AgriLife Research Scientist Helps Crack Genome of a Soft Tick

Scientists at Texas A&M AgriLife Research and Baylor College of Medicine have sequenced the genome of the disease-spreading soft tick.

The ectoparasite carries human relapsing fever and is a potential vector of the African swine fever virus, which threatens the $62 billion U.S pork industry.

The team collaborated with the U.S. Department of Agriculture Agricultural Research Service, USDA-ARS, SCINet project and Ag100Pest Initiative to develop the first complete genome assembly for Ornithodoros turicata. Their findings were published in G3: Genes | Genomes | Genetics.

The completed genome will allow researchers to investigate chromosomes linked to specific biological functions, said Pete Teel, Ph.D., co-author, AgriLife Research entomologist and Regents Professor in the Texas A&M Department of Entomology. This creates opportunities for researchers to develop novel tools to control the pest.

“Physiology, development and reproduction of these ticks is all based on their genetics, and this allows us to open the doors to those discoveries,” Teel said. “Those discoveries create opportunities for novel genetic tools we might use for surveillance and control measures against the tick and the pathogens it carries.”

Study Unlocks Mystery of Soft Tick Species

Job Lopez, Ph.D., senior author of the study and associate professor in the National School of Tropical Medicine at Baylor College of Medicine, said the genome opens up a whole new avenue of research to study the tick, its life cycle and biology, as well as tick-pathogen interactions.

Researchers Alexander Kneubehl, Baylor College of Medicine, and Amanda Stahlke, Ph.D., Colorado Mesa University, as well as Mackenzie Tietjen, Ph.D., David Luecke, Ph.D., Perot Saelao, Ph.D., Sheina Sim, Ph.D., Scott Geib, Ph.D., Brian Scheffler, Ph.D., and Anna Childers, all with USDA-ARS, contributed to the study.

“It’s one thing to generate a genome, but there’s a lot of postprocessing once you’re done with sequencing,” Lopez said. “Our goal was to generate a high-quality, chromosome-level genome instead of a highly fragmented genome that is challenging to work with.”

These ticks are cavity dwellers, meaning they live in animal burrows, caves and root hollows, as well as under pier-and-beam buildings. They are secretive and blood-feed on animals that visit those environments.

They can live longer than five years without a blood meal, and female ticks can transfer pathogens to their offspring. This enables pathogens to be passed across tick generations, which not only makes the ticks important vectors of disease but also as pathogen reservoirs.

Teel said there are many gaps in knowledge of this tick. For example, very little is known about how they disperse from one cavity to another and how closely related they are across their wide geographical range.

We have a basis now to really look at population genetics, and we are looking at the vast geography that we’re discovering for this species from Florida all the way through the High Plains and throughout the southwest and down to Mexico,

This is a huge step toward the connection of surveillance and development of control.”

Pete Teel, Ph.D., Co-author, AgriLife Research entomologist and Regents Professor in the Texas A&M Department of Entomology

Complete Genome Key to Understanding Tick Biology

Researchers established a colony from Ornithodoros turicata ticks discovered in a cave in Travis County over three decades ago, following a local case of relapsing fever at the time. They have maintained a colony for research.

The biology of soft ticks is complex and distinct from that of hard ticks, which are more extensively studied. The gender of hard ticks can be easily determined from a nymphal stage, but in soft ticks, gender determination is more challenging.

The egg hatches into a larva, which then feeds and molts into a nymph. Nymphs have between five to six instar stages. Each instar stage takes a blood meal to molt into the subsequent stage, and it can take about a year to progress from a larva to an adult. Finally, when they molt into adults, sex can be determined.

A primary objective of this genome project is to determine how sex is identified at a genetic level and pinpoint the chromosome involved in sex determination. Chromosomes linked to other important biological and developmental functions can also be explored.

Protecting People and the Pork Industry

Soft tick relapsing fever cases in the U.S. are rare. Between 2012 and 2021, there were 251 confirmed cases. There have been four confirmed cases since, including one in Colorado last year.

Most cases occurred in western states and were related to overnight stays in rustic cabins or forests. Symptoms range from fever, headache, muscle aches and chills to joint pain, nausea, vomiting and rare instances of facial paralysis and vision changes.

While soft tick relapsing fever poses a threat to human health, African swine fever represents a potential catastrophe for the U.S. pork industry.

About a decade ago, African swine fever spread globally due to the trade of infected domestic swine that directly transmitted the virus to each other. The disease gets its name “African swine fever” from the natural cycle involving ticks, the virus and warthogs, a cycle in which warthogs do not get sick but maintain the virus.

When the virus enters a domestic swine population, the mortality rate is nearly 100%. It poses a serious threat to the U.S. pork industry, which generates $62 billion in value-added economic impact, including more than 570,000 jobs, according to the National Pork Producers Council.

African warthogs are now present in Texas, and concerns exist that these ticks and feral hog populations may introduce the African swine fever virus to the U.S.

“Texas has all the puzzle pieces for the emergence of a natural cycle for the African swine fever virus,” Lopez said. “We have the tick and African warthogs in Texas, and one of the largest populations of feral hogs on the continent. That’s where the significance of this tick vector comes into play.”