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After two more infections, experts advise cattle producers to protect their herds from tick-borne parasite
Theileriosis, which affects cattle, is spread through Asian longhorned ticks
Jul. 14, 2025 5:17 pm, Updated: Jul. 15, 2025 8:51 am
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More Iowa herds have tested positive for a tick-borne parasite that can sicken and kill cattle, but experts said they still do not expect the illness to become widespread across the state.
Grant Dewell, associate professor and beef veterinarian with Iowa State University Extension and Outreach, said four cattle herds in southeast Iowa are confirmed to have theileriosis, a disease that spreads through parasites carried by Asian longhorned ticks.
Two herds were confirmed to have theileriosis last month.
Dewell said a fifth herd has been confirmed to have the tick, and there are clinical signs the cows have theileriosis, but a blood sample from the herd had not yet confirmed the parasite infection.
Theileriosis infects both red and white blood cells in cows, leading to anemia and difficulty breathing. The disease can be fatal in some cases.
So far, Dewell estimated that about 30 cows in Iowa have died from the disease. Twenty of those animals were from the first herd, which numbered 100 head. Dewell said two to three cows from each of the other infected herds also have died.
Dewell said the first herd’s death toll was more severe than others because the cows were purchased from Montana about six months ago and had not previously been exposed to Iowa’s environment in the summer.
“They were just extremely naive cows,” he said.
The parasite in Asian longhorned ticks was first detected in Iowa in mid-June. According to ISU Extension and Outreach, it is common to see a surge of infestations when cattle are first introduced into a new pasture.
When the ticks find a new host, a surge of ticks typically follows, as the females will begin laying thousands of eggs.
The disease posts no health risk to humans.
‘Coming in from the edges’
Asian longhorned ticks were first confirmed in the United States in 2017, although they likely were in the country before being confirmed. Since then, they have been spreading west.
Teresa Steckler, Illinois Extension commercial agriculture specialist in cattle, said the tick’s presence has been confirmed in just one county, making Illinois the 20th state to have it.
It’s likely there are more counties that have it, she said, but they aren’t confirmed.
Dewell said Missouri had its first case of theileriosis about three years ago.
The ticks likely made their way to Iowa via Illinois and Missouri, Dewell said. Their confirmed presence in Iowa has been confined to the southeast corner, in Van Buren, Davis and Appanoose counties.
Despite the new cases, Dewell said the disease isn’t likely to become widespread throughout Iowa.
“Just because our router transmission is via tick, it's going to take a while for that tick to spread beyond the few counties that it's in,” Dewell said.
“It's basically coming in from the edges,” Dewell said. “And so, it's going to take it a while to move beyond those first-tier counties.”
Protecting the herd
Generally, Dewell said the guidance for producers in caring for their herds has not changed.
Producers — especially those in Iowa’s southern counties — are advised to use more insecticides to control the tick population. They can spray or pour insecticides onto cattle to prevent infection.
With four — and possibly a fifth — confirmed infections, the state is starting to more aggressively work to control the tick, Dewell said. This includes outreach with producers about how to protect their herds, helping to confirm cases, and spraying insecticides in tall grass areas in southern Iowa and along the Mississippi Valley.
Dewell said the Iowa Department of Agriculture will continue monitoring the tick population and reevaluate it.
There is no treatment for the disease, only supportive care. Dewell said this can include giving the cattle more fluids, and in some cases, blood transfusions.
He said that infected cattle will be lethargic or exercise-intolerant because their red blood cell count is low. He said the cow’s mouth and eyes might look pale and have a “jaundice look” to them.
Considering a cow’s size and how much blood they have, Dewell said a few ticks isn’t a problem. But when a cow has thousands of ticks on them, it’s a bigger concern.
Besides spraying insecticides, Dewell said herd owners also can prevent infection by mowing their tall grasses down, since ticks often prefer tall grasses for finding a host.
He said that burning pastures in the early springtime also can be an option for some, but it must be in a controlled environment.
Managing this will “take some planning ahead of time. It's kind of tough when it showed up in June and trying to change that,” Dewell said. “But next year, we can have some better plans for using pastures in a different way that we might be able to suppress that tick environment a little.”
Olivia Cohen covers energy and environment for The Gazette and is a corps member with Report for America, a national service program that places journalists in local newsrooms to report on under-covered issues.
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Comments: olivia.cohen@thegazette.com
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