A New Threat from an Old Enemy

By Sharla Ishmael

Somewhere in the recesses of a government laboratory, scientists are busy squashing tick larva into test tubes and extracting DNA, looking for mutants. It’s the next line of defense between your cattle herd and an old nemesis – cattle fever ticks (Boophilus microplus and B. annulatus).

An eradication program has been in place since 1906 for this non-native species, brought over by explorers and colonists way back when. Dr. Felix Guerrero, a physiologist at the USDA Knipling-Bushland U.S. Livestock Insects Research Laboratory, Kerrville, Texas, says by the end of the 19^th century, bovine babesiosis, the disease transmitted by these ticks, was a major hindrance to the development of a strong cattle industry in the southern United States.

He adds that direct and indirect costs back in 1906 were estimated at more than $1 billion (adjusted for inflation to 1976 dollars). Today, he says, "A recent cost-benefit analysis to control a hypothetical tick outbreak in Texas estimated that a mandatory dipping program for cattle would cost at least $1.3 billion in the first year."

Cattle entering the United States from Mexico are treated by dipping them in a solution of coumaphos, explains Dr. Ron Davey at the Cattle Fever Tick Research Lab in Mission, Texas. While this has been an effective solution to the problem for more than 30 years, coumaphos-resistant ticks have been found in several locations in Mexico.

Then you have the fact that coumaphos is the only pesticide approved for use in the Cattle Fever Tick Eradication Program …..

Cattle fever ticks have been eradicated in the United States for decades, thanks to the $3 to $4 million spent annually on an eradication program. And though we have multiple measures in place to protect the U.S. herd -- including "scratch" inspections at the border for all cattle entering from Mexico; mandatory dipping of those animals; and tick riders patrolling the Rio Grande -- researchers believe resistance to pesticides could undermine those safeguards.

If cattle fever ticks pose such a big threat to U.S. cattle, why is there only one pesticide which can be used in the eradication program?

"Coumaphos has been the pesticide of choice in the program since 1968, when the general use of organophosphate pesticides became commercially available," explains Davey. "Coumaphos also has a zero-day withholding period, meaning you can treat animals and then take them directly to slaughter.

"Coumaphos is the only pesticide, out of a number of pesticides that were approved at one time, that is still on the market in the United States," he adds. "Other compounds that were approved for use at one time, such as dioxathion and prolate, (both organophosphates), toxaphene (organochlorine) and arsenic have been banned from use in the United States for many years because of their adverse effects on the environment and on people.

"In addition, all of the other compounds that were ever approved had a withholding period of at least 14 days," Davey says. "That is why none of them ever received wide use in the program."

Guerrero explains pesticide resistance develops through the ability of an individual tick to survive exposure to a pesticide at a level that would kill a non-resistant individual tick. "As the tick senses the presence of a pesticide, its genetics and metabolic responses, which at this point are poorly understood by scientists, determine whether it will survive the pesticide exposure," he says.

Pesticides inhibit proteins that are critical to the proper functioning of a tick’s nervous system and needed for the insect to survive.

"Pesticides target specific gene products in an organism that are critical to survival. Although there are several ways of gaining pesticide resistance, the two most common methods are target site resistance and metabolic resistance," Guerrero explains. "Target site resistance arises when a mutation occurs in the target protein of the pesticide, which alters the capacity of the pesticide to inhibit the normal function of the protein.

"The mutation might occur initially in only one or a few individuals in a population; however, since individuals that do not possess the resistance-conferring mutation are killed by the pesticide, a population may quickly become dominated by individuals that do possess the mutation in their gene," he says.

Metabolic resistance involves overproduction of naturally occurring enzymes capable of degrading toxins, including pesticides. Guerrero says these overproduced enzymes can act to degrade the pesticide directly or they can bind the pesticide and prevent its action on the target protein.

"Breeding ticks lay an enormous quantity of eggs and the embryonic development process has a natural capacity to generate mutant individuals," Guerrero says. Mutants with the enhanced ability to survive pesticide exposure will be selected in the presence of the pesticide. Eventually, pesticide resistance should be expected with any product, even under the best of conditions.

Davey says the greatest threat to the U.S. eradication program comes not from adult ticks, but rather the immature stages – nymphs and larvae. Furthermore, this may require more thorough inspections of cattle attempting to be crossed at the border.

He explains, "The inspection process is highly effective, probably greater than 99 percent, in detecting adult ticks on the animal. Adult ticks are relatively large in size and are easily seen and/or felt during the inspection. So, the likelihood of allowing cattle that are infested with adult ticks to enter the United States is very slight.

"However, the immature stages of the tick, the nymphs and larvae, are very small," Davey says. "They are very difficult or impossible to see or feel on the animal unless the inspection is extremely thorough – parting the hair and looking at the animal intensely – which is normally not done by the inspectors.

"Before resistance became widespread, the issue of missing the detection of immature ticks was not considered much of a problem because dipping the cattle at 0.3 percent a.i. (active ingredient) coumaphos was sufficient to kill 100 percent of the immature stages of the tick," Davey explains. "Resistance has dramatically changed this situation. Our research has clearly shown that a single dipping will not kill all the nymphs and larvae of a strain that is even moderately resistant. Thus, the risk of dispersing viable ticks to uninfested areas has increased dramatically."

Davey says the "scratch" inspections – where a USDA inspector appraises the animal visually and runs his hands over the animal’s body to feel for adult ticks – will no longer be sufficient. Instead, he thinks inspectors will have to conduct a more thorough examination by parting the hair and looking for the immature stages.

"This will, no doubt, have an adverse impact on the port operations because fewer animals will be able to be so thoroughly inspected, so fewer cattle will be crossed," he says. "But this is the only way that we can be absolutely sure that no viable ticks are being allowed to enter the United States."

Besides the border inspections and mandatory dipping, researchers are working on another front in the battle against pesticide-resistant cattle fever ticks – identifying them quicker and more accurately.

Guerrero and Dr. John Pruett, a microbiologist at Knipling-Bushland, have developed new tests for both coumaphos resistance and pyrethroid resistance. (Coumaphos is an organophosphate chemical; pyrethroids are another class of chemicals that could be used to control organophosphate-resistant ticks.)

It takes about six weeks to test for coumaphos resistance at the border, according to Agricultural Research, an ARS publication. Pruett has developed a one-day test that distinguishes coumaphos-resistant ticks from nonresistant ticks. Guerrero has developed a new rapid test that indicates resistance to pyrethroids. He’s also working on a field detection kit.

"Resistance is tested for by a larval packet test," explains Guerrero. "This test exposes tick larva to various doses of pesticide for a specific amount of time. The percentages of mortalities that result are statistically analyzed on a computer to determine the amount of active ingredient needed to kill 50 percent of the larvae in the test. These types of tests can take up to eight weeks to complete and can only be done on fertilized female ticks.

"Our research has developed a DNA-based test, which can assay for a pyrethroid-resistant gene mutation in an individual tick, and results can be obtained in a single day. Other DNA-based tests are being developed to speed the pesticide-resistance assays," he adds.

Guerrero says with few chemical pesticide products available for tick control and little expectation that the chemical industry will develop new chemical acaricides, they are investigating the application of technologies used in the human genome project to solve problems caused by livestock pests. "The identification of genes that regulate a tick’s response to pesticides may lead to novel methods of tick control with tick-specific toxicity, low persistence and minimal environmental impact."