There is a new flu virus going around. It initially looked quite lethal, and caused panic. Now it is clear that it has killed relatively few victims - and many of those have underlying conditions. It is particularly dangerous to be the possessor of a pushed-in nose - that is, to be a Pekingese, a pug or a Shi-Tzu.

It is the H3N8 dog flu. The virus, scientists believe, jumped from horses to dogs at least five years ago, but it has never infected a human.

Last week, the United States Department of Agriculture announced that it had approved the first vaccine for it.

While fears of a flu pandemic among humans have shifted from the lethal H5N1 avian flu to the relatively mild H1N1 swine flu, the H3N8 canine flu has been a quiet undercurrent in the United States, rarely discussed except among veterinarians and dog owners in the few areas where it has struck hard: Florida, New York City's northern suburbs, Philadelphia and Denver.

In line with the virologists' adage that the only predictable thing about flu viruses is that they are unpredictable, the dog flu has baffled those following it.

"I don't think we know what this virus is going to do yet," said one of its discoverers, Dr. Cynda Crawford of the University of Florida veterinary school.

When Dr. Crawford began studying it in January 2004, it had come to her notice as a mysterious cough and pneumonia that killed a third of the greyhounds at a Florida dog track. By the next year, she had found it in seven states and had shown that it could be passed by dogs who just rubbed noses on the street or shared a water dish, and that humans could carry it on their clothes. There was a brief flurry of fear that it would kill 1 percent to 10 percent of the country's 70 million dogs.

It has proved about as deadly as Dr. Crawford predicted. She estimates that by itself, it kills 5 percent of the dogs that catch it. Add the deaths at shelters that eliminate the virus by killing all their dogs and disinfecting their cages, and the total mortality rate is 8 percent.

（By contrast, the mortality rate of the 1918 Spanish flu in humans was about 2 percent.）

But it has not spread nearly as vigorously as she expected. It has now been found in 30 states, but almost exclusively in settings where dogs live closely together: shelters, pet stores, kennels and dog schools. Because the owners of these establishments have learned to turn away sick dogs just as school principals facing swine flu send home sick children, the disease's progress has been slowed.

"Probably over 10,000 dogs have been infected," Dr. Crawford said, "but I can't say whether it's 20,000 or 30,000. In a population of 70 million, that's a drop in the bucket."

Dr. Edward J. Dubovi of the veterinary school at Cornell University, another discoverer of the virus, said it is "probably not as well adapted to dogs as it could be." It took five mutations to let it jump to dogs from horses, where it had circulated for 40 years.

Another mutation or two "could make it a very serious issue," he said, but at the moment, "it takes a certain density of dogs to keep it going."

Some veterinarians have found that the dogs that tend to die from it are the "brachycephalics" - dogs with short snub noses.

Just as obesity has proved dangerous to human flu victims because of the weight on their chests, being bred to have a short, bent respiratory tract is dangerous for dogs.

"It really puts a strain on their ability to breathe," Dr. Crawford said. "They can't move air in and out of their lungs."

一种新的流感病毒在四处流传。最初看上去它非常致命，并造成了恐慌。现在清楚了，新流感病毒造成死亡的受害者相对较少，尽管许多受害者具有受传染的基本条件。特别危险的是受害者全有一个探来探去的鼻子，也就是说，那是一只北京哈巴狗，或叫狮子狗。

这就是H3N8狗流感。科学家认为，至少在5年前，这种病毒就从马传给狗了，但它还从没传染给人。

美国农业部上周宣布，它已批准了首批狗流感疫苗。

虽然对人传人流感爆发的恐惧已从致命的H5N1禽流感转向了危害较轻的H1N1型猪流感，美国的H3N8狗流感却已悄然爆发，但除了狗流感流行严重的佛罗里达州、纽约市郊北部、费城和丹佛等少数地区的兽医和养狗人之外，很少有人谈论它。

恰如病毒学家所言，对流感病毒唯一可预见的就是，它们流行起来出人意料，狗流感却阻挠了这些病毒成为下一个流行病源。

狗流感病毒的发现者之一、佛罗里达大学兽医学院的辛恩达·克劳福德博士说："我认为我们还不知道这种病毒传染的走向。"

克劳福德博士2004年1月间开始研究狗流感病毒，当初引起她注意的是，它会引发莫名其妙的咳嗽、肺炎，并导致佛罗里达一场赛狗会1/3的灰狗死亡。隔年，她在7个州发现了狗流感，这表明，狗流感是通过狗在街上嗅来嗅去或是共用饮水盘传播的，人的衣服也会携带病毒。有种恐慌忧虑简言之就是，狗流感会使美国7000万只狗当中1%至10%的狗一命呜呼。

事实已残酷地证明了克劳福德博士的预言。她估计，单是狗流感病毒，就会使5%染上狗流感的狗死掉。把狗全部扑杀以便消灭病毒、消毒笼舍的那些收容所，死狗数量增加，总死亡率为8%.

（相比之下，1918年西班牙流感造成人类死亡的比率约为2%.）

而狗流感的传播远非她预料的那样厉害。现有30个州已发现了狗流感，但几乎完是发生在狗群密集的环境里：收容所、宠物商店、养狗场和训狗学校。由于这些场所的业主已懂得隔离病狗，犹如校长对付猪流感时会将生病的儿童送回家，狗流感的发作已被延缓下来了。

克劳福德博士说，"大概1万多只狗已被感染了。但是我不能说是2万还是3万只。在7000万只狗里，受感染的数量不过是九牛一毛而已。"

狗流感病毒的另一位发现者、康奈尔大学兽医学院的爱德华·J·杜波维博士说，病毒"可能对狗适应得并不那么理想".病毒经过5次变异才从马传染给狗，而在马之间的传播已有40年了。

他说，再有一两次变异"可能问题就很严重了，"但是目前"病毒需要在狗群有一定密度的情况下才能存活下去。"

一些兽医发现，死于狗流感的狗往往是"短鼻品种",即短狮子鼻的狗。

正如肥胖已被证明由于胸腔承重过大，对患流感的人是相当危险的一样，对狗来说，培育呼吸道又短又弯的品种也是相当危险的。

克劳福德博士说，"对狗的呼吸能力来说，这实际上是处于一种紧张状态。呼吸道又短又弯无助于空气进出肺部。"