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: http://1.usa.gov/1lfwhRz Emerging Infectious Disease, online
August 13, 2014.
