A Permanent Quarantine Zone buffer along the Texas/Mexico border works to keep southern cattle fever ticks (Rhipicephalus [Boophilus] microplus from re-establishing in Texas after being eradicated from the U.S. in the 1940s. Periodic outbreaks in south Texas are attributed to a unique landscape suitable for the ticks and their wild hosts, such as white-tailed deer and exotic nilgai antelope. (Photo by iNaturalist user José Edilson Espitia Barrera, CC BY-NC)

Melissa Mayer

Southern cattle fever ticks (Rhipicephalus [Boophilus] microplus, SCFT) carry disease-causing pathogens with the potential to ravage livestock. Back in 1906, cattle production losses were so serious—a staggering $3 billion from babesiosis alone, adjusted for inflation—that it prompted the introduction of the USDA-APHIS Cattle Fever Tick Eradication Program.

That program established a Permanent Quarantine Zone buffer along the Texas/Mexico border, and the United States declared cattle fever ticks eradicated in 1943. However, cattle fever ticks pose an ongoing threat to the U.S. cattle industry because the ticks persist in Mexico. This issue came into sharp focus in 2014 with outbreaks of SCFT among cattle and non-domesticated ungulates within and just north of the eastern portion of the Permanent Quarantine Zone.

In their Forum article published in late April in Environmental Entomology, Allan Showler and Adalberto Pérez de León, both with the USDA-ARS Knipling-Bushland U.S. Livestock Insects Research Laboratory, describe the landscape features and host ecology that affect the recent SCFT resurgence in the South Texas coastal plain wildlife corridor and discuss ideas for mitigation.

An Exotic Complication

The affected area is a unique habitat that includes beaches, coastal prairies, mud flats, thorn thicket, and fresh- and saltwater wetlands. The wildlife corridor is home to undomesticated cattle fever tick hosts, namely native white-tailed deer (Odocoileus virginianus) and exotic nilgai antelope (Bosephalus tragocamelus).

Allan Showler, Ph.D.

Hunting regulations, conservation efforts, restocking, and the eradication of the New World screwworm fly have shored up the white-tailed deer population. While white-tailed deer only travel modest distances, they can leap over barbed wire cattle fencing into rangeland and pasture areas, bringing ticks with them.

“[SCFT] has made incursions, partly because the host ecology has changed,” explains Showler. “Now we have larger numbers of more protected deer populations, which of course favors the dissemination of cattle fever ticks. And, with feral nilgai populations, we have an additional complication.”

Ranchers brought nilgai, which are native to the Indian subcontinent, to South Texas as trophy game in 1924—and escaped nilgai quickly spread in Texas and Mexico. They likely co-evolved with cattle ticks, which came from the same area. Since wild nilgai enjoy large home ranges and wander freely between wildlands and unfenced ranchlands on both sides of the border, they are considered key players in SCFT outbreaks.

Showler and Pérez de León say detailed landscape ecology studies could confirm a report that the ticks are completing their entire life cycle at the Permanent Quarantine Zone overlap with wildlife refuge land—with white-tailed deer and nilgai serving as hosts for the ticks. They could also show how wild ungulates and livestock move and interact, reveal the actual distribution of SCFT pockets within the wildlife corridor, and help adapt eradication approaches to address the resurgence.

In the South Texas coastal plain, an exotic antelope called nilgai species, Bosephalus tragocamelus, originally from India and introduced to Texas by game hunters in the 1920s, serves as a host for southern cattle fever ticks (Rhipicephalus [Boophilus] microplus ), which pose an ongoing threat to the cattle industry in the area. (Photo by iNaturalist user ralphxxx, CC BY-NC)

So Salty

Another unique aspect of the area’s ecology is Laguna Madre bay, where wind tides push hypersaline water up into some areas of the flatlands—and that’s bad news for ticks. Between egg-destroying surges of seawater and egg-consuming mudflat fiddler crabs, Showler says that soil salinity helps predict the distribution of ixodid (hard) ticks like SCFT.

And since the sea oxeye daisy dominates the saline areas, Showler used its distinct coloration to key in on saline areas with satellite imaging to explore tick distribution. “What we found was these areas, constituting about 25% of the wildlife corridor, harbor negligible numbers of ticks—because it’s saline and because the mud flat fiddler crab is a highly efficient predator of ixodid eggs,” explains Showler. “That shows us where the ticks are … so we have a distribution idea based upon that.”

Researchers can overlay the wildlife data on that distribution and see SCFT population differences among wild ungulates. In one largely saline area called Bahia Grande, less than 2 percent of the nilgai hosted SCFT, but a whopping 91 percent of nilgai had SCFT in a less saline area just north of there.

The factors playing into those differences are complex, but Showler is particularly interested in one of them: isolation. “These animals don’t migrate, but they can move around, so why are these populations of tick infestations so discrete? Well, it’s because of the salinity, crabs, and incidental man-made barriers, I think,” says Showler. “Then I started to think, ‘Well, why don’t we use barriers?'”

Good Fences Make Good Neighbors

Barriers like the chain link fencing recently erected to protect endangered ocelots in the wildlife corridor from highway traffic—combined with freeway infrastructure, residential areas, and the Brownsville Ship Channel—all work together to keep the nilgai in Bahia Grande isolated. Those are the nilgai with the low tick counts. The authors think movable barriers might temporarily seal off areas, enabling step-wise, area-by-area tick eradication efforts without the interference of those wandering wild ungulates.

When it comes to treatment, Showler is investigating desiccant dusts comprised of silica gel and diatomaceous earth—both lethal to larval and nymphal ixodids—as well as combinations of desiccant dusts with plant-based substrates, such as pyrethrins and thyme oil. These combos pack a one-two punch: the pyrethrin or thyme oil takes out ticks while feeding, and the fine powder interferes with the waxy cuticle, drying up mobile ticks before they can feed.

Desiccant dusts don’t degrade in the environment like most toxins, so they can probably provide long-lasting prophylactic protection of livestock. These dusts could work to spot-treat sites with clutches of tick eggs or even be applied directly to livestock and wildlife—possibly via low-cost, passive methods like dust bags that deliver a puff when animals brush up against them or by treating bands of vegetation the animals walk through.

And cattle fever ticks have a vulnerability: they’re a one-host tick species. Unlike many disease-spreading ticks, which drop off and switch hosts between molts, cattle fever ticks stick with one host for the entirety of their lives—so they would be particularly susceptible to those treatments, especially during the larval stage when they are most exposed and vulnerable.

Showler and Pérez de León say the SCFT resurgence in South Texas appears to be increasing and point out knowledge gaps. Their careful examination of the interactions between the ticks, the tick hosts, and the local landscape begins to fill in those gaps and offers a roadmap for future ecology studies.

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