Do Certain Vertebrates Transmit Heartland Virus to Humans Via Tick Bites?

Although Lyme disease gets the lion’s share of headlines when it comes to tick-borne illnesses, a more recently discovered disease has scientists digging for further information. The phlebovirus, known widely as Heartland virus (HRTV), was first identified in two people in Missouri in 2009. Since then, HRTV has yielded five new cases in that state along with one additional case in Tennessee. Now, scientists from the Centers for Disease Control and Prevention (CDC), the US Department of Agriculture, and Colorado State University are studying whether animals play a role in HRTV’s transmission to humans.

Besides fever, fatigue, headache, nausea, and muscle pain, HRTV is noteworthy in that it causes leukopenia (low white blood cell count), thrombocytopenia (low platelet count), and modestly elevated liver enzymes. Most of the patients who contract HRTV have been hospitalized, and one, who had pre-existing health conditions, died. In five of the six newer cases, the patients were known to have been bitten by a tick before the onset of illness.

The scientists had reason to suspect that vertebrates living in close proximity to humans could be susceptible to HRTV themselves, and act as a conduit for the virus to travel from ticks to humans. “The purpose of this study was to determine the capability of different vertebrate hosts to develop virus levels in their blood that would allow ticks to become infected when they feed on these hosts,” said Aaron C. Brault, PhD, a research microbiologist at the CDC’s Division of Vector-Borne Diseases and an author of the study. Dr. Brault noted that raccoons found near the home of one of the HRTV patients were “quite heavily” infested with Lone Star ticks, which also prey on deer. If a tick bit one of these raccoons or deer and became infected, it might then go on to bite a human and cause an HRTV infection.

Taking a blood sample from an infected human, the scientists inoculated a number of mice, chickens, hamsters, goats, rabbits, and raccoons with the HRTV virus. None of these animals went on to show any detectable level of virus in their blood, with the exception of mice that had been bred to lack interferon receptors, producing an altered immune response.

Although the animals in this experiment did not display elevated levels of HRTV in their blood after inoculation, the researchers know there is more work to be done when it comes to the potential role of wild animals in human HRTV infection. “A number of factors really need to be answered to fully assess the potential health risk of HRTV for humans,” said Dr. Brault. “Principally, what is the clinical disease rate in exposed humans (healthy versus individuals with comorbidities) and what is the exposure rate of people? These will likely have to be addressed by serosurveys.”

So far, all eight of the patients diagnosed with HRTV disease in the United States have been white men over the age of 50, and all became symptomatic between May and October. Due to their leukopenia and thrombocytopenia, along with somewhat elevated liver enzymes, clinicians first assumed the patients had ehrlichiosis, another tick-borne illness. When the men did not improve after being given courses of doxycycline, doctors had to look for a different explanation. There is no routine test available for HRTV and no medications that can treat it; most patients have fully recovered as a result of supportive care.

Laurie Saloman, MS, is a health writer with more than 20 years of experience working for both consumer and physician-focused publications. She is a graduate of Brandeis University and the Medill School of Journalism at Northwestern University. She lives in New Jersey with her family.

Feature Picture Source: Katja Schulz / flickr / Creative Commons.