Growing Beef Newsletter
August 2025, Volume 16, Issue 2
Asian longhorned ticks and Theleriosis
Grant Dewell, ISU extension beef veterinarian
In June, the Iowa Department of Agriculture and Land Stewardship confirmed a case of Theleriosis transmitted by the Asian longhorned tick in a herd of cattle in southeast Iowa for the first time. Currently, seven herds have been diagnosed with the disease, all in the southeast corner of Iowa. Going forward, Iowa cattle producers will have to deal with this issue as there is not a reasonable means to eliminate the problem. Based on the number of herds we have seen with the problem and extensive tick infestations within these herds, we can conclude that both the tick and protozoan blood parasite have been in Iowa for a year or two and have now reached the point where obvious clinical disease is noticed.
Theileria is a protozoan parasite that infects red and white blood cells leading to anemia and death in some cases. The parasite is carried by Asian Longhorned Ticks (ALHT) which amplify the disease as the parasite goes through a life stage in the digestive tract of the tick. Typically, cattle affected by Theileriosis will show signs of lethargy, anemia and difficulty breathing. They may develop ventral edema, exercise intolerance, jaundice and abortions. There are some models that predict southeast Iowa and the Mississippi River valley as the potential habitat for the tick. Although central, western and northern Iowa may not be ideal habitat for the ALHT, there is definitely suitable habitat in these areas that will support the tick, so expect the tick and the parasite to slowly spread across Iowa. Southeastern Iowa will remain the hot spot for the next few years, but it will migrate due to movement of cattle, deer and birds.
For the rest of the summer and this fall, southeast Iowa should be aggressive at tick control. Fall-calving cows are susceptible to late-term abortions and stillbirths as well as clinical disease as they near the end of gestation. Young calves are also at risk as colostral immunity plays out and calves are stressed around weaning. Backrubbers or sprays will be most effective at keeping ticks under control in late summer and the fall. Also, it's important to make sure that cattle’s general health is good to allow them to fight off the clinical disease until they can mount an immune response. Cows should be in a good plane of nutrition with access to a good trace mineral supplement and receive a basic herd health program.
Next spring, the first step will be to limit tick numbers in the environment. Burn pastures in late spring before they green up, then keep them mowed as best you can. Remove or fence cattle out of brushy areas to limit contact with ticks. I recommended that cows be poured with a pyrethroid product that has a label for tick control prior to pasture turnout. Monitor cows regularly and when ticks become evident, implement use of backrubbers to get more dispersion of insecticides on cattle. If tick numbers continue to increase, switch to an organophosphate insecticide, and if necessary, treat cattle with an injectable avermectin product. If you suspect either Theileria or ALHT, have your veterinarian collect appropriate samples and submit to the ISU Veterinary Diagnostic Lab.
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