La que sigue: The one that follows. Once upon a time, I used this phrase often when I spent most of my professional time toiling in arm-length plastic sleeves behind hundreds of Holstein and Jersey cows lined up in Hatfield lockups. 

It functionally meant “next cow,” but what it really meant when I stood up straight after emerging victorious from a reproductive examination on a dairy cow and said, “La que sigue,” was “that’s one down, bring on the next one.” 

Lately, it’s taken on a new meaning for me. Now, when I recall the words, “La que sigue,” I often wonder about the next pathogen. What infectious agent will be the next pathogen that challenges our systems and exposes our vulnerabilities in the cattle industry? How will we respond to meet that challenge?

As far as potential candidates for the one that follows, there are a few to choose from. Our colleagues in the dairy industry recently had a scare with H5N1 avian influenza. The specter of New World screwworm (NWS) has also been looming large for several months now. There is plenty of discussion worth having regarding both of these emerging disease threats. However, there is one pathogen that has been dominating my thoughts more than any other these past few months: Theileria orientalis genotype Ikeda.

Theileria (pronounced “tie-leer-ee-uh”) is a genus of protozoal, tickborne parasites. Native species of Theileria have existed in the United States for several years, and most of them are largely harmless. However, the Ikeda genotype emerged in the late 2010s. It was first detected in Virginia in 2018 and has since rapidly spread across the United States. 

The primary vector for this strain of Theileria is the Asian longhorned tick (Haemaphysalis longicornis) which is a foreign, invasive tick species that seemed to arrive in the United States about the same time as T. orientalis Ikeda. The arrival of the Asian longhorned tick is a bad enough threat on its own, though with regards to the cattle industry the onslaught of rising T. orientalis Ikeda infections seems to be the most consequential point of impact.

The symptoms

T. orientalis Ikeda, plus a few other genotypes, infect the red blood cells of cattle, which eventually leads to red blood cell destruction, fever, anemia, abortion in females and often death from hypoxia (inability of the blood to carry oxygen). 

When T. orientalis causes this constellation of clinical signs, we call it theileriosis. The clinical presentation of theileriosis is very similar to the clinical presentation of anaplasmosis, though there are two features of theileriosis that distinguish it from anaplasmosis. 

First, anaplasmosis is caused by a parasitic bacterium (Anaplasma marginale) and has some susceptibility to antibiotics such as tetracyclines. 

On the other hand, Theileria are protozoans, which means that even though they are single-celled organisms, their genetics and metabolism are sufficiently different from bacteria that most antibiotics are largely useless against them. The second distinguishing characteristic is that — while young cattle are pretty much immune to the severe clinical effects of anaplasmosis — young cattle are just as susceptible to theileriosis as are older cattle.

While the Asian longhorned tick is the primary biological vector for this disease, other tick species may also be competent vectors, and there is evidence that biting flies, lice and needles may also be able to transmit Theileria. Many cattle carry Theileria organisms and show no signs of clinical disease. Those that do come down with theileriosis and recover often become lifelong carriers. This ability to set up a carrier state can allow this parasite to spread effectively throughout cattle populations. While the hallmark features of theileriosis are red blood cell destruction, anemia and hypoxia, there is another potential aspect that may be more troubling, and unfortunately, we don’t know a whole lot about it currently.

Recently, many of our industry partners in the feedlot industry have reported abnormally high levels of disease outbreaks and death in fall placed cattle. I am hesitant to oversimplify the problem, but it suffices to say that T. orientalis Ikeda has been isolated from many of the cattle that died in these wrecks. To be clear, most of the cases I have firsthand knowledge of also had evidence of extensive respiratory disease on postmortem examination. 

While T. orientalis Ikeda has been implicated as a potential source of increased risk of respiratory disease in feeder cattle, the evidence supporting that claim is spotty at best. Notwithstanding, I have spoken to several veterinarians and cattle feeders that are greatly concerned about this emerging pathogen, and the demand for a treatment is growing.

Unfortunately, the only drug that is supposedly effective against T. orientalis is buparvaquone, which is not approved for use in the United States in cattle; nor is it likely to be in the near future. 

An emerging challenge

Buparvaquone has some efficacy against theileriosis, but it also has a tendency to persist in the treated animal’s system for several weeks. This means any approved use of buparvaquone would require lengthy slaughter withdrawal periods, thereby decreasing its potential utility.

Currently, the best tools available in the United States for mitigating any possible effects of T. orientalis infections in feeder cattle are greater biosecurity and biocontainment.

Unfortunately, T. orientalis Ikeda is rapidly spreading across North America, so it may become increasingly difficult to source cattle that are at a negligible risk of carrying the infection. Switching needles between injections when processing cattle will likely help, but it doesn’t eliminate transmission due to ticks, biting flies and/or lice, placing added importance on insect control strategies.

While this emerging challenge may seem daunting, it is important to remember that because we know so little about possible contributions of T. orientalis Ikeda to respiratory disease during the feeding period, we should not automatically assume theileriosis is behind any increases in disease or death we observe; even if the organism itself is detected in cases of disease. The recognized pattern of destruction with T. orientalis Ikeda is red blood cell destruction and anemia; if these signs are absent, then any connection between the detection of the parasite and the cause of disease is purely speculative.

It’s difficult to say at this stage how big of an impact T. orientalis Ikeda will have going forward. It is a keen area of research interest for me and my colleagues; stay tuned for new information as it emerges. In the meanwhile, be aware that this pathogen is out there, and that it may be worth looking into if your operation is afflicted by an inexplicable rise in cases of disease and death, though any conclusions drawn from the detection of this organism in these types of cases should be tempered with an open mind and careful deliberation with your veterinarian.  

Editor’s note: Todd Gunderson is a clinical assistant professor, beef production medicine, at Kansas State University in Manhattan, Kan.