Veterinarians from the Texas Animal Health Commission (TAHC) are worried because fever ticks (Boophilus microplus) capable of carrying and transmitting deadly cattle “tick fever” protozoans Babesia bigemina or Babesia bovis, have been detected on livestock or wildlife in 139 Texas pastures during the past 12 months. More than one million acres in Texas is under quarantine, according to a statement from the TAHC.

Horse owners are worried because this tick species can also spread Babesia equi and B. caballi protozoal parasites that attack and destroy red blood cells in horses. The resulting disease is known as piroplasmosis. (There are Babesia parasites that affect horses, cattle, dogs, cats, mice, humans, and other mammals.)

“In July 2007, the first preventive quarantine was established–39,325 acres in Starr County–to enable the U.S. Department of Agriculture’s Tick Force and the TAHC to inspect and treat livestock moved from the area, get ahead of the fever tick, and push it back across the quarantine line,” said Bob Hillman, DVM, Texas’ state veterinarian and head of the TAHC, the state’s livestock and poultry health regulatory agency, in the release. “Now, a year later, we have more than a million acres under preventive quarantines in Starr, Zapata, Jim Hogg, Maverick, Dimmit, and Webb Counties, in addition to the half-million acres in the permanent fever tick quarantine zone that runs alongside the Rio Grande, from Del Rio to Brownsville.”

The fever tick, which can survive winters from coast to coast and as far north as Washington, D.C., was successfully pushed back into Mexico in 1943, according to the TAHC. Periodic tick incursions since then have occurred in Texas, but only one, in the 1970s, eclipsed the current outbreak for the number of premises infested, and that outbreak took six years to eradicate.

“Historically, fever ticks preferred cattle, and sometimes hitched a ride on horses,” the TAHC statement noted. “Now fever ticks are being detected not only on white-tailed deer and nilgai (an antelope species), but also on aoudad sheep, fallow, axis, and red deer, and elk. Fighting fever ticks on a variety of species–especially free-ranging animals that don’t respect fences–makes this battle much more difficult,” said Hillman.

B. equi and B. caballi can be transmitted by adult and nymphal ticks of several genera, including Dermacentor, Hyalomma, Rhipicephalus, and Boophilus.

Piroplasmosis

Equine piroplasmosis is a tick-borne protozoal infection of horses (EPM, equine protozoal myeloencephalitis, is another disease caused by a protozoal parasite, Sarcocystis neurona, although it is transmitted differently). It can be difficult to diagnose piroplasmosis since the parasites cause a wide variety of clinical signs, including acute fever, lack of appetite, anemia, jaundice, chronic weight loss, poor exercise tolerance, and sudden death. The disease can be fatal in up to 20% of previously unexposed animals. The tick vectors exist in the United States, and cases of piroplasmosis were seen in Florida in the 1960s. It took until 1988 before piroplasmosis was eradicated from Florida, according to Peter Timoney, MVB, PhD, FRCVS, a researcher at the University of Kentucky’s Gluck Equine Research Center.

Recovered horses become chronic carriers without clinical signs.

The only treatment is a potent type of chemotherapy that should eliminate clinical signs of disease; however, it won’t necessarily eliminate the parasites from infected horses. Recent research at Washington State University found that administration of a specific drug can effectively clear B. caballi from infected horses.

Piroplasmosis is found in nearly every country in the world except the United States, Canada, Australia, England, Ireland, Japan, and Iceland. It is estimated that only 10% of the world’s horse population is naive (has not been exposed to or developed antibodies to one or both parasites). Therefore, it is crucial that the naïve U.S. horse population avoids exposure.

Equine piroplasmosis can also be spread by contaminated needles and syringes. Piroplasmosis can infect a fetus in utero, particularly if it’s caused by B. equi. After recovery, equids can become carriers for long periods of time, probably lifelong in many cases, said Timoney. Semen from chronically infected stallions that might show no outward signs of disease could potentially spread the parasites if semen is contaminated with infected blood.

The incubation period for piroplasmosis varies between five and 21 days.

Equine piroplasmosis is considered a foreign animal disease by the USDA and, therefore, should be reported immediately to state and federal authorities, noted Timoney.

Texas’ Financial Fight

“Early this year, the fever tick program received $5.2 million of the $13 million of federal funds requested to fight the tick, and while appreciated and used, it is not enough to win this battle,” said Hillman in the release.

“It’s really a ‘pay now or pay later’ scenario, because this tick won’t be stopped with less than an all-out assault that requires adequate personnel, sufficient treatment products, and enough equipment, such as portable dipping vats or portable spray boxes for cattle, and treatment equipment for deer and other wildlife hosts,” continued Hillman. “Texas has a ticking time bomb in south Texas. So far, we have had only two of the three elements for a ‘tick fever’ outbreak–fever ticks and fever tick hosts, including cattle, horses, and several species of wildlife. If, however, some of these fever ticks carry Babesia, a blood parasite deadly to cattle, the equation would be complete and we could see livestock death losses.”

“Fighting fever ticks may seem simple, but it’s not easy … and it’s never cheap,” said Hillman.

References

Equine Piroplasmosis; Iowa State University College of Veterinary Medicine, Center for Food Security and Public Health; with the Institute for International Cooperation in Animal Biologics.

Equine Babesiosis–A Review; Russell Z. Edwards, DVM; Holly Moore, DVM; Bruce E. LeRoy, DVM, PhD; and Kenneth S. Latimer, DVM, PhD; Class of 2005 (Edwards) and Department of Pathology (Moore, LeRoy, Latimer) College of Veterinary Medicine, University of Georgia.

TheHorse.com articles on Piroplasmosis

Personal communications with Dr. Peter Timoney, University of Kentucky Gluck Equine Research Center