Degradation of forest contributes to the spread of spotted fever
June 13, 2012
By Elton Alisson
Agência FAPESP – In the regions to the north and south of the São Paulo metropolitan zone, where fragments of the Atlantic rainforest remain, a type of tick called Amblyomma aureolatum can be found, known as the yellow canine tick, that is one of the transmitters of Brazilian spotted fever (or “tick fever”).
However, although a large number of cases have been registered since the 1920s to the south in the municipalities of Diadema, São Bernardo and Santo André, there have been no reports of the zoonosis in the north in the Cantereira Mountains and municipalities of Mairiporã, Arujá and Nazaré Paulista.
A study carried out by researchers at the Universidade de São Paulo (USP) School of Veterinary Medicine and Zootechny, together with the Superintendence of Endemic Control (Sucen), part of the São Paulo State Secretariat of Health, formulated a hypothesis to explain the difference in the number of cases of the disease between the two regions of the city.
The researchers observed that the occurrence of Brazilian spotted fever is related to the state of conservation of the patches of forest in which the transmitting tick lives.
In areas where the patches of forest are better conserved and that have a greater diversity of animal species, such as those in the north of São Paulo, no cases of the disease were found. However, in areas where the vegetation had been destroyed and few species of animals remained, such as in the southern part of the city, there was a greater incidence of the fever.
The results of the study, which was funded by FAPESP, were published in the beginning of May in the periodical Parasitology and will provide direction to Brazilian spotted fever control in the São Paulo metropolitan region.
“We compared wildlife diversity between the two regions of the São Paulo metropolitan zone and found that in the north, where there are no cases of the disease, the areas of vegetation are much better preserved and have more animal diversity. In the southern municipalities, there are patches very poor in animal species. This could be a prevalence factor for the disease,” said Maria Halina Ogrzewalska, author of the study, to Agência FAPESP.
According to the Polish researcher, who performed the study through a Post-Doctorate FAPESP Fellowship, in well-preserved forest patches with a broad diversity of species, the Amblyomma aureolatum tick is parasitic to different types of wild animals, such as squirrels, birds and rats, whose abilities to transmit the Rickettsia rickettsii bacteria varies.
However, in more degraded areas of forest, from which most of the animals have disappeared due to environmental destruction, the researchers suspect that there is a greater chance for the tick to parasitize generalist species, which could be animals with a greater capacity to become infected with the bacteria that causes Brazilian spotted fever.
“Greater diversity of wildlife species in a region reduces the possibility of a tick becoming infected with the pathogen because it has a greater chance of feeding off an animal with reduced capacity for increasing the bacteria, resulting in lower rates of infection among the ticks in the region. This could be one of the reasons why regions with well-preserved forest have not reported cases of Brazilian spotted fever,” noted Ogrzewalska.
The illness is transmitted to humans by ticks from domesticated dogs, which run free in the communities on the edges of the forest, such as those in outlying areas of the São Paulo metropolitan region.
When they enter the forest, the animals can be parasitized by the tick and bring them into homes, in which the ticks can bite and infect people with the spotted fever-causing bacteria.
However, domesticated dogs have specific antibodies against R. rickettsii bacteria and therefore do not usually get sick and are spontaneously cured. That is why they are considered perfect “sentries”.
However, if infected humans are not treated in time, Brazilian spotted fever can be lethal. “We estimate that in these areas of the São Paulo metropolitan region that have reported cases of the disease, if the patients aren’t treated in time, the death rate is nearly 100%,” said Sucen researcher Adriano Pinter.
Exception to the rule
The researchers collected samples from wild animals and domestic dogs over a year-long period to evaluate whether there were differences in the number of cases of the disease and the diversity of tick host animals in the patches of forest in the northern and southern São Paulo metropolitan regions.
Samples were collected in the cities of Diadema, São Bernardo, Santo André, Mairiporã, Arujá, and Nazaré Paulista and the region of the Horto Florestal in the north end of the city to study the ticks identified and to verify whether they were infected by the bacteria.
Laboratory analyses revealed that the ticks collected in São Bernardo and Diadema were infected by R. rickettsii and that the dogs captured in the two municipalities also had the disease, as detected by the presence of antibodies in their blood.
However, the ticks and dogs captured in Mairiporã, Arujá, Nazaré Paulista and the Horto Florestal region and Santo André did not have the bacteria. In Santo André, this result was a surprise for the researchers.
“We also expected to find the disease in Santo André, which is in an area known for transmission of Brazilian spotted fever and located only 4 kilometers from another patch of forest where the bacteria was found,” affirmed Ogrzewalska.
Comparing the state of conservation of the patch of forest in Santo André with the state of forest conservation in the other six municipalities evaluated in the study, the researchers found that it more resembled the forest in the north end of São Paulo in terms of diversity of wildlife.
In addition, the Santo André patch of forest was less isolated than the others in the south end, which are smaller and more spread out, with ecological corridors that make the flow of animals between forest patches possible.
“The areas with the most Brazilian spotted fever in the São Paulo metropolitan area coincide with these areas with very fragmented patches of forest, isolated and without connections that enable the animals to move about,” affirmed Pinter.
The researcher said that based on the results of the project, Sucen is analyzing images of the forest fragments in the entire metropolitan region to find areas with the same profile of fragmentation as the areas where the illness was found so that disease control can be better managed.
The article “Epidemiology of Brazilian spotted fever in the Atlantic Forest, state of São Paulo, Brazil” (doi:10.1017/S0031182012000546) by Ogrzewalska and others can be read by Parasitology subscribers at the following webpage: http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8560121&fulltextType=RA&fileId=S0031182012000546.
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