Researchers have identified the common tick species Amblyomma americanum, also known as the lonestar tick, as the source of a novel tick-borne disease dubbed ‘Heartland virus’ (HRTV).
In 2009, two Missouri were hospitalized with high fever, fatigue, diarrhea, thrombocytopenia and leukopenia after being bit by ticks in the weeks prior to admission. Investigators suspected the illness could be a tick-borne disease, so Harry Savage, PhD, of the CDC in Atlanta, and colleagues collected samples of ticks from the farms of the two men and six nearby sites.
Analyses revealed genetic sequences from A. americanum were at least 97.6% identical to those isolated from the two men, the researchers reported online in the American Journal of Tropical Medicine and Hygiene.
“It’s pretty strong evidence that the virus is persisting from season to season in tick populations and that these ticks play an important role in disease transmission,” Savage said in a statement.
The tick-borne bacterium Ehrlichia chaffeensis was initially suspected as the cause of the mens’ illness, but after finding no signs of the bacterium the researchers isolated the HRTV novel virus. A member of the Phlebovirus genus, HRTV is most closely related to a virus recently discovered in China called severe fever with thrombocytopenia syndrome virus (SFTSV).
“The goals of this project were to collect and test arthropods (ticks and mosquitoes) to determine if HRTV persisted at the two case-patients’ residences, and to incriminate potential arthropod vectors for this newly described human pathogen,” the researchers wrote.
Using CO2-baited tick traps—and sometimes simply pulling them off of dogs and horses—the researchers were able to collected 56,428 ticks from the property of the two men and the surrounding areas, spanning four Missouri counties.
The samples encompassed three tick species, the most common of which was the lone star tick, named such for its signature single white spot on its back. More than three quarters of the ticks collected were larvae, although nymphs and adults were also gathered.
Ten samples tested positive for HRTV, all of them consisting of lone star tick nymphs. Nine of those pools were collected from the residence of a case-patient. Eight produced a viable virus, translating to approximately a one in 500 likelihood that a lone star tick will carry the virus. Although these odds are not as high as those associated with Lyme disease, the risk is still significant given the range and abundance of the lone star tick species.
The researchers also collected 758 mosquitoes specimens, representing 12 species, but none carried the novel virus.
The researchers hypothesized HRTV is most likely transmitted to humans during the summer, when nymphs are widespread and seeking a host. Although the study marks a breakthrough in the continuing quest to pinpoint the cause of HRTV, many questions about the virus remain.
Ticks have not been confirmed as the only carrier of the virus. Many other tick-borne diseases have other hosts that serve as a reservoir. The researchers have not yet identified such a host, so person-to-person transmission cannot be ruled out.
For now, HRTV can be added to the list of dangerous tick-borne illnesses. Although the lone start tick is widspread in the United States, to date there have been no other reports of HRTV outside of the two Missouri cases. The CDC recommends people going outside in the woods or tall grass use insect repellent, wear pants and long sleeves, check for ticks daily and shower soon after being outdoors.