Theileria is a genus of protozoan parasites (family Theileriidae, suborder Piroplasmorina) transmitted by ticks. Several species cause theileriosis in ruminants by multiplying in the white blood cells, causing tumor-like growth, and then invading the red blood cells. The Wellcome Trust estimates that 240 million cattle in tropical and subtropical climates are at risk from bovine theilerosis caused by a single species of theileria.
Life Cycle of Theileria
Larval or nymphal ticks ingest parasite-infected red blood cells. In the gut of the tick, the parasite develops and, as the tick matures, the parasite travels to the salivary glands. The infective parasite is transmitted to animals in the saliva when the nymphal or adult tick feeds. The parasite infects the host’s lymphocytes, where the parasite replicates when the infected cells divide. Differentiated and mature parasite is released by the lymphocytes in the bloodstream, where they infect the erythrocytes (red blood cells) to be taken up by another tick and begin the cycle again. The International Livestock Research Institute has a diagram illustrating the infection process. Most species of the parasite do not cause symptoms in infected animals.
Theileria parva - East Coast fever
A form of bovine theileriosis called East Coast fever is caused by Theileria parva. This parasite is most often transmitted by the brown ear tick (Rhipicephalus appendiculatus) in Africa, from southern Sudan to South Africa and Zaire. The parasite infects cattle, African buffalo, waterbucks, and Indian water buffalo, with nonnative herds being at particular risk. Though mortality is high among infected animals, the disease is considered to be relatively mild compared to that caused by the annulata species.
Theileria annulata – Tropical theileriosis
Also known as Theileria dispar, the annulata species is transmitted by Hyalomma ticks and results in the destruction of red blood cells in infected cattle and water buffalo. This parasite is found in southern Europe and the Mediterranean coast into the Middle East, Asia, and northern Africa. In addition to hemorrhaging, neurological signs often lead to tropical theileriosis being called the “turning sickness”, as described by Iowa State University’s College of Veterinary Medicine. This species of the parasite is considered to be the most concerning for the cattle industry.
The lestoquardi species, previously known as hirci, is the most virulent among sheep and goats. According to Iowa State University, documented cases have been noted in Asia, the Middle East, Africa, and Europe. The parasite is now known to be transmitted by the Hyalomma species of ticks, though Rhipicephalus bursa was once considered to be the vector. Disease and mortality reflects that of East Coast fever in cattle.
Other Animals Infected by Theileria
Camels – Theileria camelensis
Deer – Theileria cervi, appears only in splenectomized animals
Platypus – Theileria ornithorhynci
Reindeer – Theileria tarandi, transmitted by Taiga ticks (Ixodes persulcatus)
Theileria in the Southern Hemisphere
Though most species of the parasite are found in the Northern Hemisphere, a few species have been identified in Australia and New Zealand. A disease similar to East Coast fever, known as corridor disease or buffalo disease, is caused by the lawrenci subspecies of Theileria parva transmitted from buffalo to cattle by brown ear ticks as reported by the New Zealand agriculture agency.
Theileria buffeli (or buffali) is found in cattle and buffalo in Australia, transmitted by Haemaphysalis longicornis and Haemaphysalis bancrofti ticks, but it only results in sporadic disease.
Once thought to be nonpathogenic except for anemia in imported cattle, Theileria orientalis has been linked to outbreaks of theileriosis in Australia, as described in the journal Parasites&Vectors in February 2011.
Mildly or Nonpathogenic Theileria Species
Other Theileria species in goats and sheep include separata and ovis. Both are found in Asia and are primarily transmitted by Hyalomma ticks according to Iowa State University. Ovis can sometimes cause mild disease. Other species that can cause anemia in cattle are sergenti (in Asia) and mutans. Theileria mutans is transmitted by Ambylomma ticks and was found in the United States from 1950 to 1975, though it is now limited to Africa and the Caribbean. Species that are generally nonpathogenic in cattle are taurotragi and velifera.