* Senegal is fifth West African country to confirm case
* Ebola rumour sparks riot in Guinea's second city
* Medical charity MSF calls for U.N. Security Council action
* Graphic on Ebola outbreak: http://link.reuters.com/jab72w (Adds Sierra Leone minister's dismissal, lifting of West Point quarantine, discharge of ZMapp patients)
By Diadie Ba and Saliou Samb
DAKAR/CONAKRY Aug 29 (Reuters) - The West African state of Senegal became the fifth country to be hit by the world's worst Ebola outbreak on Friday, while riots broke out in neighbouring Guinea's remote southeast where infection rates are rising fast.
In the latest sign that the outbreak of the virus, which has already killed at least 1,550 people, is spinning out of control, the World Health Organisation (WHO) said that Ebola cases rose last week at the fastest pace since the epidemic began in West Africa in March.
The epidemic has defied efforts by governments to control it, prompting the leading charity fighting the outbreak, Medecins Sans Frontieres (MSF), to call for the U.N. Security Council to take charge of efforts to stop it.
Including the fatalities, more than 3,000 have been infected since the virus was detected in the remote jungles of southeastern Guinea in March and quickly spread across the border to Liberia and Sierra Leone. It has also touched Nigeria, where six people have died.
Sierra Leone's President Ernest Bai Koroma dismissed his Health Minister Miatta Kargbo on Friday over her handling of the epidemic, which has killed more than 400 people in the former British colony.
Liberia's Information Minister Lewis Brown said that two African healthcare workers treated with the experimental ZMapp Ebola drug would be released from hospital on Saturday, after making a full recovery.
Scientists on Friday also reported that ZMapp, the drug that last week cured two American aid workers who contracted the disease in Liberia, had cured all 18 lab monkeys infected with the virus in laboratory tests.
Senegal's first case is a student from Guinea.
Senegalese Health Minister Awa Marie Coll Seck said the man turned up for treatment at a hospital in the capital Dakar on Tuesday, concealing the fact that he had had close contact with victims in his home country.
"We are tracing his whole itinerary and also identifying anyone who had contact with the patient, who now that he has been diagnosed is much more cooperative and supplied all the necessary information," the minister said.
A Health Ministry official, who asked not to be named, said that the 21-year-old crossed into Senegal via its southern border with Guinea and had been living in the densely populated Dakar suburb of Parcelles Assainies for weeks. He added that the man appeared to have a good chance of recovering.
The man had been under surveillance by health authorities in Guinea because of his contact with Ebola victims but escaped to Senegal, Seck said.
Residents in Dakar reacted with anger and concern. "When you are sick, why do you leave your own country to export the disease to another?" asked radio host Taib Soce on RFM, a popular station owned by Senegalese music star Youssou N'dour.
In an attempt to prevent the spread of the virus, Senegal last week banned flights to and from three of the affected countries and shut its land border with Guinea.
The country, a regional hub for U.N. agencies and aid groups, has also refused to give clearance for U.N. aid flights to Ebola-hit countries in a move that humanitarian workers say is hampering their ability to respond to the epidemic.
The director of the United States Centers for Disease Control (CDC) warned on Friday of a "catastrophe" if emergency action were not taken immediately to reverse the trend of rising cases.
"There is time to avoid a catastrophe but only if immediate and urgent action is taken at every level," Tom Frieden said in the Sierra Leone capital Freetown.
The World Health Organization (WHO) said on Thursday that the actual number of Ebola cases could be up to four times higher than reported and said 20,000 people in total could be infected before the outbreak ends.
In the remote southeastern Guinean city of Nzerekore, riots broke out on Thursday night over rumours that health workers had infected people with Ebola, a Red Cross official and residents said.
The government of Guinea says it has the epidemic under control, but the number of cases has flared up in southern Guinea, a trend the government blames on people spilling over the borders from Liberia and Sierra Leone.
A crowd of young men, some armed with clubs and knives, set up barricades across Nzerekore on Thursday and threatened to attack the hospital before security forces moved in to restore order. Gunshots were fired and several people were injured, said Youssouf Traore, president of the Guinean Red Cross.
"A rumour, which was totally false, spread that we had sprayed the market in order to transmit the virus to locals," Traore said. "People revolted and resorted to violence, prompting soldiers to intervene."
Local Red Cross workers had to flee to the military camp with their medical equipment. Another resident said the security forces were preventing people leaving their neighbourhoods overnight. More than 400 people have died in Guinea, though the infection rate is slower than in Liberia and Sierra Leone.
The Liberian government said it would end its quarantine of the oceanfront West Point neighbourhood of its capital Monrovia at 0600 GMT on Saturday after residents there cooperated with authorities in putting in place health checks and public education about the disease.
Soldiers opened fire on angry West Point residents last week when the quarantine was imposed, following an attack days before on a holding centre for Ebola victims in the teeming district.
The WHO, on Thursday, unveiled a $490 million road map to bring the outbreak under control over the next nine months.
The International Monetary Fund (IMF) has said it may give more support to affected countries. "We're working on a financing package subject to the approval of the IMF Executive Board to help Liberia along with Guinea and Sierra Leone mitigate any socio-economic impacts of the epidemic," IMF Liberia representative Charles Amo-Yartey said on Friday.
In Freetown, a new WHO-backed mobile laboratory opened this week, speeding up the time needed to test suspected cases.
But often financial pledges have not translated into more clinics and staff on the ground, said Jorge Castilla, epidemiologist with the European Commission's Humanitarian Aid and Civil Protection Department.
"I've seen many declarations, I see treatment centres on the maps but I know they are not working," he said in an interview after a trip to the affected countries.
Suspicion of healthcare workers has dogged government responses to the Ebola outbreak across West Africa.
Frightened by the sight of healthcare workers clad from head to toe in plastic protective gear and wearing protective masks, many locals have shunned their assistance, often preferring to die in their own homes.
So far, more than 120 healthcare workers have died in the epidemic. Liberia reported five new cases of infection among them in a single day this week. (Additional reporting by Emma Farge in Dakar, James Giahyue Harding in Monrovia and Umaru Fofana in Freetown; Writing by Emma Farge; Editing by Daniel Flynn and Tom Heneghan)