Posts Tagged ‘West Nile epidemics in the United States’

West Nile: background and 2013 updates

Monday, July 22nd, 2013

We were a ways up in the mountains for a dinner yesterday and I cautioned the people who were hosting that mosquitoes could easily be found at their 7,600 foot level. I said the West Nile season was usually in August and September, but one of my gourmet group told me there had already been a first case in Colorado and West Nile positive mosquitoes in our county. What I discovered online was 94% of West Nile symptomatic cases occur between July and September.

Heed the warning!Today I found the West Nile advisory page for Larimer County where we live and the CDC's West Nile statistics that mention 23 cases nationwide through July 16th with three deaths. Forty-three percent of the patients had West Nile Neuroinvasive Disease (WNND); the others had West Nile Fever (WNF). Twenty-nine states and the District of Columbia have reported human disease thus far.

And it's still early in the Summer of 2013.

Then I went back to the last of the four articles in the July 17th edition of JAMA; this one covers information intended for physicians and goes into more depth than the others. Its title is West Nile Virus: Review of the Literature. I thought there would be a few nuggets of information that might be useful for an extended audience

To start with, West Nile is endemic in every state in our country except Alaska and Hawaii. That means it is found in each of those regions on an ongoing basis without the need to be imported each year. An example I found said chickenpox is endemic in the UK, but malaria is not. Canada has had no cases of either WNF or WNND thus far in 2013, but had 428 cases last year with five provinces reporting patents with the virus.

Here it is a much more frequent disease problem that has led to the three worst outbreaks of arbovirus neuroinvasive disease in US history, each leading to ~3,000 cases of encephalitis, meningitis or sudden onset of severe muscle weakness (AKA acute flaccid paralysis). Arboviruses are those carried by ticks, mosquitoes and similar species. Older adults don't have a greater chance of developing WNF, but a markedly increased chance of WNND. One of fifty who catch West Nile and are over 65 get this dire form; that's sixteen times the rate for those age 16 to 24. The death rate in patients with WNND is ~10%, lower in relatively young patents, higher in the 65+ age bracket: one series reported 0.8% mortality for those under 40 and 17% for patients over 70. It's unknown how the virus crosses the blood-brain barrier, the super-tightly-packed cells that line the brain's blood vessels, preventing passage of most substances.

2012 was a really bad year for human disease in this country. The CD's final summary for that year included 5,674 cases, with 2,873 of those being WNND, and 286 deaths. The state of Texas had 37% of all reported cases and California, Louisiana, Mississippi, Michigan, Oklahoma and South Dakota were also hot spots for West Nile.

The virus has been found in over 325 bird species in the US and 65 different mosquito species, but only a few members of the Culex mosquito family have been shown to transmit West Nile to humans. Culex mosquitoes bite us from dusk to dawn, so, as I mentioned in my last post, I've changed what I wear when outside during those hours.

Passerine (perching) birds can infect mosquitoes; the robin isn't as plentiful as some species, but has a high serum viremia (lots of the virus in its blood), so is an important reservoir. If we are bitten by an infected mosquito and go on to develop WNF we'd have a low serum viremia, so a second mosquito biting us won't get the virus from us. Mosquito bites are responsible for almost all human cases; rarely one can occur after an organ transplant or a transfusion.

Higher-temperature areas both shorten the time a mosquito infected with the virus becomes infectious when it bites again and also improve the efficiency of it transmitting the virus to a bird. Of the 5 lineages of the virus only two have caused significant human outbreaks, but the 1999 New York City lineage has genetically altered subsequently to improve viral transmission. I don't know if this is true worldwide, but there have been significant outbreaks in Russia, Israel, Greece and Romania since the early 1990s.

Most who get a West Nile infection, as I've mentioned before, remain subclinical (e.g., without enough symptoms to need medical attention). In a study of blood donors who had West Nile viremia, but no significant symptoms initially, 38% eventually saw a physician and 2% were hospitalized. Only 5% of those seeking a diagnosis got the correct one!

West Nile in pregnancy, in one study of 71 women who delivered 72 babies, did not result in any malformations or infected infants.

Fatigue may last a long time.

Fatigue may last a long time.

Full recovery is the norm for those who have either uncomplicated WNF or meningitis, but they may be fatigued for a considerable length of time. In a 2004 study of 98 patents, almost all had this symptom and its median duration was 38 days. The patient's age didn't predict the duration of symptoms.

There are four licensed equine vaccines, but none for us. Several candidate human vaccines have gone through early trials, but no large-scale clinical trials have been attempted. Given that, if amazes me that even during outbreaks few of us use insect repellents that have proven efficacy.

If this West Nile season is like last year's, please heed the warnings, empty the water containers at intervals, avoid gardening at dusk or later and use an insect spray.

The brain and even the life you save may be your own.




An enemy returns: East of the Sun and far West of the Nile

Friday, July 19th, 2013
epidemic warnings come late; prevention is better

epidemic warnings come late; prevention is better

We came home from a short trip to Texas and I realized my weekly copy of JAMA had four articles on West Nile disease, its complications and possible prevention. After several years of relative quiescence, West Nile was back with a vengeance in 2012. Between the original New York City outbreak in 1999 and 2010 the virus has been endemic in the United States with intermittent flares, overall infecting an estimated three million Americans, causing West Nile fever (WNF) in 780,000, the much more severe West Nile neuroinvasive disease (WNND: e.g., involving the brain and other portions of the nervous system)  in 16,196 and killing 1549 of those.

Until 1999, the West Nile virus lived, mostly in birds, in Africa, Europe and Asia, but was not known to be in the Western Hemisphere. Its hosts (reservoirs) were bitten by mosquitoes whose name comes from the Spanish or Portuguese for "little fly," but some think this insect is the most dangerous creature on earth as it also transmits malaria, yellow fever and a variety of encephalitis-causing illnesses (inflammation of the brain). The mosquito in turn can pass the virus on to another bird or to a human

A 2001 article in the New England Journal of Medicine reviewed the original Western Hemisphere outbreak, the 1999 epidemic in New York City. It began with two patients hospitalized for encephalitis in the New York City borough of Queens. A preliminary epidemiologic search of nearby hospital admissions revealed six more cases. All eight lived in a sixteen square mile area in northern Queens and all had participated in evening outdoor activities (e.g., working in their gardens).

A search of their environments showed breeding sites for the particular species of mosquitoes known to carry St. Louis encephalitis (its vector), so the mini-epidemic was presumed to be an arthropod-bourne disease (The term arthropod include insects, spiders and crustaceans such as lobsters and crabs.) Mosquito control measures were promptly begun.

Eventually fifty-nine New Yorkers were hospitalized in August and September of that year with this outbreak. Although their ages varied from 5 to 90, the median age of these severely ill patients was 71 (NB. The "mean" is the "average" you're used to, where you add up all the numbers and then divide by the number of numbers. The "median" is the "middle" value in the list of numbers, so in this case there were as many patents over 71 and under that age).

The overall attack rate of West Nile in this outbreak went sharply up with age. Nearly two-thirds of those affected had encephalitis with altered mental status along with fever and abnormalities in their cerebrospinal fluid (CSF, the fluid that cushions the brain and spinal cord, bring it nutrients and removes waste products). Some of those had moderate to severe muscle weakness and/or meningitis (inflammation of the membranes that cover the brain and spinal cord) with fever, abnormal CSF findings, headache and stiff neck symptoms.

Seven of the fifty-nine (12%) of those with the disease died and those over age 75 were especially prone to death in this epidemic. Diabetes was another risk factor for mortality.

At the same time, there was a seemingly unrelated fatal illness occurring in the area's birds, especially in crows. The connection initially was missed as St. Louis encephalitis doesn't usually kill off its avian reservoir hosts.

Eventually the West Nile virus was identified in tissue specimens taken from both crows and a Chilean flamingo that had died at a zoo in the vicinity.

Within five years the virus had spread to all forty-eight contiguous states and several provinces in Canada. There were major flares of WNF and WNND in 2002 and 2003 with almost 3,000 cases of WNND in those years.

Then things calmed down and, as is typical, the money available for mosquito control became less abundant. The disease spread to Argentina by 2005 and eventually to most of the Western Hemisphere countries. Strangely only the United States (and Canada) suffered major epidemics.

And then came 2012.

Dallas County had a population of nearly two and a half million humans; of those 255 developed WNF, another 173 got WNND and 19 of those died. The review article, published in the July 17, 2013 edition of JAMA, states there was "An usually rapid and early escalation of human cases (which) followed infection trends in mosquitoes."

As in the rest of the country, Dallas had experienced a considerably warmer than usual winter. Ever since 1966, when aerial spraying for mosquitoes was instituted for an epidemic of St. Louis encephalitis, researchers in the metropolitan area had a heightened awareness of arthropod-bourne disease. Blood studies of residents in a Dallas area prone to flooding had shown there had been recurring epidemics of related virus-caused illness.

As West Nile virus supplanted its predecessor, Dallas began to experience cases of WNND and to prepare for its flares. In May 2012 a mosquito trap yielded evidence of West Nile-infected insects and all area physicians were sent a recommendation to test patents with symptoms of potential WNND for the virus.

A "vector index" was developed and calculated weekly; the specific of the math doesn't matter, but it related to how many of the particular mosquito species which was the region's primary carrier of West Nile virus, Culex quinquefasciatus, were found and their infection rate. Human cases of WNND were noted to follow the vector index peaks by one to four weeks.

One means of prevention: avoid being bitten

One means of prevention: avoid being bitten

An 8-day aerial spraying of insecticide did not lead to an increase in emergency room visits for asthma or other respiratory problems, or, for that matter, to skin rashes.

As of today I'm unaware of any human vaccine for West Nile or any treatment for the disease other than supportive measures.

So what can we do to prevent WNF or WNDD?

At present our only method is to control the mosquito population and avoid being bitten. I decided to change the drinking water we put out for the birds we feed and any dog water that's outside every other day. And we don't garden in the evening.

I'll mention one other, low-tech preventive measure in my next post.