COLLEGE STATION — With U.S.-Mexico trade increasing, the old problem of “Texas fever” has become new again — but Texas Agricultural Experiment Station researchers hope to see another happy ending to the latest version of the story.
“I think there will be a variety of solutions that will come about. But we are being challenged to look at alternative strategies and deal with problems in ways we haven’t had to before,” said Dr. Pete Teel, a professor and associate department head in etnomology at Texas A&M University and an experiment station researcher.
He and others are working with several Mexican and U.S. agencies to combat cattle tick fever, also known as Texas fever or babesiosis. Included are a vaccine and a computer model that will help authorities make decisions on fighting the Boophilus ticks that can cause the disease.
Tick fever was a problem that plagued the U.S. cattle industry in its early years but was declared eradicated in the United States in 1943. It was often called Texas fever because resistant Texas cattle carried the ticks north with them to midwestern markets and spread the disease.
A program of quarantines, vacating pastures and dipping cattle into large vats of pesticide-treated water virtually wiped out the disease in the United States. Today, a two-mile wide strip of quarantined land along the Texas border from Del Rio to the mouth of the Rio Grande serves as a buffer zone to prevent re- establishment of the ticks in Texas.
But the possibility of bigger markets in Mexico, where the disease is still prevalent and many cattle have built up a resistance to it, has the scientists working on the problem once more. If U.S. cattle could be vaccinated or otherwise protected against the disease, U.S. producers could sell more cattle to Mexico.
“Even today, we have situations where large numbers of cattle are moved into Mexico and 40 to 50 percent of them die from tick- borne diseases within a few months,” said Dr. Gale Wagner, a professor and coordinator of international programs for Texas A&M University’s College of Veterinary Medicine.
That’s where he and Teel come in with a host of graduate students, technicians and others working on the problem. Wagner is leading an effort to develop a vaccine for tick fever. Teel is coordinating development of computer models that will help decision-making on various disease eradication methods.
The two started working together against tick fever in 1978. Their work has at times involved cooperative efforts with the USDA’s Agricultural Research Service and APHIS, the Interamerican Institute for Cooperation in Agriculture, the Office of International Coordination and Development and the U.S. Agency for International Development, as well as several Mexican government agencies.
Wagner’s work may have already produced a temporary solution to some of the problem of exporting cattle: a “live” vaccine produced in small quantities that successfully protected 400 susceptible animals exported to Mexico. The vaccine is produced in cultures as a weakened strain of the parasite causing babesiosis and given to the cattle in the hope that it will help them produce immunity to the disease.
“So far, we’ve been very successful, even more than we expected,” he said. “But the danger is that if ticks are also present on the animals, they could transmit the disease strain to other animals, and it could revert to virulence.
“We’d like to think we can reach the same immunity by using isolated antigens that we perhaps can produce by relatively inexpensive recombinant techniques.”
Wagner thinks his group has already identified two antigens and suspects they may need a combination of three or four antigens for a successful vaccine.
“If everything worked well, we might be able to test a vaccine within five or six years. But it’s a lot of work to identify the antigens and develop the technique to reproduce them.” Meanwhile, Teel continues leading an interdisciplinary effort to develop computer simulation models to study the interaction of ticks with their animal hosts and rangeland landscapes. Also involved are Texas A&M’s departments of statistics, rangeland ecology and management, and wildlife and fisheries sciences. Teel said understanding these interactions is the key to predicting how cattle tick populations are maintained or spread.
“These models could provide information to producers and regulatory agencies for speedy and efficient elimination of infestations,” Teel said. The models incorporate previous research by Texas A&M scientists and others. They consider various vegetation zones and temperature ranges at different stages of tick development.
The team also will consider factors like the hydrology and topography of an area through use of a geographic information system. Such systems help researchers relate and interpret a wide range of data, giving them a better idea of what outcomes to expect under a variety of circumstances.
The data can be used to determine how quarantines are used, which is especially important because current quarantines for tick fever last between six and nine months and require dipping every 14 days.
One of the key factors Teel’s models will consider is the impact of wildlife, such as white-tailed deer, Nilgai antelope and European red deer on tick populations and spread of the disease.
“They are a tremendous worry. They can carry ticks, but don’t respect the fences that have been a tool in eradication and control of the tick,” Teel said.
The deer are a problem that researchers didn’t have to face earlier in the century, Teel said, because the screwworm — another eradicated pest — kept whitetail populations down. Now the deer are an economic commodity in South Texas’ traditional hunting areas.
In addition to the work of Teel and Wagner, others are working on the problem. For example, Texas A&M graduate student Rodrigo Rosario Cruz is collaborating in a program directed by Drs. Julie Scott and John George at the USDA’s Knipling-Bushland U.S. Livestock Insects Research Laboratory. The work involves development of a DNA probe, a genetic method for identifying ticks resistant to coumpaphos. Coumpaphos is the key ingredient in the cattle dip used to eradicate fever ticks.
For all the tools in the battle against tick fever, there is one factor that can’t be considered even in Teel’s models, however.
“Attitudes about tick fever are different. We’ve lost the memory for the battles fought in the past,” explained Teel, who thinks that will make it more difficult to garner support for anti- tick efforts.
“There are now relatively small numbers of people involved in the great burden of fighting these ticks,” he said. “In essence, however, they are protecting the entire southeastern United States.”