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."

    一種新的流感病毒在四處流傳。最初看上去它非常致命，并造成了恐慌。現在清楚了，新流感病毒造成死亡的受害者相對較少，盡管許多受害者具有受傳染的基本條件。特別危險的是受害者全有一個探來探去的鼻子，也就是說，那是一只北京哈巴狗，或叫獅子狗。

    這就是H3N8狗流感。科學家認為，至少在5年前，這種病毒就從馬傳給狗了，但它還從沒傳染給人。

    美國農業部上周宣布，它已批準了首批狗流感疫苗。

    雖然對人傳人流感爆發的恐懼已從致命的H5N1禽流感轉向了危害較輕的H1N1型豬流感，美國的H3N8狗流感卻已悄然爆發，但除了狗流感流行嚴重的佛羅里達州、紐約市郊北部、費城和丹佛等少數地區的獸醫和養狗人之外，很少有人談論它。

    恰如病毒學家所言，對流感病毒唯一可預見的就是，它們流行起來出人意料，狗流感卻阻撓了這些病毒成為下一個流行病源。

    狗流感病毒的發現者之一、佛羅里達大學獸醫學院的辛恩達·克勞福德博士說："我認為我們還不知道這種病毒傳染的走向。"

    克勞福德博士2004年1月間開始研究狗流感病毒，當初引起她注意的是，它會引發莫名其妙的咳嗽、肺炎，并導致佛羅里達一場賽狗會1/3的灰狗死亡。隔年，她在7個州發現了狗流感，這表明，狗流感是通過狗在街上嗅來嗅去或是共用飲水盤傳播的，人的衣服也會攜帶病毒。有種恐慌憂慮簡言之就是，狗流感會使美國7000萬只狗當中1%至10%的狗一命嗚呼。

    事實已殘酷地證明了克勞福德博士的預言。她估計，單是狗流感病毒，就會使5%染上狗流感的狗死掉。把狗全部撲殺以便消滅病毒、消毒籠舍的那些收容所，死狗數量增加，總死亡率為8%.

    （相比之下，1918年西班牙流感造成人類死亡的比率約為2%.）

    而狗流感的傳播遠非她預料的那樣厲害。現有30個州已發現了狗流感，但幾乎完是發生在狗群密集的環境里：收容所、寵物商店、養狗場和訓狗學校。由于這些場所的業主已懂得隔離病狗，猶如校長對付豬流感時會將生病的兒童送回家，狗流感的發作已被延緩下來了。

    克勞福德博士說，"大概1萬多只狗已被感染了。但是我不能說是2萬還是3萬只。在7000萬只狗里，受感染的數量不過是九牛一毛而已。"

    狗流感病毒的另一位發現者、康奈爾大學獸醫學院的愛德華·J·杜波維博士說，病毒"可能對狗適應得并不那么理想".病毒經過5次變異才從馬傳染給狗，而在馬之間的傳播已有40年了。

    他說，再有一兩次變異"可能問題就很嚴重了，"但是目前"病毒需要在狗群有一定密度的情況下才能存活下去。"

    一些獸醫發現，死于狗流感的狗往往是"短鼻品種",即短獅子鼻的狗。

    正如肥胖已被證明由于胸腔承重過大，對患流感的人是相當危險的一樣，對狗來說，培育呼吸道又短又彎的品種也是相當危險的。

    克勞福德博士說，"對狗的呼吸能力來說，這實際上是處于一種緊張狀態。呼吸道又短又彎無助于空氣進出肺部。"