Ticks may transmit disease faster than currently thought
NEW YORK (Reuters Health) – Brazilian ticks that carry Rocky Mountain spotted fever passed the disease to animal hosts in as little as 10 minutes if they had recently fed on another animal, a new study found.
“The current literature, including medical textbooks and guidelines for the general public, has repeatedly advised that an infected tick requires a minimum feeding period varying from 2 to 10 hours to transmit Rickettsia rickettsii - the bacterium that causes Rocky Mountain spotted fever - to humans,” Marcelo Labruna told Reuters Health in an email.
“We believe our results will change some of our current recommendations for the prevention of Rocky Mountain spotted fever in endemic areas,” said Labruna, a researcher at the University of São Paulo in Brazil and senior author of the study.
Rocky Mountain spotted fever is the deadliest known rickettsial disease spread by several species of ticks that carry the bacteria and transmit them to the hosts they feed on, including dogs and humans.
In Brazil, where the disease is known as Brazilian spotted fever, 20-40 percent of people who are infected die as a result, Labruna and his colleagues write in the journal Emerging Infectious Disease.
Ticks transmit infections when they feed on a host, which they only do during short periods during their life cycles. The rest of the time, they are “free living” and their metabolism slows down to a semi-dormant state, the authors write. Reawakening to feed involves physical changes in the tick, which may alter the tick’s ability to transmit disease at first.
To see whether the ticks’ ability to transmit disease differs depending on whether it is unfed or has already fed, the researchers experimented with Amblyomma aureolatum, the main carrier of Rocky Mountain spotted fever in the Sao Paulo area.
First, they allowed hungry bacteria-infested ticks to feed on guinea pigs for 2 to 48 hours.
Next the study team allowed infected ticks to feed on rabbits for 48 hours before transferring them to guinea pigs. These ticks were left on the guinea pigs for anywhere from 1 minute to 168 hours.
All the guinea pigs were then monitored for signs of the disease. Of the 24 guinea pigs exposed to unfed adult ticks,10 remained uninfected after 10 hours of tick exposure.
Guinea pigs exposed to ticks that had first fed on the rabbits remained uninfected when ticks were removed after less than five minutes of feeding time, but one became infected when the tick was left to feed for just 10 minutes.
Labruna said it’s not known for sure how often ticks that have fed on other animals bite people but epidemiological data indicates that it is very common in some areas.
“For example, in the metropolitan area of São Paulo, nearly 70 percent of the Rocky Mountain spotted fever cases have occurred in children and adult women, who usually did not enter the forest habitat of the tick as frequently as did adult men,” he said.
He added that 93 percent of the cases in that area have been associated with direct contact with dogs.
“Our results highlight that dogs are much more important than previously thought in the epidemiology of Rocky Mountain spotted fever in Brazil,” he said. “Within endemic areas, efforts to keep dogs free of ticks should be much more intensive.”
In the U.S., a different tick, known as a wood tick or dog tick, is the primary carrier of Rocky Mountain spotted fever. Patrick Liesch, an entomologist with the University of Wisconsin in Madison, said the simplest way for people and pets to avoid tick-borne disease is to avoid areas where ticks are likely to occur.
“Weedy areas and wooded areas are favorite hangouts for ticks,” he told Reuters Health in an email. He added that dressing accordingly can help prevent ticks from getting to your skin and can make it easier to see them.
“Wearing light-colored, long-sleeved shirts and pants can make it easier to spot ticks before they have a chance to bite,” he said. “Tucking pants into socks can also limit the ability of ticks to get down to skin level.”
Leisch also suggests using tick repellents, such as DEET and permethrin and performing a thorough tick check after visiting tick-infested areas.
“Educating yourself about ticks is a final important consideration,” Leisch said. “By thoroughly ‘knowing your enemy,’ you'll have a better understanding of what to keep an eye out for.”
He suggests the University of Rhode Island's ‘Tick Encounter’ website at www.tickencounter.org.
“There's a wealth of information on the site, and you can even access a tick identification chart, which highlights the main species of ticks in your region of the U.S., so you can learn what to watch for,” Leisch said.
Labruna added that people should be “re-educated” to know that a tick does not necessarily need as long as two hours of attachment for transmission of R. rickettsii.
“Actually, they must have in mind that a much shorter period, namely 10 minutes, could result in successful inoculation of the bacterium,” he said.
Labruna also said the general recommendation of searching the body for attached ticks every one or two hours while walking within endemic areas may not be sufficient if the person is in direct contact with dogs.
SOURCE: 1.usa.gov/1lfwhRz Emerging Infectious Disease, online August 13, 2014.