MEXICO CITY (AFP) - Mexico said its swine flu epidemic had peaked and was now in decline Sunday as a diplomatic storm brewed over Chinas decision to quarantine dozens of Mexicans who have shown no signs of infection. US health authorities said swine flu was now present in more than half of its states while Colombia became the 19th country to verify a case on its soil. But although the latest figures worldwide underpinned fears of a spread of the disease, Mexico, at the epicentre of the outbreak, expressed confidence that the worst was now over. The epidemic is in its phase of decline. The national peak was between April 23 and 28, Health Minister Jose Angel Cordova told reporters. The number of confirmed deaths from the virus was unchanged from late Saturday at 19, while the number of confirmed infected cases had risen by 33 to 487, he added. Containment measures, including the publics wearing of facemasks and regular hand washing, had been effective in limiting the flus spread, said Cordova. But it was important not to lower ones guard, he said. Health authorities in the US, the only other country where a patient has died of the virus, said that swine flu was now present in 30 states with 226 confirmed infections in 30 states. But Secretary for Health and Human Services Kathleen Sibelius saw grounds to be cautiously optimistic, saying the situation seems to be lessening. More than 400 schools around the United States have shut their doors, dismissing nearly a quarter of a million students as fears grew over the spread of a potentially deadly strain of flu. Schools are not only very good for education but also for the transmission of illness, said Fran Phillips, deputy secretary for public health in the state of Maryland, where nine probable cases of the H1N1 virus have been detected, including several among school students. Officials in Forth Worth, Texas, closed all 144 schools in their district and cancelled extra-curricular activities after a case of the H1N1 virus was confirmed. In Spain, the number of confirmed swine flu cases rose to 40, double the previous number, Madrids health ministry announced, but added that only six victims were still being treated in hospital. The number of swine flu sufferers in Britain meanwhile rose to 18 as a man who had returned from the United States and two schoolchildren tested positive, health authorities said. In China, Mexicos ambassador complained that Mexicans were being treated unfairly after learning that about 50 were being held in isolation across the country. China has cancelled a chartered flight to pick up around 120 stranded Chinese citizens in Mexico, state media reported Sunday. Guangzhou-based China Southern Airlines said it would not fly to collect the Chinese nationals in Mexico as earlier planned, official news agency Xinhua reported. The airline added that it was fully prepared for the charter mission, but it was up to the government as to if and when to implement it, the report said. Ambassador Jorge Guajardo succeeded in visiting 10 Mexicans detained in a hotel in Beijing, but only after a brief standoff with a Chinese official who did not want him to enter. The ambassador said a Mexican diplomat living in the Chinese city of Guangzhou had been subjected to special testing after his return from Cambodia which has no confirmed swine flu cases just because he was Mexican. We of course object every time somebody is singled out for their nationality, and for no other reason, especially when they have no symptoms or when they are coming from a country that has absolutely no cases, he said. No other country in the world is implementing this type of actions, and it is something that concerns the Mexican government a lot. We have stated our objections. Health officials in Hong Kong where tropical temperatures and dense population put it at risk said they were testing 10 more suspected cases, as the search for anyone who may have come into contact with the citys only positive case went on. Thomas Tsang, controller of the Centre for Health Protection, said 15 new cases of people with flu-like symptoms who had travelled to countries with confirmed swine flu outbreaks had been tested since Saturday. Five were negative, while the others were undergoing further tests, Tsang said. Hong Kong was put on its highest health alert after a 25-year-old Mexican man was found to be carrying the A(H1N1) virus on Friday, in the first confirmed case of swine flu in Asia. The World Health Organization (WHO) said Saturday that the virus had not spread in a sustained way outside of the Americas, a condition necessary to declare a full global pandemic. World health experts have also said there is no evidence that the virus is transmitted by eating pork but Egyptian authorities are nevertheless pushing ahead with the mass slaughter of the nations 250,000 pigs. In Cairo, Egyptian riot police clashed on Sunday with stone-throwing pig farmers who were trying to prevent their animals being taken away for slaughter as part of a mass nationwide cull. Between 300 and 400 residents of the hilly Moqattam slum district, where mostly Coptic Christian scrap merchants raise pigs, hurled stones and bottles at police. Anti-riot police replied by firing rubber bullets and tear gas to disperse the demonstrators, most of them youths.