美遭遇史上最严重西尼罗河病毒流行季节

目前美国正处于最严重的西尼罗河病毒流行季节，确诊病例数量已达1,118例，其中41例死亡，所有死亡均由神经侵袭型病毒引起。最近几周内，感染病例数量急剧增加，联邦卫生官员称这是美国自1999年发现该病毒以来最严重的暴发。

美国疾病预防控制中心(CDC)虫媒传染病部主任Lyle R. Petersen博士表示，这是8月前3周报告的最高感染病例和死亡病例数量。7月前3周报告的感染病例数量仅为25例。共38个州出现人类感染病例；47个州在鸟、蚊子和人类中发现该病毒。最令人不安的是，629例中的56%感染的是神经侵袭型病毒。其余44%感染非侵袭型，表现为发热和肌痛。神经侵袭型病例较易被发现，因为这类病例多数会到医院就诊。而发热相关病例数量显然被明显低估。Petersen博士认为，在所有感染非神经侵袭型病毒的病例中，实际上仅有2%~3%被报告。这是因为这类病情较轻的患者往往并不寻求医生诊治，或者医生并未意识到其为西尼罗河病毒感染病例。大部分(75%)病例出现在德克萨斯州、密西西比州、路易斯安那州和俄克拉何马州，其中50%出现在德克萨斯州。

德克萨斯州卫生服务部负责人David Lakey博士表示，达拉斯市和北部郊区的病例数量特别高。目前已确诊586例病例和21例死亡病例。下次德克萨斯州病例更新报告中将包括至少4例新增病例。

Petersen 博士表示，2003~2011年期间仅发现10例死亡，而目前的病例多于过去几年的总和。随着西尼罗河病毒流行季的继续，病例数将可能持续增加，高峰通常出现在8月中旬，但是患者一般在感染数周后才出现症状并寻求诊治，这时也才有病例上报，因此预计将有更多病例被上报，并且感染风险将持续至9月底。

实验室数据显示，炎热天气可增加蚊虫传播病毒的可能性。此次西尼罗河病毒大流行可能是因为去年冬季暖和、入春早及夏天特别炎热造成的。欧洲和非洲的最大规模暴发都是在异常炎热时出现。

CDC目前正在调查该病毒是否已突变成更危险类型，但尚未发现相关证据。目前尚无针对性治疗，在暴发期间不推荐进行常规检测。血液供体筛查项目已在26个州发现西尼罗河病毒感染血液。目前正在对所有献血者进行该病毒的筛查。

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By: MICHELE G. SULLIVAN, Ob.Gyn. News Digital Network

West Nile virus has never struck the U.S. as hard as it is striking now.

The number of confirmed cases now reaches 1,118 – including 41 deaths, all from the neuroinvasive form of the disease. The number has risen dramatically in recent weeks, leading federal health officials to call the outbreak the worst since the virus arrived in the United States in 1999.

"This is the highest number of cases and deaths that has ever been reported in the period of the first 3 weeks of August," said Dr. Lyle R. Petersen, director of the CDC’s Division of Vector-Borne Infectious Diseases. By the third week in July, only 25 cases had been identified, he added.

Human cases occurred in 38 states; 47 states have found the virus in birds, mosquitoes, or in humans, Dr. Petersen said during a press briefing.

Most troubling is that 56% of the cases (629) have been the more aggressive neuroinvasive disease, he said. The remaining 44% were the noninvasive type characterized by fever and myalgia.

Neuroinvasive cases are somewhat easy to track, since most people who develop that form end up in a hospital. The number of fever-related cases is almost certainly a vast underestimate.

"We think that only 2%-3% of all people with [nonneuroinvasive disease] actually get reported. The reasons are very simple. Many people with milder disease don’t go to the doctor or the doctor does not recognize it as a case. There’s no specific treatment for it, and even during an outbreak, routine testing is not recommended," Dr. Petersen said.

Most of the cases (75%) have occurred in Texas, Mississippi, Louisiana, and Oklahoma, with 50% in Texas alone. "People are understandably very concerned, especially in the hardest-hit areas," Dr. Petersen said.

The case load is particularly high in the city of Dallas and its northern suburbs, said Dr. David Lakey, commissioner of Texas Department of State Health Services.

"We are talking about a disease that’s impacting the lives of hundreds – even thousands – of people," Dr. Lakey said at the briefing. "As of right now we have confirmed 586 cases and 21 deaths. ... I know of at least four more," that will be included in the next Texas case update.

From 2003 to 2011, there were 10 deaths. "So now we have many more cases" than in all the previous years of West Nile combined. "This will certainly be our worst season ever." Cases will probably continue to rise as the West Nile season wears on, Dr. Petersen said.

"The peak usually occurs in mid-August, but it takes a couple of weeks before people become sick, go to the doctor, and get diagnosed and reported. So we expect many more cases to occur and the risk of infection will be with us through the end of September."

A mild winter, early spring, and exceptionally hot summer have probably contributed to the high West Nile activity. "It’s a complicated cycle within the interaction of birds, mosquitoes, and people," Dr Petersen said. "Hot weather seems to promote major outbreaks. The biggest outbreaks in Europe and Africa have occurred during periods of abnormally hot weather. And we know from lab experience that heat can increase transmissibility through mosquitoes."

The CDC is investigating whether the virus has mutated into a more dangerous form, but so far, Dr. Petersen said, there is no evidence to support this.

The blood donor screening program has identified West Nile-infected blood in 26 states. "All donors are now screened for West Nile virus, and by deferring those, we have taken an important step in protecting the blood supply," he said.