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West Nile Virus Update

Saturday, November 17, 2012 // Uncategorized

A patient told me that a friend of his was hospitalized and in a coma due to West Nile Virus. Since the number of cases reported has peaked with cooler weather, this disease has not been in the news, but it is important to remember that it is still out there.  This has been a record year for the virus due to a variety of reasons which are outlined in the following interview with Dr. Lyle Peterson of the Center for Disease Control  in the November 14 issue of JAMA.

Medical News and Perspectives | November 14, 2012

Record Heat May Have Contributed to a Banner Year for West Nile Virus

Bridget M. Kuehn, MSJ
JAMA. 2012;308(18):1846-1848. doi:10.1001/jama.2012.13495.
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With nearly 4000 cases of West Nile virus reported to the US Centers for Disease Control and Prevention (CDC) as of early October, more than 5 times the number of cases reported in 2011, the 2012 West Nile virus season has been one of the worst since the virus emerged in the United States in 1999.

This surge in cases—which was concentrated in Texas, Mississippi, Michigan, South Dakota, Louisiana, Oklahoma, and California—likely resulted from a confluence of ecological factors, including higher-than-normal temperatures, that may have influenced mosquito and bird abundance, the replication of the virus in its host mosquitoes, and interactions of birds and mosquitoes in hard-hit areas, according to Lyle Petersen, MD, MPH, Director of the CDC’s Division of Vector-Borne Diseases. Petersen discussed this year’s resurgence of West Nile virus with JAMA.

 
 

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Ecological changes likely led to outbreaks of West Nile virus infection in 2012, said Lyle Petersen, MD, MPH, Director of the Division of Vector-Borne Diseases at the Center for Disease Control and Prevention.

Higher temperatures and moist conditions can promote West Nile virus outbreaks by facilitating mosquito breeding and speeding viral replication.

JAMA:Was what happened this year unexpected?

Dr Petersen: Outbreaks of arboviruses, like West Nile virus, historically are very episodic in nature. When West Nile virus marched across the country, we didn’t really know what was going to happen, but St Louis encephalitis, a similar arbovirus tracked since the 1930s, is a good model of how West Nile virus may behave in the United States. In some years, there are very few cases of St Louis encephalitis, but it periodically causes focal or regional outbreaks big and small, including a huge epidemic in the Midwest in the 70s. People are still trying to figure out why. It’s very difficult to predict when and where arbovirus epidemics will occur.

JAMA:What trajectory has West Nile taken since it was introduced here?

Dr Petersen: From its 1999 discovery in New York City through 2001, West Nile virus spread across the eastern United States but caused relatively few human cases. Big outbreaks occurred in 2002 and 2003, when the virus spread into the Midwest and mountain states where all the birds were susceptible, sufficient water existed to breed many mosquitoes, and weather was warm. It was the perfect storm. The virus spread to the West Coast in 2003 and has become endemic nationwide. But in the last few years, West Nile virus seasons have been less active, so this year’s outbreak came as a surprise to those not familiar with the sporadic nature of arboviruses. The bottom line is that West Nile virus is here to stay and it will cause large and small outbreaks for years to come.

JAMA:Has West Nile virus behaved differently in the United States than in other regions where it is endemic?

Dr Petersen: West Nile virus is endemic in Africa. What is interesting is that big outbreaks only happen in more temperate climates, and the reasons for this aren’t clear. The big outbreaks haven’t been in equatorial Africa but have occurred in more temperate climates in South Africa, Europe, and now in the United States and southern Canada, which seem to have an environment that is particularly conducive to outbreaks. Similar to the situation in Africa, the virus has not caused outbreaks in tropical America.

JAMA:Why do you think we saw an uptick in cases this year?

Dr Petersen: We really don’t know. West Nile virus has a complicated ecological cycle between birds and mosquitoes. The number of susceptible birds, the number of vector mosquitoes, the rate of viral replication in mosquitoes, the average length of survival of mosquitoes, and the interaction of birds and mosquitoes are all important factors. All of these factors are influenced by weather. The first 8 months of 2012 were the hottest on record in the United States. That could have been an important factor in this year’s outbreak.

JAMA:Have previous outbreaks occurred in hot years?

Dr Petersen: The initial outbreak in New York City and the ones in 2002 and in 2003 were associated with warmer weather. Warmer-than-average weather does a couple of critical things. It increases viral replication in mosquitoes, making them more infectious. It also shortens the extrinsic incubation period—the time from when the mosquito takes an infectious blood meal to when the mosquito can become infectious by biting a person or bird. The shorter that period, the better it is for viral transmission. But high temperatures can also dry up mosquito breeding sites, reducing mosquito breeding. Temperature and rainfall patterns have a complicated relationship with mosquito breeding cycles.

JAMA:Why did we see an uptick only in certain states?

Dr Petersen: There was a huge swath of the country with high temperatures. But if you look at places with the big outbreaks, they mostly didn’t experience the severe drought other areas had. They had relatively normal rainfall. Most places with severe drought didn’t experience outbreaks. So it was probably a combination of enough rain to produce mosquitoes, along with a conducive temperature for West Nile virus transmission.

 
 

Image not available.

Ecological changes likely led to outbreaks of West Nile virus infection in 2012, said Lyle Petersen, MD, MPH, Director of the Division of Vector-Borne Diseases at the Center for Disease Control and Prevention.

Higher temperatures and moist conditions can promote West Nile virus outbreaks by facilitating mosquito breeding and speeding viral replication.

JAMA:Do we know if there were more neurologic cases this year?

Dr Petersen: Over all, about 1 in 150 to 250 persons who are infected will develop severe neurological disease. We have no reason to believe that this ratio has changed this year. Our surveillance data tell us that cases of West Nile virus neuroinvasive disease, particularly encephalitis, are related to increasing age and that people with kidney disease, diabetes, immunosuppressing conditions, and hypertension are also more likely to get neuroinvasive disease. Because people with neuroinvasive disease generally are hospitalized, the rate of testing for West Nile virus is very high. We believe that people with severe neuroinvasive disease are well captured in our surveillance, and that’s how we monitor the severity of a West Nile epidemic.

Among persons with West Nile fever, there’s quite a spectrum of illness, ranging from barely any symptoms to severely ill for weeks to months. Thus, many people don’t go to the doctor or, if they do, simply get sent home to rest, since we don’t recommend routine testing for these patients. The end result is only about 2% to 3% of West Nile fever cases are captured by our surveillance. So fever cases are grossly underestimated by surveillance.

JAMA:Is there any reason to believe that the outbreaks were caused by changes in the virus?

Dr Petersen: There is no reason now to believe that the virus’ virulence has changed, but we are looking to make sure that this isn’t the case. But the most likely explanation was that ecological conditions led to big outbreaks.

JAMA:Do you think cuts to public health and municipal services have contributed to the resurgence?

Dr Petersen: I don’t think they have necessarily contributed to the resurgence. But they have undoubtedly contributed to changes in the response to the resurgence.

We don’t collect data on local mosquito control, but anecdotally, we know a lot of jurisdictions have cut back on entomological services and don’t have sufficient response capability when an outbreak occurs. An effective response relies on a certain amount of expertise. Mosquito control activities and surveillance require experienced, well-trained people and special equipment. Once those capabilities erode, they can’t be easily regathered in the course of an epidemic. It takes a long-term perspective to maintain those services, and in this fiscal climate that is hard to do.

JAMA:What do we know about the long-term effects of West Nile virus infection?

Dr Petersen: We know many people with encephalitis will have long-term cognitive or neurological problems. Among people who develop West Nile virus paralysis, about one-third will have significant recovery, one-third will have some recovery, and one-third will have no recovery. People with meningitis generally do recover fully.

For those with West Nile fever, there is a huge spectrum of illness severity, but I think West Nile fever is an underappreciated source of morbidity. Many persons experience illness that may last for months, and a subgroup may have symptoms over the longer term.

JAMA:What can physicians do to prepare for West Nile outbreaks next year and beyond?

Dr Petersen: Physicians should have a high degree of suspicion for West Nile–related illness from late July through September. We know that even patients with classic symptoms are going unrecognized. The symptoms of West Nile fever include fever, headache, gastrointestinal disturbance, muscle aches, fatigue, sometimes a skin rash on the trunk and extremities, and swollen lymph nodes. Symptoms of neuroinvasive disease include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis.

In terms of advice for patients, the name of the game is prevention. The best preventive measure is wearing mosquito repellent, particularly at dawn and dusk. But that is not so easy. There are repellents that work (DEET, oil of lemon eucalyptus, IR3535, and picaridin), but there are others that don’t. There are many myths about repellents, and physicians should steer patients toward products with these ingredients. Physicians should also be aware of what mosquito control is and isn’t. There is a lot of hype around municipal spraying, and patients may ask if they can be harmed by it. The bottom line is that for West Nile virus control, municipalities use ultralow volumes of pesticides sprayed by trucks or planes—less than an ounce per acre. With such a low amount, there are no known health effects. These pesticides are rapidly degraded and do not persist in the environment.

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