Find the mad-cow infection route

狂牛病の感染経路

On Oct. 18, the Health and Labor Ministry started testing all cattle for meat processing, estimated at 1.3 million head annually. It was predicted then that perhaps a few more cases might be discovered, given the highly infectious nature of the disease. Over the past month about 90,000 head throughout the country have been tested, and all but one have tested negative.

Both cases have a few things in common. The two animals that tested positive were milch cows about 5 years old, and they were both raised in Hokkaido. BSE has an incubation period of two to eight years. So it is possible that the cattle feed used in the region contained prions, the protein agent that causes the brain-wasting malady.

In particular, the second case should provide important clues to the infection route. The first case has proved something of a mystery in spite of extensive investigations. Following the second outbreak, though, further analysis will hopefully lead to the discovery of a common source of infection. One vital question is whether the feed contained meat-and-bone meal purchased from suspect countries such as Britain. The question must be exhaustively addressed by tracing all of the feed imported over the past five years or more.

BSE is so infectious that a cow fed with meat-and-bone meal containing just 0.1 gram of infected cow-brain matter may contract the disease. To dispel consumer unease once and for all, the infection source must be identified as soon as possible. The blanket testing that started in October was aimed primarily at preventing contaminated meat from entering the distribution system. As a result, beef from cattle less than 3 years old — which are thought to be safe — hit the market first. Dairy cattle more than 5 years old were not shipped because of fears they might be infected.

But making such an arbitrary distinction is practically impossible when large numbers of cattle are involved. Lifting artificial limits on the shipment of older dairy cattle should also help to determine the extent of BSE infection here. What needs to be done, first and foremost, is to unravel the route and the means of infection. For that, it is essential to test all cattle.

Beyond that, all cattle that die of various diseases, estimated at 160,000 head annually, should also be tested. The selective testing now in practice will not do. It is also essential to check for BSE-causing pathogens in all older cattle that die of illness.

The discovery of the second BSE case is disturbing, but not the sort of thing that should send alarm bells ringing across the nation. All cattle are now being tested for the disease. Meat in distribution does not include infection-prone parts, such as brains. The use of meat-and-bone meal in animal feed is entirely prohibited. On the whole, the safety measures here are more severe than even those in Europe, where mad cow disease has reached serious proportions. There is no reason why consumers here should stop buying beef.

Nevertheless, the government should do everything it can, including testing, to restore consumer confidence. Intrusive inspections may be warranted to make sure that cattle feed does not contain dangerous ingredients, such as meat-and-bone meal. Steady confidence-building efforts are also needed to prevent damage due to unfounded rumors.

The second cow in question, unlike the first one, showed no visible symptoms of the disease, such as inability to stand. The fact that an infected cow does not always exhibit such signs has grave implications: Meat from apparently normal but actually infected cattle could have entered the market. It is indeed possible that infected meat may have reached the table in the past several years because of failure to detect BSE pathogens in cattle feed.

BSE may be transmitted to humans, causing Creutzfeldt-Jakob disease, the fatal human equivalent of mad cow disease. In Britain, a total of 180,000 cattle reportedly have become infected since the 1980s, and about 110 people have contracted CJD. How many heads of cattle in Japan are infected, or likely to become infected, is hard to tell at the moment. But the number is thought be dozens at the most, according to reports. The danger of human transmission is considered extremely low but cannot be entirely ruled out. We had better be vigilant.

The Japan Times: Nov. 24, 2001
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      陽性と判定された二頭には共通点がある。5歳前後の 乳牛で、北海道で飼育されていた。狂牛病の潜伏期間 は2年から8年だから、飼育地で使われた飼料に狂牛 病の病原体プリオンが含まれていた可能性がある。一 例目の調査では成果がなかったが、二例目と合わせて 分析すれば、共通の感染源の発見につながるかもしれ ない。問題は英国などから購入した肉骨粉が飼料に含 まれていたかどうかだ。これについては、過去5年よ り以前に輸入された飼料の追跡調査を行うべきだろう。 狂牛病の感染性は高く、感染した牛の脳がわずか0.1 グラム含まれた肉骨粉を与えられた牛は感染の危険が ある。消費者の不安を払拭するためにも感染源の特定 を急がねばならない。全頭検査は感染した牛の肉が流 通機構に入るのを防ぐことを目的としており、それに より安全とされる3歳以下の牛が市場に出回り、感染 の危険性が高い5歳以上の牛は出荷されない。

      しかし多数の牛が対象になると、そうした恣意的な 区別は不可能になる。5歳以上の牛の出荷制限を解除す れば、感染の範囲を特定する助けになるだろう。今す べきことは、感染の経路と実態を明らかにすることだ。 病気で死ぬ牛は年に約16万頭いるといわれるが、これ らについても抜き取り検査でなく、病原体プリオンの 有無の確認を含めた全頭検査を実施すべきだろう。 二例目の感染の発見は重大な危機というわけではな い。全頭検査が行われている上、流通する食肉は脳な ど感染しやすい部分を含んでいない。飼料としての肉 骨粉の使用は禁止された。日本の安全対策は、感染が 拡大している欧州のそれよりも厳しい。消費者が牛肉 を買い控えなくてはならない理由はない。

      それでも政府は、消費者の信頼回復のため、厳重な 検査により飼料の安全性を保障するなど、打てる策は すべて打たなくてはならない。風評被害を防ぐ努力も 必要だ。狂牛病は人間に感染するとクロイツフェルト･ ヤコブ病を引き起こす。英国では110人が感染したと いう。日本国内での感染は何十頭のレベルと推定され る。人間への感染の危険性は低いが、全くないという わけではない。注意を怠らないようにしたい。

The Japan Times Weekly
Dec. 1, 2001
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