Theileria is a genus of parasites that belongs to the phylum Apicomplexa, and is closely related to Plasmodium. Two Theileria species, T. annulata and T. parva, are important cattle parasites. T. annulata causes tropical theileriosis and T. parva causes East Coast fever. Theileria species are transmitted by ticks. The genomes of T. orientalis Shintoku, Theileria equi WA, Theileria annulata Ankara and Theileria parva Muguga have been sequenced and published.
Theileria equi infects equid blood cells causing equine piroplasmosis. The disease presents with a variety of clinical conditions, such as fever, depression, jaundice, cramps, haemolytic anaemia, hemoglobinuria and even death, but asymptomatic infections are frequently observed. The most common vectors are the ticks Dermacentor nitens and Rhipicephalus microplus but Amblyomma cajennense was also implicated in the disease transmission.
Vaccines against Theileria are in development. In May 2010, a vaccine that was reported to protect cattle against East Coast fever had been approved and registered by the governments of Kenya, Malawi, and Tanzania.
Species in this genus undergo exoerythrocytic merogony in the lymphocytes, histiocytes, erythroblasts, and other cells of the internal organs. This is followed by invasion of the erythrocytes by the merozoites, which may or may not reproduce. When merogony does occur, no more than four daughter cells are produced. The frequent occurrence of elongated bacillary or "bayonet" forms within the erythrocyte is considered as characteristic of this genus.
The organism is transmitted by various tick species, including Rhipicephalus, Dermacentor, and Haemaphysalis. The organism reproduces in the tick as it progresses through its life stages. Both T. annulata and T. parva induce transformation of infected cells of lymphocyte or macrophage/monocyte lineages. T. orientalis does not induce uncontrolled proliferation of infected leukocytes and instead multiplies predominantly within infected erythrocytes.
Following infection with Theileria equi, horses may develop detectable antibodies, with seroprevalence varying widely across regions. Serological surveys have revealed a high prevalence of T. equi antibodies in horses and other equids, including 39.8% in Central-Southern Italy and 33.4% in Northern Brazil.
The genomes of T. orientalis Shintoku, Theileria equi WA, Theileria annulata Ankara and Theileria parva Muguga have been sequenced. Genomic data can be accessed though PiroplasmaDB which is part of the Eukaryotic Pathogen Database).
The genus is thought to have first appeared in ruminants during the Miocene. It is named for parasitologist Gertrud Theiler, daughter of Arnold Theiler.
Theileria spp. can be transmitted to cattle through tick bites, including the brown ear tick, a Rhipicephalus sp.
In North America, Theileria orientalis (specifically the virulent Ikeda genotype) has emerged as a significant One Health challenge, representing a complex intersection of invasive species biology, agricultural stability, and wildlife livestock interfaces. Since its first detection in a Virginia cow-calf herd in 2017, the parasite has undergone a rapid geographical expansion across the United States. As of early 2026, the Ikeda genotype has been confirmed in at least 25 states, including recent expansions into Missouri, where it is considered endemic in over half of the state's counties, as well as first-time detections in Ontario, Canada in late 2025.
The primary driver of this emergence is the invasive Asian longhorned tick (Haemaphysalis longicornis). This vector is uniquely suited for rapid dispersal due to its parthenogenetic nature, allowing a single asexual female to establish a massive localized population without a mate. While the disease primarily impacts cattle, the "Animal" pillar of the One Health triad extends to wildlife; 2026 studies have detected T. orientalis Ikeda DNA in ticks collected from various non-bovine hosts, including raccoons, opossums, and domestic cats. Although clinical disease has not been documented in these wildlife species, they facilitate the spread of the vector across fragmented landscapes.
Although T. orientalis Ikeda does not currently pose a direct infection risk to humans, it significantly impacts human well-being through threats to food security and agricultural economics. In newly affected regions, naive cattle herds can experience mortality rates of up to 5%, while surviving animals often suffer from chronic anemia, reduced weight gain, and late-term abortions. The lack of an FDA-approved treatment or vaccine in the United States as of 2026 has forced a management shift toward aggressive tick suppression and biosecurity, with annual industry losses estimated in the millions of dollars.