前幾天有朋友拿出一個衛福部的報告說,同性戀得 AIDS 的比例比較高,而我記得之前有個 seminar 的講者也有講到(雖然我忘了那次演講的題目是什麼),不過以病毒學的角度來看,這不合理啊,因為病毒傳染是 species-specific,不是 sex-specific,如果是一樣的傳染途徑,應該是不管性別,所有的性行為都是一樣的機率,於是剛剛就孤狗了一下看有沒有全球性的報告,而不是只有台灣的。全球的當然就是先到 WHO 官網上去找找看,結果發現因為 AIDS 的最大人口都是在非洲,所以主要以非洲為主,不過大家以為的 MSM (men who have sex with men) 為主要傳染途徑其實是因地而異的 [1],在非洲或東南亞國家,愛滋是因 MSM 傳染的比例比較少 [2],主要是以多重性伴侶和 drug injection 為主,而在西方國家或高收入國家,才是以 MSM 為主要傳染途徑。

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為什麼會有這種區別呢?以目前的資料看來,是因為非洲和東南亞的性行為的安全措施(protected)觀念還不普遍,所以在各種傳染途徑都是相同競爭的情況下,多重性伴侶(也就是 heterosexual 為主)的愛滋人口還是最多的,在高收入高開發國家,因為性行為的安全措施觀念已經普及,所以這條傳染途徑被大大降低了,但是為什麼 MSM 得愛滋的比例還是比較高呢?Cold Spring Harbour 的研究報告中說 [2]:

"Both behavior and biology are relevant to the MSM epidemic. Multiple sexual partners, use of recreational drugs and alcohol proximate to sex, and the practice of unprotected anal intercourse all increase HIV risk markedly. Rectal exposure to HIV is more likely to infect than vaginal exposure (Powers et al. 2008; Boily et al. 2009; Baggaley et al. 2010). This is likely owing to the large surface area and thin epithelial layer of the rectum, its capacity for fluid absorption, trauma during anal sex, and the high numbers of immunological target cells in the gastrointestinal tract (Dandekar 2007; Brenchley and Douek 2008)."

這也是為什麼 AIDS 在高開發國家的傳染途徑,即便是在同性戀之間,也還是以男性為主,而不在於女性之間。總結來說,愛滋的傳染在性行為上,跟是否 heterosexual 還是 homosexual 沒太大關係,主要是因為沒有安全措施的性行為,如果 MSM 的保護措施普遍化的話,應該是可以降到和 heterosexual 差不多,根據 Cold Spring Harbor 的資料:

"Despite the worrisome trends among MSM as a whole, the proportion of all reported HIV/AIDS cases that have occurred among White MSM in the United States has dropped markedly from the 80% range in the early 1980s to 25% in 2010."

所以其實拿同性戀的愛滋比例高來反對是沒有道理的,因為重點不在是同性還是異性,而是在於性行為是否有做安全措施。(如果大家這麼怕得到愛滋,可以萬人連署,請總統下令全面禁止性行為。)

"It is thought that the more pragmatic sexual education approaches in Europe and Australia, with structural changes like widespread provision of condom dispensing machines in bathrooms, may explain why the heterosexual epidemic among heterosexuals has been lower than in the United States where comprehensive sex education and condom advertising and distribution have been comparatively curtailed (Dworkin and Ehrhardt 2007)."



參考資料:

1. WHO UNAIDS Global AIDS Update 2016

2. WHO: Understanding the modes of transmission model of new HIV infection and its use in prevention planning

3. SH Vermund and AJ Leigh-Brown, The HIV Epidemic: High-Income Countries. Cold Spring Harb Perspect Med (2012)







 

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