Quarantined cow goes through
a tick treatment bath at an
APHIS facility in McAllen, Texas.
Cattle fever tickswhich include the southern cattle tick, Boophilus
microplus, and the cattle tick, B. annulatusbecame
infamous in the years after the Civil War, when southern ranchers drove
cattle to northern markets. Southern cattle had acquired immunity to
bovine babesiosisthe disease transmitted by these ticks. But the
cattle drives carried the disease and the ticks to vulnerable northern
Although cattle fever ticks were eradicated from the United States
in about 1943, they are plentiful in Mexico today. A quarantine zone
in South Texas along the Mexican border is currently the only barrier
to their reentry into the United States, where all cattle are susceptible
to the disease.
Cattle entering the United States from Mexico are routinely dipped
in the organophosphate coumaphos, the only pesticide approved for use
in dipping vats. "Coumaphos is the most effective product we have
to use, but concerns about coumaphos resistance have prompted a search
for alternatives. Our project focuses on resistance to several classes
of pesticides," says ARS
physiologist Felix D. Guerrero. He and ARS microbiologist John H. Pruett,
at the Knipling-Bushland U.S. Livestock Insects Research Laboratory
in Kerrville, Texas, are finding ways to identify pesticide resistance
in cattle fever ticks.
They have identified two independent mechanisms by which ticks become
resistant to pyrethroids. They've found one strain of resistant Mexican
tick possessing a gene that produces a large amount of a specific esterase
protein, called CzEst9, which is involved in the breakdown of pyrethroids.
The protein's name reflects the Mexican city of Coatzacoalcos, where
Mexican scientists originally collected these ticks. Pruett isolated
CzEst9 from this strain of tick, which is reared at ARS' quarantine
facility in Mission, Texas. Pruett's work dovetailed with subsequent
studies in which Guerrero cloned the CzEst9 gene. Together, Guerrero
and Pruett are working on a quick test to measure the amount of CzEst9
in ticks as an indicator of resistance to pyrethroids.
Guerrero has already devised a rapid test for pyrethroid resistance.
A tick larva is squashed in a tube to extract its DNA, which is then
analyzed by polymerase chain reaction followed by gel electrophoresis.
A diagnostic band on the gel, produced by resistant ticks, indicates
the presence of a DNA mutation responsible for conferring pyrethroid
Though widespread, pyrethroid resistance is not ubiquitous in Mexico.
So pyrethroids can still play a role in killing ticks that are resistant
to organophosphates, like coumaphos.
At the border, it takes at least 6 weeks to test ticks for coumaphos
resistance, because many live tick larvae are needed. Pruett has adapted
a faster laboratory test that uses color to distinguish coumaphos-resistant
ticks from nonresistant ones. This test takes only a day to complete
and can be performed on either crushed tick larvae or a dissected adult
tick brain. If the tick is resistant to coumaphos, the medium in the
test vial turns yellow; if not, it stays clear.
Meanwhile, Guerrero is striving to develop a field detection kit to
identify resistant Mexican ticks. "Someday we hope to have a laboratory
in a suitcase for identifying resistant tick strains in the field, so
that we won't need to transport them or rear them in a lab," he
says.By Linda McGraw, formerly
This research is part of Arthropod Pests of Animals and Humans,
an ARS National Program (#104) described on the World Wide Web at http://www.nps.ars.usda.gov.
Felix D. Guerrero and John
H. Pruett are at the Knipling-Bushland
U.S. Livestock Insects Research Laboratory, 2700 Fredericksburg
Rd., Kerrville, TX 78028-9184, phone (830) 792-0308, fax (830) 792-0314.