Malaria is a major cause of death and disability in the developing world. Control of the disease is a thus a major public health goal. Inexpensive and effective means of prevention are the subject of much investigation.
Researchers led by Mark Rowland, Ph.D., from the London School of Hygiene, report in the June 9 issue of the British medical journal, The Lancet, that an effective method involves treating domestic animals instead of people or their homes.
Malaria is caused by one-celled parasites belonging to the Plasmodium group of organisms. The parasites are transmitted by mosquitoes that have previously bitten an infected person or animal. As part of their life cycle, the parasites invade red blood cells and destroy them, causing anemia that can be severe or even fatal.
Standard means of preventing the spread of malaria usually involve treating houses with insecticides to kill malaria-carrying mosquitoes, or having people sleep under insecticide-treated nets. While both methods can be effective, the authors of the paper state, they are often too expensive for the very poor (or for agencies that help them) to support.
The investigators tried a new method in six refugee settlement areas in a region of Pakistan where malaria is common. Residents of the villages, as in many areas of rural south Asia, keep their domestic livestock, mostly cattle and goats, in the same compounds where people live and sleep. Thus, the animals can serve as a source of infection if they are bitten by mosquitoes that also bite people. Some mosquitoes prefer animals, others prefer people; the type typically found in the area studies preferred animals.
The project ran for three years; in the first and third years, three of the villages sponged insecticide on their cattle; in the second year the other three villages received the insecticide. The villagers sponged deltamethrin, an insecticide which kills mosquitoes on cattle for over a month, on the animals once every six weeks during peak mosquito season between July and October.
Over the course of the project, the researchers conducted surveys of the village populations every October, the peak time for malaria transmission, to measure the frequency of new cases of infection by the parasite. The overall occurrence of malaria in the villages was also tracked by free clinics in each village.
Dr. Rowland and colleagues report that treating livestock every six weeks significantly reduced the overall incidence (new cases) of malaria caused by two species of parasite common in the area under study — one type was reduced by 56 percent, the other by 31 percent. These results were documented by the village clinics.
In addition, yearly testing of the participants also revealed a reduction of between 40 and 54 percent in the occurrence of the two main types of parasite.
The livestock-sponging campaign was popular with the villagers, the authors reported. They attributed this to the fact that the insecticide also killed ticks and other parasites infecting the animals, and thus raised the production of milk by the treated cattle.
In their discussion, Dr. Rowland and coauthors conclude that “livestock sponging is a new way to control malaria,” especially in areas where livestock are the primary sources of food for mosquitoes carrying the disease, as they are in this area of Pakistan. Thus they recommend that livestock sponging be substituted “for indoor spraying wherever malaria transmission due to zoophilic [animal-preferring] mosquitoes is a risk to public health.”
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