Cuban health authorities launched small-scale fumigation efforts in Havana on Friday to fight the spread of the Oropouche virus, but a rainy Caribbean summer, fuel shortages and growing roadside trash heaps are complicating those efforts, workers and officials said.

More than 500 cases of the virus have been registered since May when the disease was first detected in far-eastern Cuba, health officials said this week.

The virus, also known as sloth fever, is transmitted by the bite of mosquitoes and midges and has spread quickly across all of the country’s provinces and major cities, including the capital Havana. Patients complain of fever, body aches and nausea, though the disease is rarely fatal.

Fumigation workers using hand-held, gas-powered blowers fired smoke into dark corners and alleyways in parts of Havana on Friday, though those efforts have been stymied, in part, by limited resources.

“In the past, all the blocks were fumigated every week ... but now, due to fuel (shortages), they focus on specific cases where (fever outbreaks) occur,” said Havana fumigation worker Luís Aguilar.

U.S. health authorities earlier this week said 21 US citizens who had visited Cuba during the summer months returned home with cases of Oropouche.

Cuba has reported relatively few cases compared to other countries, including Brazil, where the virus is also present, and no fatalities.

But economic crisis and shortages of fuel, food and medicine have hampered efforts to control the spread of such mosquito-borne illnesses as Oropouche and Dengue fever, authorities have said.

Frequent power outages mean many sleep with windows open during the hot Caribbean summer, and few Cubans have access to insect repellent.

Trash heaps have grown in size on street corners, which, combined with summer rains, provide ample breeding grounds for biting insects, authorities said.

There are no specific treatments or vaccines available for Oropouche.