Hepatitis C virus can survive in syringes for up to 63 days

Hepatitis C virus (HCV) might still be transmissible via syringes long after their first use, according to a US study presented at the 17^th Conference on Retroviruses and Opportunistic Infections (CROI) last week in San Francisco. Larger syringe size and colder ambient temperature were associated with longer survival, up to two months.

A large proportion of people with chronic hepatitis C contract the virus through shared drug injection equipment. HCV infection rates range from around 30% to as high as 90% in different groups of injection drug users, much higher than the prevalence of HIV.

While harm reduction measures such as needle exchange programmes have dramatically reduced new HIV infections among injectors, they have had a lesser impact on hepatitis C. Furthermore, HCV transmission occurs ten times more often than HIV transmission from an accidental needle stick.

Elijah Paintsil and colleagues from Yale School of Medicine in Connecticut designed a study to test the hypothesis that this greater likelihood of infection might be due to longer survival of HCV in syringes.

The researchers developed a new laboratory test to assess the viability of HCV in residual blood in syringes. Because HCV taken directly from infected people cannot be grown in the laboratory, they used a special genotype 2a "reporter" virus that can live in cell cultures.

The investigators first prepared syringes by loading them with HCV-spiked blood. They looked at two types of syringes and blood volumes: a low-volume insulin syringe with a permanently attached needle filled with 2 µL of blood, and a higher-volume tuberculin syringe with a detachable needle filled with 32 µL of blood. People who inject hormones – for example, transgender people or body-builders – usually use larger syringes.

Some syringes were tested immediately and others were stored for periods ranging up to two months. Syringes were stored at three different temperatures: 4ºC (similar to an average refrigerator), 22ºC (room temperature in a temperate climate) and 37ºC (body temperature or a very warm climate). The syringes were then flushed out and the recovered virus was tested for infectivity in cell cultures.

The researchers found that cell cultures showed varying levels of HCV infectivity. Looking at the proportion of syringes containing infectious virus, in the low-volume scenario the likelihood of finding infectious virus fell rapidly in syringes stored at 37ºC, and none contained viable HCV after one day of storage.

At 22ºC, one-third of syringes still had infectious virus at day one, but none did at day three. At 4ºC, viable HCV remained in about two-thirds of syringes at day one, about one-quarter at day three and about 5% at day seven.

The pattern was not as consistent for the high-volume scenario. At the coldest temperature, nearly all syringes still had viable HCV at day seven, about half did at day 35, and a small proportion did even at day 63 (nine weeks).

The number of syringes containing infectious virus initially decreased more rapidly at the two higher temperatures, but then levelled out. At room temperature, about 70% had viable HCV at day seven and about 40% did at day 35. At 37ºC, the proportion was just over 50% at day seven and only slightly lower at day 35. Again, a small proportion still contained infectious HCV after 63 days.

Turning to the titre or amount of viable HCV, in the low-volume scenario the amount of virus in the syringes showed a biphasic rate of decay, with an initially rapid decline followed by a slower decrease. Again, the amount of virus varied according to temperature. Infectious HCV fell to an undetectable level between days two and three at 37ºC and by day three at 22ºC; at 4ºC, however, a small amount remained at day seven.

In the high-volume scenario, viable virus levels declined until about day seven at the two higher temperatures and about twice as long at 4ºC. Levels then remained low but stable for the remainder of the time studied at all temperatures.

The researchers concluded that survival of infectious HCV is dependent on syringe type and size, with detachable-needle syringes and high-volume syringes being more likely to transmit the virus. Further, lower temperature preserved viable HCV in low-volume syringes more than in high-volume ones.

Comparing with HIV, they found that HCV and HIV in low-volume syringes show a similar time course, but HCV appears to survive longer than HIV in high-volume syringes.

Speaking at an accompanying press conference, Dr Paintsil said these findings have implications for harm reduction efforts, suggesting that it is probably advisable for needle exchange programmes to provide insulin syringes rather than larger ones. However, he emphasised, "the bottom line is not to re-use syringes", and programmes should give out enough so that users do not have to share.

In response to a question, he said that more studies should be done to determine whether HCV transmission via injection drug use varies between warm and cold climates, or between summer and winter.

Further information

You can view the abstract on the official conference website.