American dog tick (Dermacentor variabilis). Photo courtesy of Jim Occi, BugPics, Bugwood.org.

By Melissa Mayer

Melissa Mayer

Most tick-borne infections have a grace period, or noninfectious buffer zone between the time the tick inserts its mouth parts and the time it transmits infection. This makes sense because many pathogens need time to migrate from the tick’s internal tissues to its salivary glands. Lyme disease, for example, usually requires the tick to be attached for 36-48 hours before the the Lyme disease bacterium can be transmitted, according to the CDC.*

Scientists used to think the pathogens that cause spotted fevers—such as Rocky Mountain spotted fever, which is caused by the bacteria Rickettsia rickettsii—also needed a solid 48 hours to reactivate from dormancy after tick attachment. The evidence for this is almost 100 years old, well before the discovery of the molecular methods researchers use today.

A new study in the Journal of Medical Entomology aimed to update the data and reveal the minimum length of attachment required for transmission of Rocky Mountain spotted fever by American dog ticks.

In the Tick Lab

To do this, the research team allowed American dog ticks to attach and feed on guinea pigs in the lab. They removed individual ticks at various time points and ran PCR testing on each tick to measure the amount of bacterial DNA in their salivary glands. They also measured rickettsial loads in unfed American dog ticks from the same infected colony to check for differences between ticks who have fed and those who have not.

In a parallel experiment, researchers took small biopsies of the guinea pigs’ skin at and near the site of tick attachment to see how far the bacteria could spread after transmission. The research team also delivered an injection made from the infected salivary glands of American dog ticks to yet another group of guinea pigs and monitored those animals for infection before eventual euthanasia and necropsy.

This Data Bites

American dog tick (Dermacentor variabilis). Photo courtesy of Susan Ellis, USDA APHIS PPQ, Bugwood.org.

PCR revealed that both unfed and fed ticks carried infection in their salivary glands. When it came to clinical signs of infection, time mattered. Guinea pigs exposed for very short times developed some symptoms while those exposed for 8 hours had mild fevers and those exposed 12 hours or longer had high fevers. The guinea pigs who received injections were even sicker—as ill as their lab mates exposed to ticks for more than 16 hours.

For the guinea pigs who underwent immediate biopsies, removing both the tick and the site of the bite within 30 minutes of exposure, the news was good; they didn’t get sick at all. When tick removal and bite site excision happened 4 or more hours after attachment, infection took its usual course.  Even more interesting, the infection spread pretty quickly in the skin of guinea pigs; within 4 hours, scientists could detect it at least 30 mm from the site of the initial bite.

The Bloody Takeaway

These findings suggest that American dog ticks can transmit Rocky Mountain fever almost immediately upon attachment—with almost no grace period. Somewhere between five and 30 minutes after piercing a host’s skin with their mouth parts, ticks begin secreting a cement cone that helps the tick stay put while feeding. This, plus the data showing that some guinea pigs became sick after only 30 minutes of exposure, leads scientists to think the pathogen could be present in the very first bits of tick saliva.

“Previously, research indicated that it took 24 to 48 hours after tick attachment for Rickettsia to reactivate from a dormant state and become infectious,” says Michael Levin, Ph.D., of the Centers for Disease Control and Prevention and one of the study’s authors. “Our new study shows that viable R. rickettsii can pass to an animal host in less than 4 hours.”

They also think that the longer the tick remains attached—and therefore pumping infected saliva into the host—the more severe the resulting illness. That doesn’t mean that every single tick bite will sicken the host, however. There are likely pathogen-level variations in the bacteria and immune system differences that affect illness. Plus, it’s hard to say how accurate a guinea pig model is for predicting human reactions.

Still, the idea that the grace period for Rocky Mountain spotted fever could be very short or even nonexistent means that prompt tick checks and removal of any attached ticks is the best way to avoid falling ill.

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.

*Editor’s Note: This sentence was edited to add a link to the CDC resource referenced by the author, and to better reflect the wording of CDC’s information, in response to a question from an Entomology Today reader.