It's a blood-sucking parasite with the potential to spread a fatal disease to U.S. cattle and cost the beef industry billions of dollars. Thanks to a successful eradication program in the U.S., that hasn't been the case.
But cattle fever ticks still infest northern Mexico and parts of Texas, and keeping the parasite under control in the southern U.S. is becoming increasingly difficult.
Over the last 20 years, several factors have been contributing to this complex situation, says John George, laboratory director for the USDA's Knipling-Bushland U.S. Livestock Insects Research Lab in Kerrville, TX.
For starters, Mexico has an abundance of cattle fever ticks, and changes in the ecological environment in south Texas have improved the habitat for the ticks. In particular, the ticks are better able to survive without a host animal because they're protected from exposure to dryness and heat by buffel grass, a very drought-resistant African species transplanted into Texas, as well as the thick shrubs and woody plants that cover many parts of the state, he says.
Besides improved habitat, both sides of the border now have dense populations of white-tailed deer and other wildlife that can carry the tick.
“Normally, if a ranch is found to be infested with one of two species of cattle fever ticks, you could go in and remove all the cattle from the premises for six to nine months… and the ticks would die because there weren't enough alternative hosts to sustain the population,” George explains. But this “pasture vacation” approach doesn't work anymore due to abundant deer and wildlife populations, he says.
A bigger concern is that some populations of the parasite are developing resistance to acaricides — the main chemical agents used to keep them out of the U.S.
“This has created a huge problem for treating cattle that are brought from Mexico to the import facilities to be exported to the U.S.,” George says.
At the border, the Animal and Plant Health Inspection Service (APHIS) inspects all cattle, rejecting those that are tick-infested. All animals that pass inspection then swim a dipping vat containing 3,000 ppm of an organophosphate product called coumaphos — the only acaricide approved for use in the eradication program, he says.
But continued use of coumaphos and other organophosphates is evolving tick strains resistant to even high concentrations of the acaricide.
“There are southern cattle ticks in Mexico so resistant to very high concentrations of coumaphos that a large percentage of the adult ticks — about two-thirds — would survive treatment in the vat,” George says. “And about 10% of resistant ticks in the nymphal stages can survive.”
No alternatives to coumaphos are available because the ticks also have intense resistance to the only other chemicals that can be used in a dipping vat — a class of chemicals called pyrethroids and a single chemical called amitraz.
What's more, no new classes of chemicals are coming along to replace these acaricides. The animal health industry isn't interested because it costs tens of millions of dollars to develop and market a new product, George says. “And even before the patent runs out on it, there likely will be populations of ticks that are resistant to it,” he adds.
Without effective acaricides or alternative technologies, cattle fever ticks could disperse and re-infest the southern U.S. For the cattle industry, the economic impact of such a scenario would be significant, says Gary Weber, executive director of regulatory affairs for the National Cattlemen's Beef Association (NCBA).
“Obviously, re-infestation would lead to increased morbidity and mortalities,” he says. “It would force a lot of movement restrictions on cattle from Texas and any state that would get it — which would impede the efficient commerce that we have today.”
According to USDA, virtually all adult and yearling cattle are susceptible to the cattle fever parasites, and mortality rates from the disease in these animals could exceed 90%. (See “The Tick's Wrath.”)
Even worse, George says it's unlikely the ticks could be eradicated if they become widespread again.
“The white-tailed deer would be a huge factor and would probably prevent eradication,” he says.
Acaricide resistance, pesticide use restrictions and public resistance to widespread pesticide use also would make eradication more difficult and expensive to accomplish than the initial eradication campaign carried out from 1906 through the mid 1940s. (See “Wiping Out Ticks.”)
In the meantime, besides needing effective alternative acaricides, the eradication effort needs better methods to diagnose acaricide resistance, George says. And he says it's time to invest in long-term, high-risk research to develop novel techniques for eradicating the ticks.
The program needs practical tick control strategies that minimize the number of acaricide applications and integrate other control approaches. This includes anti-tick vaccines and tick-resistant cattle breeds, he explains.
“One of the things that's been proposed is an initiative to do fundamental research on the genomics of ticks to unravel some things and identify places where we can develop genetic control methods or other novel approaches to eradicating ticks,” George says. “Our agency is seeing a need for us to do this, and people in the cattle industry also have indicated they would like to see us open up this new area of research.”
In fact, Weber says NCBA is lobbying to get funding and resources for this type of research.
“We're increasing our efforts to acquire additional federal research dollars to find ways of dealing with this,” he says.
The effort is two-fold. First, NCBA wants to ensure that the Agricultural Research Service and the land-grant universities have adequate financial resources to do this work. Secondly, NCBA wants a coordinated, targeted effort to address the fever tick issue in particular, Weber explains.
For information about available procedures for fighting fever ticks, visit the Texas Animal Health Commission's Web site at www.tahc.state.tx.us.
Wiping Out Ticks
When 14 southern states were infested with two species of cattle fever ticks at the beginning of the 20th century, the direct and indirect economic losses for the cattle industry were an estimated $130.5 million. That would be about $3 billion today.
A cooperative state and federal effort, the USDA's Cattle Fever Tick Eradication Program (CFTEP) began in 1906 and eradicated the tick from the U.S. in the 1940s.
Because the tick still exists in Mexico and comes across the border on wildlife, the eradication program is ongoing. It saves the U.S. cattle industry approximately $1 billion annually, according to estimates by the USDA's Animal and Plant Health Inspection Service.
The two major concerns of the program are eradicating outbreaks and ensuring that imported cattle don't carry the tick. Activities include maintaining a permanent quarantine zone between Texas and Mexico as well as patrolling the U.S.-Mexico border for stray or smuggled Mexican livestock or native livestock that cross into Mexico and return.
The Tick's Wrath
Blood from animal hosts — usually cattle, but sometimes deer, horse, mules, sheep or goats — is vital for cattle fever ticks to complete their life cycles. Also known as Boophilus microplus and B. annulatus, these ticks attach to soft skin on the animal's thigh, flanks and forelegs or along the belly and brisket. Then they engorge.
After the tick begins feeding, the first symptoms of cattle fever can show up in as few as 10 days in the summer or as many as 90 days in the winter.
The acute form of cattle fever is usually contracted during summer and can be fatal in three to four days. Signs of the disease include high temperatures, loss of appetite, constipation followed by diarrhea, decreased milk production and bloodstained urine.
The chronic form is usually contracted in fall or winter and can last many weeks. It includes a mild fever, anemia and weight loss. In fact, a 1,000-lb. steer with the disease can lose 200 lbs. in a year, according to USDA. Most cattle recover from this form of the disease, but they often suffer nervousness and relapses and are susceptible to other health problems.