Credit CDC
# 8720
Despite the considerable public health problems that West Nile Fever causes in this country, and around the world, humans (and horses) are essentially a dead-end for the virus. We are `incidental’ hosts, as we don’t produce enough of the virus in our bloodstream to allow another mosquito to transfer it on to others.
West Nile virus is actually a disease of birds, which serve as amplifying hosts for the virus, and human infection is basically a case of unintended collateral damage (see graphic below).
Photo Credit CDC
The vast majority of people infected by WNV are asymptomatic, or end up with mild nonspecific symptoms reminiscent of a summer `cold’. The mild form of the disease is called West Nile Fever, and only 1%-3% who experience it are ever diagnosed, although estimates are 100,000 Americans or more are infected each year.
Neuroinvasive cases (which present with meningitis, encephalitis, or flaccid Paralysis) are severe enough that they nearly always result in hospitalization and diagnosis, and so they are considered the best indicator of the scope of each year’s epidemic.
In 2012, the United States saw nearly 3,000 cases of neuroinvasive WNV, and 286 deaths (see DVBID: 2012 Record Number Of West Nile Fatalities).
An impressive tally for a disease that infects humans – strictly speaking – only by virtue of happenstance. As you can see by the maps below, after its introduction in 1999, WNV spread across much of the United States in a matter of a few short years. Today is is found in all 48 contiguous states.
From the USGS Factsheet on West Nile Virus
Chikungunya, on the other hand, has evolved to become a `humanized’ virus. Outside of Africa (where it also resides in non-human primates), humans are the primary host for the virus, and once infected, can `amplify’ the virus efficiently for days.
According to the CDC: The risk of a person transmitting the virus to a biting mosquito or through blood is highest when the patient is viremic during the first 2–6 days of illness.
Unlike with WNV, the majority of people infected with Chikungunya will become symptomatic. In their information for healthcare providers, the CDC lists:
Clinical findings
- Majority of infected people become symptomatic
- Incubation period usually 3–7 days (range 1–12 days)
- Acute onset of fever and polyarthralgia are the primary clinical findings
- Joint symptoms usually symmetric and often occur in hands and feet; they can be severe and debilitating
- Other symptoms: Headache, myalgia, arthritis, conjunctivitis, nausea/vomiting, maculopapular rash
- Lymphopenia, thrombocytopenia, elevated creatinine, and elevated hepatic transaminases are the most common clinical laboratory findings
Clinical course and outcomes
- Acute symptoms typically resolve within 7–10 days
- Rare complications include uveitis, retinitis, myocarditis, hepatitis, nephritis, bullous skin lesions, hemorrhage, meningoencephalitis, myelitis, Guillain-Barré syndrome, and cranial nerve palsies
- Persons at risk for severe disease include neonates exposed intrapartum, older adults (e.g., > 65 years), and persons with underlying medical conditions (e.g., hypertension, diabetes, or cardiovascular disease)
- Some patients might have relapse of rheumatologic symptoms (e.g., polyarthralgia, polyarthritis, tenosynovitis) in the months following acute illness
- Studies report variable proportions of patients with persistent joint pains for months to years
- Mortality is rare and occurs mostly in older adults
Treatment
- No specific antiviral therapy
- Supportive care with rest and fluids
- Non-steroidal anti-inflammatory drugs (NSAIDs) to relieve acute pain and fever
- Persistent joint pain may benefit from use of NSAIDs, corticosteroids, or physiotherapy
Over the past six months we’ve watched as Chikungunya has spread across much of the Caribbean, infecting well over 100,000 people. While seemingly better equipped to spread across the United States than West Nile Virus – which, despite its limitations, has done a pretty good job – we really don’t know how much of a problem Chikungunya will pose to Americans this summer.
We’ve had similar concerns with the expansion of Dengue – another `humanized’ arbovirus that has made great strides around the globe – and has seen recent introductions into the United States (see MMWR: Dengue Fever In Key West).
While now the most common mosquito-borne virus in the world (causing up to 100 million infections a year), Dengue has managed to do little more than spark a few limited outbreaks in North America, despite an abundance of the right mosquito vectors.
In 2003, a CDC EID study looked at this paradox, and found that economics and lifestyle, may have more do to with our success against Dengue, than anything else.
Texas Lifestyle Limits Transmission of Dengue Virus
Paul Reiter* , Sarah Lathrop*, Michel L. Bunning*, Brad J. Biggerstaff*, Daniel Singer*, Tejpratap Tiwari*, Laura Baber†, Manuel Amador*, Jaime Thirion‡, Jack Hayes§, Calixto Seca¶, Jorge Mendez‡, Bernardo Ramirez#, Jerome Robinson†, Julie Rawlings¶, Vance Vorndam*, Stephen Waterman*, Duane Gubler*, Gary Clark*, and Edward Hayes*
Abstract
Urban dengue is common in most countries of the Americas, but has been rare in the United States for more than half a century. In 1999 we investigated an outbreak of the disease that affected Nuevo Laredo, Tamaulipas, Mexico, and Laredo, Texas, United States, contiguous cities that straddle the international border. The incidence of recent cases, indicated by immunoglobulin M antibody serosurvey, was higher in Nuevo Laredo, although the vector, Aedes aegypti, was more abundant in Laredo. Environmental factors that affect contact with mosquitoes, such as air-conditioning and human behavior, appear to account for this paradox. We conclude that the low prevalence of dengue in the United States is primarily due to economic, rather than climatic, factors.
Whether our luck will hold with Chikungunya (or continue to hold with Dengue) is anyone’s guess. The global track record with both of these diseases has been impressive to date, and so one is not inclined to bet against their future success.
The overall risk of contracting a mosquito-borne illness anywhere in the United States remains very small, but it has increased in recent years, with the emergence of new threats like WNV, Dengue, and Chikungunya.
While there are no vaccines available for these viruses, these are still largely preventable diseases. Local Health departments urge people to always follow the `5 D’s’ of prevention:
