MOSCOW, Russia (AFP) — The promise of a spectacular World Championship showdown between the USA and Jamaica has dissolved in a spate of doping cases that leave the Moscow sprints looking more likely to be a showcase for Usain Bolt.

A resurgent Tyson Gay had sent a ripple of excitement through US athletics in winning the 100m at the US trials at Des Moines in 9.75sec.

Gay, whose personal bests of 9.69 in the 100m and 19.58 in the 200m had long made him one of the few who looked capable of challenging Bolt, appeared poised to do just that having moved beyond a string of debilitating injuries.

But Gay won’t race in Moscow, having tested positive for a banned drug not just once but apparently several times this season.

His disgrace is one of a welter of positive drug tests that have cast a shadow over the World Championships, with former 100m world record-holder Asafa Powell of Jamaica and compatriots Sherone Simpson and Veronica Campbell Brown also caught out.

Justin Gatlin, the 2004 Olympic gold medallist who finished runner-up to Gay at the US trials, will spearhead the American challenge to Bolt.

Gatlin’s best time this season is 9.98sec, but he did hand Bolt a rare defeat in Rome earlier this year and the 31-year-old will be looking to rehabilitate himself totally after his four-year ban for doping which ended in 2010. Jamaican Nesta Carter, whose 9.87 in Madrid on July 13 makes him the second-fastest man in the field, should also be in medal contention.

Another Jamaican, 2012 Olympic bronze medallist Warren Weir, looks the only man capable of getting close to Bolt over 200m. He won the 200m title at the Jamaican trials in 19.79, Bolt skipping the event since his world title assured him of a berth.

Gatlin, twice banned for doping himself, said he wouldn’t allow himself to be distracted by the doping scandal.

“The show must go on,” Gatlin said. “I think a lot of athletes think like that. Staying concentrated and winning for your country, that’s what counts.”

Carmelita Jeter, America’s reigning women’s 100m world champion, left a press conference in Monaco rather than discuss the doping cases. Jeter didn’t compete at the US trials as she continued her recovery from a quadriceps injury, but the 33-year-old will head a strong US challenge to Jamaica’s two-time Olympic champion Shelly-Ann Fraser-Pryce in the women’s 100m.

English Gardner, who turned professional in June and began training with coach John Smith after winning the US collegiate 100m crown, won the women’s 100m at the US trials in 10.85sec, and Barbara Pierre has twice posted the same time this season.

Fraser-Pryce leads the world in both the 100m (10.77sec) and 200m (22.13) this season and with Campbell Brown unable to defend her 200m title, after her positive test for a banned drug, heads Jamaica’s bid in the half-lap sprint.

American Allyson Felix has focused all her attention this season on regaining the 200m world title she won 2005, 2007 and 2009. However, Felix admitted her training got off to a slower start than usual after she took a post-Olympics break, and she finished second at the US trials to collegiate champion Kimberlyn Duncan. Felix’s top time in 2013 of 22.36 puts her just seventh-fastest for the season, with Fraser-Pryce and Murielle Ahoure leading the way at the distance so far this year.

Gay’s absence will leave the United States struggling to reassert their authority in the flat sprints, but the Americans remain dominant in the hurdles.

David Oliver, Aries Merritt and surprise trials winner Ryan Wilson will attempt a first US sweep of a global 110m hurdles final since the 1960 Olympics.

Rising US star Brianna Rollins, who matched the fourth-fastest 100 hurdles time in history at the US trials, is set to challenge Australia’s Olympic champion Sally Pearson, who won her first Diamond League race of the season in London after being hindered by injury earlier in the season.