Intravenous drug users (IDUs) appear to have been the first major group severely affected by HIV both in Myanmar and in China, particularly in Yunnan province 
. Previous studies have shown that 37.5% and 54% IDUs are infected with HIV in the two countries because of frequently sharing the needles among the users 
. Moreover, it has been indicated that the co-infection of HIV-infected patients with hepatitis viruses especially HCV or/and HBV is very common, although the co-infection ratios vary depending on the geographic regions, risk groups, and the type of exposure involved 
. The first large outbreak of HIV in China was identified in 1989 among the IDUs in Dehong Prefecture, Yunnan province on the Myanmar border in southwest China 
. Dehong Prefecture is proximal to one of the world's largest illicit drug production and distribution center, the “Golden Triangle”, and is an important transfer station for drug trafficking 
. Because of the geographic reasons mentioned earlier, Yunnan province is one of the most severe HIV/AIDS epidemic provinces in China, while Dehong prefecture is one of the most prevalent regions in Yunnan province 
. Therefore, a transnational study was carried out in 2009 to investigate whether there was a difference in HIV, HBV, or HCV epidemic among the IDUs from the proximal regions in China and Myanmar. 403 IDUs were recruited from Yingjiang county, Dehong prefecture and 318 from northeastern Myanmar quite adjacent to Dehong prefecture. ELISA screening showed that the infection rates among IDUs were 33.7% for HIV, 69.0% for HCV, 51.6% for HBV, 31.8% for HIV-HCV co-infection, 20.1% for HIV-HBV co-infection, 19.1% for HIV-HBV-HCV triple infection, and 15.6% for HIV-HCV-HBV negative in China; the counterparts in Myanmar were 27.0%, 48.1%, 43.1%, 23.9%, 11.3%, 10.4%, and 29.9%, respectively ( and ).
From the results, it is obvious that the epidemic situation of infection and co-infection of HIV, HCV and HBV was more common among the IDUs in southwest China than in northeastern Myanmar ( and ), though the difference of the HIV epidemic between two countries is not statistically significant. There are few works about co-infection of IDUs from both two countries, especially Myanmar, but all showed HIV was very prevalent among the IDUs in both southwest China and northeast Myanmar 
. The trends of the HIV prevalence among the two countries' IDUs indicated a decrease in the overall prevalence of HIV in recent years 
. In Myanmar, the HIV prevalence among the IDUs peaked in the early 1990s at over 70% before beginning a slow but steady decline during 2005–2006 
. The HIV prevalence among the Burmese IDUs in 2008 was 37.5% (range: 37.2–54%), in which Myitkyina located in north Myanmar is 54%, while Muse and Lashio, both located in the east Myanmar, are 43.33% and 37.43%, respectively 
. The present study found 27% northeastern Myanmar IDUs was infected with HIV (), which was consistent with the trend. In Yunnan province, located in southwest China, the average prevalence rate among the IDUs increased from 1992 to 2004 (2.7% in 1992, 15% in 1995, 30% in 1999, 32.4% in 2004) and decreased to 28.4% in 2007; while in 7 counties, including Yingjiang, the HIV prevalence rates among the IDUs exceeded 40% 
. The results show 33.7% of IDUs in Yingjiang county was HIV positive (), which was lower than the 52% prevalence in 2005 
. It was also in accord with that in Yunnan province.
HIV, HBV, and HCV have similar routes of transmission, such as sharing of needles to inject drugs 
. Among the IDUs in Myanmar, 31% in Yangon, 22% in Myitkyina, and 19% in Lashio reported sharing of needle during the last injection in 2008, respectively 
. Surveys in China indicated that 40% of the IDUs engage in needle-sharing behavior 
. Moreover, the present analysis showed that the Chinese IDUs injected drugs for a longer time than the Burmese, though the former were younger than the latter and earlier in first usage of drug injection. Many studies concluded that the high prevalence of HIV and HCV among IDUs was associated with the duration of injection of drug usage 
. More frequent needle sharing and longer time of drug injection may be the reason why HIV, HBV, and HCV were more prevalent in China than in Myanmar in this study.
Rates of HBV and HCV infection are higher than those for HIV in both China and Myanmar (, ), which is consistent with the previous reports 
. HCV infection in IDUs was the highest in both the countries, compared with other infections (, ), which may be associated with the fact that IDUs acquired HCV more rapidly than HIV and HBV after starting to use drugs 
. Evidence from needle-stick injury studies suggested the injection-related transmission probability for HCV transmission was up to 10 times greater than HIV 
. The former can be acquired after the onset of injection drug usage through sharing infected needles, whereas HIV is infected after increasing durations of injection drug usage 
. Studies of the impact of needle exchange program on the incidence of HIV and hepatitis infection in the USA showed that the needle exchange program participation was associated with 33% HIV infection and more than 80% HCV or HBV infection reduction 
. In other words, the IDUs were more prone to be infected with HCV or HBV, compared to HIV, which was similar to the present study's result with mono-infection or co-infection among all HCV, HBV, or HIV infection. The proportion with HIV mono-infection among HIV-infected IDUs was significantly lower than the HCV mono-infection or the HBV mono-infection among the HCV-infected or the HBV-infected IDUs from both China and Myanmar (). The result means that once infected with HIV, the patients had been HCV and/or HBV positive. Meantime, both China and Myanmar are HBV highly prevalent (≥8%) countries 
. These may be the reasons why the HBV infection was also higher than HIV in this study.
Proportion with mono-infection or co-infection among all HCV, HBV or HIV infected IDUs from China or Myanmar.
The prevalence of HIV, HCV, and HBV infection and co-infection was significantly higher in ethnic minorities (Dai and Jingpo) than in the Han ethnic group ( and ), which is consistent with the situation of the HIV infection among Yunnan province 
. In Yunnan province, HIV prevalence among the IDUs was higher for ethnic minorities, including Jingpo, Dai, and Yi, than for the Han ethnic group, even after controlling for different levels of education across ethnicities 
. Though the exact reason was not known, it might be because of limited access to health care, habits, and lower public health awareness leading to prevalence in the ethnic minorities.
In conclusion, this study indicates that the prevalence of co-infection with HIV and HCV and/or HBV is very common among IDUs both in southwest China and northeast Myanmar. However, the HIV epidemic appears to be in a downward trend, compared with previous studies, though the epidemic in China is more severe than in Myanmar. This decrease is a response with harm reduction programs conducted in both two countries. In China, the first needle and syringe programs (NSP) were initiated in 1999 in Yunnan and Guangxi provinces 
. The year 2004 was very important in China HIV/AIDS prevention and treatment. Many counseling and harm reduction programs, for example, free HIV Voluntary Counseling and Testing (VCT) and Methadone maintenance treatment programs (MMT), were launched 
. A report showed that the HIV-infected ratio among the IDUs from Yunnan province peaked in 2004, and then started to decrease 
, which means that harm reduction programs were very useful in reducing HIV prevalence among IDUs. However, one study demonstrated that without such programs, the HIV prevalence among the IDUs can rise to 40% or more within 1 or 2 years after the virus is introduced in the communities 
. This implies that the two countries need to enhance harm reduction programs continuously, to avoid increasing the HIV, HBV, and HCV prevalence among the IDUs again.