immunosuppressed people or very young children.
“It’s not a disease of high mortality, but it is a disease of high morbidity,” Petersen said. “People get quite sick and can’t do their typical daily living activities because they feel so horrible.”
Chikungunya virus was first recognized as a human pathogen in the 1950s in Africa, and subsequently, cases were identified in both Asia and Africa.
The disease re-emerged in 2004 and became a public health concern during an outbreak in Kenya, according to Ann Powers, PhD, chief of the alphavirus laboratory in the CDC’s division of vector-borne diseases. From there, it moved rapidly to the islands in the Indian Ocean, where 50% to 75% of the inhabitants became infected within weeks or months.
“Most people who are infected with chikungunya show symptoms, so when you have half a population that has joint pain so severe they can’t stand, much less go to work, it’s incapacitating and has a serious impact on the local economy,” Powers told Infectious Disease News.
After the epidemic started in 2004, people all over the world became aware of the virus, and health agencies began conducting surveillance for it, she said. There have been several outbreaks since the initial 2004 outbreak in Kenya. On the island of Comoros in 2005-2006, there were about 250,000 cases within a period of 4 to 5 months. On the island of Réunion, there were 250,000 to 270,000 cases in 2006. In India in 2006-2007, there were an estimated 1.5 million cases.
“With those types of numbers, it’s easy to see why public health officials were interested in the virus and looking at what they could do to prevent large outbreaks,” Powers said.
The outbreak in the Caribbean appears to be linked to Southeast Asia, as the isolates obtained from patients there are genetically similar to isolates obtained from patients in Indonesia and the Philippines. It likely found its way to the Caribbean through a traveler who became infected and was still infectious when arriving in the Caribbean. At that point, the individual was bitten by a local mosquito while viremic, which started the local transmission cycle, Powers said.
According to Thais dos Santos, adviser for communicable diseases and health analysis for PAHO/WHO for the Eastern Caribbean and Barbados, the Caribbean has a lot of experience dealing with dengue epidemics every year, which has helped in dealing with the chikungunya outbreak.
“The thing about the dengue epidemics, however, is that the magnitude of those epidemics is not as large as what we’re seeing with chikungunya,” dos Santos told Infectious Disease News. “The silver lining is that once a person becomes infected with chikungunya, they are believed to have immunity to it. We’re hoping this will work in our favor, unlike dengue, where people can continue to be infected with the different dengue types.”
The countries in the Caribbean have experience with mosquito-borne diseases, and dos Santos said in response to the outbreak, PAHO/WHO has been providing support to scale up the existing five-pillared approach to mosquito-control programs to deal with the upsurge in cases. Efforts include targeting resources for epidemiological surveillance, training physicians on diagnosing the virus, introducing laboratories to methods to diagnose the virus, providing equipment and supplies for vector control and training spokespeople to communicate with the public.
There are other concerning issues to consider, however, especially water security, dos Santos said. For example, it is difficult to advise people who do not have access to pipe water to not store water because it could be a breeding site for mosquitoes.
“We’re trying to merge the messages to advise people on how to store water safely,” dos Santos said. “Many of the affected countries, however, are in the eastern Caribbean and are middle-income countries where water security issues are not as strong as they are in resource-constrained countries. But whatever we do, we still need to be very aggressive to get the mosquito population under control.”
Aedes aegypti is primarily a tropical mosquito that is abundant throughout the Caribbean. It is also the source of chikungunya transmission throughout the region.
“It’s fortunate that chikungunya is transmitted by a mosquito that we know and know well,” dos Santos said. “A. aegypti has been in the region for quite some time, and all of the countries have programs to combat it.”
Besides A. aegypti, the A. albopictus mosquito can also transmit chikungunya. Both mosquitoes can also transmit dengue and yellow fever.
According to Petersen, A. aegypti is a daytime biting mosquito that preferentially feeds on humans in urban settings, can bite multiple humans in one blood meal and is often found indoors. In the contiguous United States, these mosquitoes are primarily found on the Gulf Coast and in limited areas in California.
A. albopictus is also a daytime biting mosquito, although not as efficient a vector as A. aegypti because it will bite anything it encounters, Petersen said. However, it is a cooler weather mosquito that is found in the eastern United States as far north as New England, and can lead to outbreaks in areas where A. aegypti mosquitoes do not exist.
The A. albopictus mosquito has a much broader presence in the United States compared with A. aegypti and may play a larger role in chikungunya transmission in the country, Powers said.
“A. albopictus mosquitoes are found as far west as the Mississippi River Valley and can reach Chicago in the summer months,” Powers said. “Its vast presence puts a large percentage of the United States at risk for local transmission during the summer months. In the South, where A. aegypti mosquitoes present, transmission of chikungunya is a year-round risk.”
Most of the cases in the contiguous United States thus far have been travel-associated. However, there has been local transmission in Puerto Rico and the US Virgin Islands. Chikungunya has been reported in 41 states and nearly one-quarter of those cases were in Florida. The cases of local transmission in the United States were also in Florida.
The likely reason is because Florida is a gateway to the Caribbean for many travelers, and Florida and the Caribbean have many social ties, according to Carina Blackmore, DVM, PhD, deputy state epidemiologist and state public health veterinarian at the Florida Department of Health.
“We have many people going back and forth, both to visit the Caribbean and because many people from the Caribbean also have family in Florida,” Blackmore told Infectious Disease News. “After the earthquake in Haiti, for example, we had many people come here to stay with family. There are many connections between the Caribbean and Florida.”
All states that have A. aegypti or A. albopictus mosquitoes are at risk for local transmission. Many state health departments have advised their residents to take precautions against mosquito bites, including wearing insect repellent, wearing long-sleeved shirts and pants when outdoors and staying behind screens or in air-conditioned buildings.
Florida is promoting a “Drain and Cover” initiative, encouraging its residents to remove standing water from flower pots, tires, etc, and covering skin with clothing and repellent, Blackmore said.
“Some local health offices have received calls from consumers regarding chikungunya, and it’s clear that they are aware of it and want to learn more about it,” Blackmore said. “We use those opportunities to reassure them that there are steps they can take to prevent it.”
Potential for local outbreaks
Phyllis Kozarsky, MD, professor in the division of infectious diseases at Emory University School of Medicine and medical director of TravelWell, the travel clinic at Emory University Hospital, said in the travel medicine clinic, she has not seen many patients concerned about chikungunya. However, most people traveling to the Caribbean do not typically visit a clinic before travel.
There have been four locally transmitted cases of chikungunya in the United States thus far.
“The disease is all over the Caribbean, and the incubation period, while usually 3 to 7 days, can be as long as 12 days, so travelers can come home before realizing that they’ve been infected,” Kozarsky told Infectious Disease News. “Because the viremic period may be long, a mosquito can then bite an infected person and transmit the virus to another person. There is no question that we may see more locally acquired cases in the United States. In the past, West Nile virus suddenly appeared and spread rapidly, and chikungunya may have that capacity as well.”
It used to be that the main mosquito-related illness related to traveling was malaria, Kozarsky said. In that case, the advice was to use insect repellent between dusk and dawn. But with chikungunya, and also dengue, the advice is different: Insect repellent needs to be used all day because the mosquitoes are active and bite during the day.
“We only know the numbers of reported cases, which means people who have gone to the doctor and had their blood tested at a state or federal laboratory,” Kozarsky said. “Compare that to the number of people who return from the Caribbean with aches and pains, but don’t even go to the doctor, or who are diagnosed based only on symptoms. My guess is that the number of cases we have documented in the United States is quite low compared with the actual number of cases that have probably occurred.”
One concern with chikungunya is that it is not easy to diagnose. The symptoms are close to dengue and to other viruses, but making a definitive diagnosis is difficult without testing, which few state laboratories have capacity to do, Kozarsky said.
“Commercial laboratories may offer testing, but the tests may not be well standardized,” Kozarsky said. “The most reliable testing is done at state laboratories or at the CDC, and it may not be that easy for many clinicians to access this testing within their busy practices.”
In the Caribbean, laboratory testing is also an issue, dos Santos said. Most bloodwork needs to be sent to a reference laboratory for testing. The regional reference laboratory for the Caribbean is in Trinidad.
Powers said testing symptomatic patients will also help the CDC and other public health officials better understand where localized transmission of the disease is occurring. Although chikungunya is not a nationally notifiable disease in the United States, physicians are encouraged to report cases through the ArboNET surveillance system.
“Having that information out there helps in a variety of public health ways to help contain the virus,” Powers said. “Physicians can alert public health officials and mosquito-control officials who can work to prevent localized transmission in areas where these mosquitoes exist.”
Blackmore also said public health officials are counting on physicians to report cases of chikungunya, as that information will help guide their efforts in public health and trigger mosquito control action.
What some clinicians and public health officials are concerned about is the possibility of mistaking dengue for chikungunya. Both diseases originate from the same mosquitoes, and clinically, both illnesses start out the same. But the outcomes are completely different, according to Petersen.
“We don’t want to see physicians mistaking dengue for chikungunya, simply because you can die from dengue with inappropriate or lack of treatment,” Petersen said. “It’s important to recognize that dengue exists in areas where chikungunya exists and the two can be confused from a clinical presentation standpoint.”
If travelers come back with a chikungunya-like illness, they should be tested for both dengue and chikungunya, Petersen said. Chikungunya needs to be on the differential diagnosis of all patients presenting with a febrile illness who recently returned from an outbreak area.
According to dos Santos, PAHO has offered training for clinicians in the Caribbean about the challenges in distinguishing dengue and chikungunya. One difference is that dengue will often cause pain behind the eyes.
In the meantime, physicians need to identify and treat, dos Santos said.
“The treatment for both dengue and chikungunya is comparable: rest, fluids and managing symptoms while monitoring for signs in people whose disease might evolve,” dos Santos said. “We try to alert clinicians of who they need to keep an eye on, and on how to manage pain and make patients as comfortable as possible.”
What is different in the United States compared with areas of the outbreak in the Caribbean is that the United States has different housing conditions, including air conditioning and screens that cover doors and windows. Because of this, the mosquitoes do not thrive as well compared with other countries in the Caribbean where the homes are open, Blackmore said. Houses with air conditioning and screens keep out the A. aegypti mosquito, which thrives indoors, and as a result, transmission is less likely to happen.
For that reason, the risk for local transmission may be lower in the United States. However, it is still there, Blackmore said, and just as there have been outbreaks of dengue, there may be outbreaks of chikungunya.
“The good thing about chikungunya is that unlike dengue, it doesn’t cause a hemorrhagic fever, and it’s not a disease with a high mortality rate,” Blackmore said. “That said, it does cause significant joint pain, which can last for months or even years. So, of course, we want to make sure we prevent any local transmission we can.”
The bad news is that the risk for local transmission still exists.
When West Nile virus appeared in the United States, it demonstrated the ability of a newly introduced arbovirus to spread quickly, Petersen said. However, the two are different because West Nile is a disease of birds. Mosquitoes are infected by birds. Humans can be infected by mosquitoes, but humans cannot infect mosquitoes.
With chikungunya, humans are reservoirs and can infect mosquitoes.
“If you have people, and you have one of these mosquitoes, you have the potential to cause a local outbreak,” Petersen said. “That can have bad consequences.” — by Emily Shafer
Carina Blackmore, DVM, PhD, can be reached at: carina.blackmore@FLHealth.gov.
Thais dos Santos can be reached through the PAHO media representative, Leticia Linn, at firstname.lastname@example.org.
Phyllis Kozarsky, MD, can be reached at: email@example.com.
Lyle Petersen, MD, MPH, and Ann Powers, PhD, can be reached through CDC’s Division of Vector-Borne Diseases, at firstname.lastname@example.org.
Disclosure: Blackmore, dos Santos, Kozarsky, Petersen and Powers report no relevant disclosures.
Should chikungunya be added to the list of notifiable diseases?
It would help inform public health planning.
Chikungunya fever is an emerging, mosquito-borne disease caused by the chikungunya virus. The first outbreak of chikungunya fever in the Western Hemisphere was reported by WHO in December, 2013. Since that time, hundreds of thousands of cases have been reported from 17 countries in the Caribbean or South America. Chikungunya infection causes a clinically significant acute illness as well as chronic infections. The first locally-transmitted chikungunya case in the United States was reported in Florida, a result of persons who acquired the infection during travel and returned to the United States, where they were subsequently bitten by local indigenous mosquitos that transmitted the virus to others in the community. The disease is transmitted predominantly by Aedes aegypti and A. albopictus mosquitoes. Both species are prevalent in many southern states and A. albopictus has expanded its range to south-central and northeastern United States, and its range of may continue to expand. Consequently, imported chikungunya cases could result in local spread of the virus in other parts of the United States. Although some states have made the disease reportable, chikungunya is not yet a nationally notifiable disease in the United States.
Surveillance, reporting suspected and confirmed chikungunya cases to state or local health departments and subsequently to CDC, is important to identify new areas of local transmission and monitor the extent of geographic spread and magnitude of disease transmission in the United States over time, characterize the clinical illness and risk groups, help inform public health and heath care system planning and response measures, and to inform public communication regarding disease prevention strategies. Until the condition is made nationally reportable, state health departments are encouraged to report confirmed chikungunya virus infections to CDC through existing mechanisms (ArboNET).
Jeffrey Duchin, MD, is chief of communicable disease epidemiology & immunization section at Public Health – Seattle & King County. Disclosure: Duchin reports no relevant disclosures.
Listing chikungunya would highlight the need for mosquito control measures.
In one sense, putting chikungunya on the list is already beginning to happen. The CDC notes that chikungunya is not specifically notifiable in the United States. But it is transmitted by mosquitoes, making it an arboviral disease, which is a category on the CDC list of reportable diseases. The term is open to interpretation, however. Some state lists, and the CDC list, include a few specific viruses. The New York City list, in contrast, is an increasingly comprehensive list, defining them as, “Arboviral infections, acute (including but not limited to the following viruses: Chikungunya virus, dengue, Eastern equine encephalitis virus, Jamestown Canyon virus, Japanese encephalitis virus, La Crosse virus, Powassan virus, Rift Valley fever virus, St. Louis encephalitis virus, Western or Venezuelan equine encephalitis virus).”
But what does it mean for a condition to be listed? It can raise the awareness of clinicians, and help to provide more accurate surveillance data on the spread and occurrence of the disease, essential for epidemiologists and researchers. But the list is long — about 70 on the CDC list and twice as many on New York City’s. It’s a lot for busy clinicians to remember. To be useful, listing must lead to appropriate response. For a mosquito-transmitted disease with no specific treatment or vaccine, this means effective mosquito control is essential.
Aggressive mosquito control programs in the early and mid-20th Century greatly diminished yellow fever and dengue cases in South America and other diseases in the United States. As these efforts started to pay off, they became victims of their own success. Many programs were reduced or eliminated, and the mosquito populations rebounded. The real significance of adding chikungunya to the list may well be to demonstrate how urgently we need effective mosquito control measures, and can’t afford to become complacent again.
Stephen Morse, PhD, is a professor of Epidemiology in Mailman School of Public Health at Columbia University. Disclosure: Morse reports no relevant financial disclosures.