In January 2007, in a village in Kenya, a Kenyan public health officer demonstrates how to spray a mixture of insecticides and water to prevent mosquitoes from entering houses during an outbreak of Rift Valley Fever

This is the VOA Special English Development Report.

Rift Valley fever mainly affects farm animals. But the virus can also infect humans, and South Africa has been experiencing an outbreak. The National Institute for Communicable Diseases reported two hundred twenty-five confirmed human cases as of July second. Twenty-five of the people died.

South Africa launched an expanded program to watch for public health threats during the World Cup. Health officials said last week that there were no cases of Rift Valley fever in tourists. The majority of farms affected by the outbreak are outside areas generally visited by travelers.

Most of the cases have been found in farm workers in two provinces: Free State and Northern Cape.

Most human infections with Rift Valley fever are caused by direct or indirect contact with diseased animals. Infected mosquitoes can also pass the disease to humans. So can drinking unpasteurized or uncooked milk from infected animals.

Most human cases of the disease are minor. Some patients do not get sick at all. Others may get flu-like symptoms such as fever, muscle and joint pain and headaches. Patients normally recover within seven days.

But patients with more severe forms can go blind. Some develop encephalitis, a brain disease that can lead to headaches, coma or seizures. And some patients bleed to death.

The World Health Organization says Rift Valley fever was first discovered in Kenya in nineteen thirty-one. In the year two thousand, it was identified in Saudi Arabia and Yemen. These were the first reported cases of Rift Valley fever outside of Africa.

Current vaccines to protect against Rift Valley fever are limited to use in animals. But researchers are working to develop the first human vaccine.

In a new study, scientists in the United States tested vaccines made with two kinds of inactivated virus. They said tests in mice showed that their new vaccines are safer than live-virus vaccines and appear to work just as well. Using live virus in vaccines can increase the risks.

The researchers are at the University of Pittsburgh and the University of North Carolina. Their study appeared in the journal PLoS Neglected Tropical Diseases, published by the Public Library of Science,.

And that's the VOA Special English Development Report, written by June Simms. You can read and listen to all of our reports at voaspecialenglish.com. You can also find us on Facebook, Twitter, YouTube and iTunes at VOA Learning English. I'm Steve Ember.