Seeking Solutions in Brazil
Dog collars, long a method for identifying and controlling domestic dogs, have been used to control ticks and fleas. Now they may be used to control sand flies, insects that carry visceral leishmaniasis (VL), a serious parasitic infectious disease affecting humans and dogs. In northeastern Brazil, where this is becoming a critical health concern in several regions of the country, researchers are hoping to move away from conventional methods of eradicating dogs, known carriers of the disease, and spraying insecticides, to a more novel and promising approach which instead uses dog collars impregnated with an insecticide.
In 1997, R. Killick-Kendrick and his colleagues at Imperial College, UK reported that polyvinylchloride (PVC) dog collars treated with the insecticide , deltamethrin, protected dogs from 96% of bites from the Phlebotomus pernicious sand fly for up to eight months. In January 1999, we began to collaborate with Dr. José Wellington Oliveira Lima, the director of the Fundaío Nacional de Saúde in Ceara, Brazil, who had received his doctorate in our department at the Harvard School of Public Health a few years back. Luisa Stamm, who had carried out her honors thesis on Leishmania in our laboratory and recently graduated from Harvard College, went to Fortaleza to help initiate and organize the study.
We hoped to determine if the same results could be obtained with two different species of sand fly in this area of Brazil, the Lutzomyia longipalpis and Lutzomyia migonei.
Four dogs wearing PVC collars impregnated with deltamethrin and three dogs wearing placebo collars were kept in separate kennels outdoors for eight months. Periodically a dog from each group was sedated and placed in a net cage for two hours. Within these cages 150 female sand flies had been released 10-15 minutes before. After the collars were attached, the two species of sand fly were alternatively added to the net cages every two weeks for 36 weeks.
The deltamethrin collars had a dramatic anti-feeding effect on both species of sand fly. Over a 35-week period, only 4.1% of the Lutzomyia longipalpis taken from the nets containing dogs wearing treated collars were engorged with blood from a meal, an average of 96%. The inhibition of feeding was 99.3% when tested four weeks after the application of the collars, 100% after eight and 12 weeks, 96% at 16 and 20 weeks, and was still 94% at 35 weeks. During this time the average sand fly mortality was 68%, with half dying in the first two hours.
In contrast, 83% of the Lutzomyia longipalpis recovered from the nets of placebo collared dogs were engorged, and the average mortality was 8.6 %, with only 1.1% dying in the first two hours.
The results were similar for the Lutzomiya migonei species of sand fly.
These studies showed that the dog collars treated with the insecticide deltamathrin could protect dogs against sand fly bites for up to eight months, the time in which sand flies typically transmit diseases they are carrying. We are now planning two large field trials, one in Fortaleza, Ceara and one in Jacobina, Bahia, to determine if controlling the sand flybites a dog results in the protection for humans. If this were the case, a disastrous trend in human infection, and costs incurred in trying to prevent infection, could be reversed.
This is particularly important in Brazil. In the past two decades, VL has spread from the rural to urban areas in northeastern Brazil, possibly as a consequence of the migration of persons leaving areas ravaged by droughts. This urbanization of VL further increases the possibility of future epidemics involving many more people than previously. Furthermore, there is a resurgence of VL in persons who have recovered from the disease in the past and have subsequently become infected by HIV
Worldwide there are 400,000 new cases of VL and 50,000 deaths each year. The World Health Organization (WHO) estimates the severity of various diseases by determining the Disability Adjusted Life Years (DALYs) of a disease. In 1990, more than two million DALYs were estimated for VL in the world, second only to malaria for parasitic infections.
Symptoms accompanying the disease are loss of appetite, gastrointestinal problems, weight loss, and a tremendous enlargement of the liver and spleen. Full-blown VL is invariably fatal unless treated. Epidemiologic studies suggest, however, that probably only one of ten people who is infected gets the full-blown disease, with others experiencing a subclinical disease that goes away without treatment.
According to a 1997 article in Revista de Patologia Tropica, from 1984-1996, the cost of programs aiming to control the disease in Brazil exceeded $96 million. In 1995 alone, the staff involved in the control program totaled 1,839 people. In seven years, nearly one million houses were sprayed and blood samples were taken from 6.5 million dogs to test for seropositivity, a sign that they are carrying the disease. In all, 153,819 seropositive dogs were destroyed. Despite this effort, morbidity and mortality in the human population remains unacceptably high.
If successful, these treated dog collars could be useful in a program to control the incidence of VL and would be much more acceptable to the population than the present methods of sacrificing seropositive dogs. We hope to intensify our efforts in experimenting with the treated dog collars in the next several years to find a practical and low-cost solution to this dire health problem.
John R. David, MD is the Richard Pearson Strong Professor, Department of Immunology and Infectious Diseases at the Harvard School of Public Health and Professor of Medicine at Harvard Medical School.
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