Experts claim disease is on the wane as Merkel and Zapatero attempt to limit diplomatic row over infection's source

Cases of infection by the deadly E coli bacterium have continued to spread around the world from its source in northern Germany, reaching a dozen countries by Friday evening as the German chancellor and Spanish prime minister moved to calm a diplomatic row over the source of the infection.

The Czech Republic and the US have joined the list of those dealing with cases amid concern that some of those infected had not visited Germany and so must have been infected elsewhere.

Angela Merkel has said she would push for EU help for farmers in Spain – whose cucumbers were wrongly blamed by German authorities for the outbreak.

Germany reported a further 200 cases diagnosed on the first two days of the month as the total number of people infected worldwide rose above 1,800. The total number of reported deaths in Germany is 19. Just 11 cases have been confirmed in England.

"All these cases except two are in people who reside in or had recently visited northern Germany during the incubation period for the infection … or, in one case, had contact with a visitor from northern Germany," said the World Health Organisation in a statement.

The spread of cases in Germany has begun to slow, however, raising hopes that the outbreak might be controlled as Germans heed warnings to wash and prepare vegetables carefully and avoid raw cucumber, tomatoes and lettuce.

"There is no reason for hysteria because it's not spreading and it's not increasing, it is decreasing," said Dr Reinhard Brunkhorst of the German Nephrology Society.

Scientists are still working to understand the new strain better. Professor James Paton, director of the Research Centre for Infectious Diseases at the University of Adelaide, said the O104:H4 strain causes disease by colonising the human gut and releasing a toxin called Shiga into the intestines.

"The toxin is then absorbed into the blood and then attacks the kidneys and the small blood vessels, resulting in a life-threatening condition called haemolytic uraemic syndrome (HUS). Damage to the blood vessels supplying the gut is also largely responsible for the severe, often bloody diarrhoea suffered by patients with ... infection," he said.

Typically, fewer than 5% of patients with diarrhoea caused by this type of bacterium develop HUS but in the current European outbreak, the proportion of people with HUS is very much higher, about 25%, indicating that it is extremely virulent, said Paton. Possible explanations for this could include higher production of the Shiga toxin, or better absorption of the toxin by the gut.

The E coli O104:H4 strain is also resistant to several antibiotics. "But this is not an issue in the current outbreak, because antibiotics are not used to treat patients with ... infection or HUS," said Paton. "Indeed, such therapy may make the disease worse … massively increasing the amount of toxin released into the gut."

According to the European Society of Clinical Microbiology and Infectious Diseases, German scientists have recommended to doctors that antibiotics should not be withheld in all situations, and that a type of antibiotic called carbapenem can probably be used without putting the patient at additional risk.

Spain wants its farmers to be compensated for losses of €200m a week as fresh vegetables are left to rot. Zapatero said the EU "should have reacted quicker and more vigorously" as soon as tests showed Spanish vegetables were not to blame.