Theileria

Theileria are tick-transmitted protozoan parasites of domestic and wild ruminants. Theileria parva and Theileria annulata are members of this genus that cause debilitating and often fatal disease in cattle. In eastern, central and southern Africa, the most important species is Theileria parva, which causes East Coast fever. In addition to cattle, T. parva infects African buffalo and waterbucks, both of which serve only as reservoirs of infection.

T. parva threatens the lives of over 25 million cattle and is a severe economic constraint to cattle farming in Burundi, Kenya,Malawi, Mozambique, Rwanda, Sudan, Tanzania, Uganda, the Democratic Republic of the Congo, Zambia, and Zimbabwe.

The existence of three subtypes of T. parva – Theileria parva parva, Theileria parva bovis and Theileria parva lawrenceicomplicates the search for better control methods. All three are transmitted by the brown ear tick, Rhipicephalus appendiculatus, but only two – T. p. parva and T. p. lawrencei produce severe disease in cattle.

T. p. parva and T. p. bovis are transmitted between cattle. T. p. lawrencei is transmitted to cattle mainly from buffaloes, which act only as reservoirs and do not show any clinical symptoms of theileriosis.

Life cycle

The life cycle of T. parva is complex. In both the tick and the mammalian host, the parasite undergoes a series of cellular transformations into different forms. Upon ingestion by R. appendiculatus tick, the parasite undergoes differentiation first in the tick gut, leading to the formation of forms called kinetes. The kinetes migrate to the tick’s salivary glands, where they differentiate into infective sporozoites.

As the tick feeds on cattle, the sporozoites are injected into the blood along with tick saliva. In the host, the parasites attach to and enter lymphocytes. Within two to three days of invading the lymphocytes, the sporozoites develop into intracellular forms called schizonts. The infected lymphocytes grow bigger and begin to divide. These enlarged lymphocytes are known as lymphoblasts. The division of the lymphocytes ensures that each daughter cell is infected. In the end, large numbers of lymphocytes are infected so that the infection spreads throughout the lymphatic system, leading to the widespread destruction of the lymphatic cells.

In the later stages of the infection, some of the schizonts differentiate into merozoites that are released into the bloodstream where they invade red blood cells. In the red cells, the parasites change into forms that are called piroplasms, which are infective to ticks. As the ticks feed on infected animals, they ingest red blood cells that are infected with piroplasms, and this completes the life cycle. The incubation period is usually 10 to 25 days

Pathogenesis and symptomatology

Typical clinical symptoms associated with theileriosis include inflammation and swelling of the lymph nodes, followed by generalized lymphadenopathy, fever, anorexia, and rapid deterioration of condition. Other symptoms include lacrimation, nasal discharge and corneal opacity, an increased respiratory rate and diarrhoea. Death is usually due to pulmonary oedema, severe dyspnoea and discharge. Some animals develop a fatal disease called ‘turning sickness’, in which infected cells block capillaries in the CNS and cause neurological signs. Some animals recover from the infection and become asymptomatic carriers; others may have poor productivity while others may become stunted in growth.

Post-mortem examination shows extensive haemorrhages in different tissues and body organs, they lymphoid system is greatly swollen but may be shrunken in chronic cases, the liver and the spleen are enlarged. The lungs are reddened and both the trachea and bronchi are filled with fluid and froth.

The main effect of theileriosis on the host is the extensive destruction of the lymphatic cells by multiplying schizonts.Morbidity and mortality vary with host’s susceptibility and the strain of parasite. The mortality can reach 100% in susceptible cattle from nonendemic areas in three to four weeks of infection. However, mortality is low in indigenous zebu cattle in endemic areas. Although animals that recover from the infection acquire immunity that may last for a long time, some of the animals act as reservoirs of infection, or show low productivity and appear stunted.

Diagnosis

Theileria should be suspected in tick-infested animals that present with fever and enlarged lymph nodes, and where mortality seems to affect mainly calves. Diagnosis is made by identification of the schizonts in thin blood smears, lymph node and liver biopsies. At necropsy, schizonts may be found in impression smears from internal organs.

Polymerase chain reaction (PCR) tests and DNA probes are sometimes used to identify Theileria species. Antibodies to T. parva, can be detected with an enzyme-linked immunosorbent assay (ELISA) and other immunological tests.

Control

In areas where East Coast fever is endemic, dipping or spraying cattle with an acaricide to kill the ticks is a sure way of controlling the disease. The dipping or spraying of the animals must be carried out regularly. Because cattle that are regularly treated with acaricide are not exposed to T. parva, they develop no immunity and thus have no protection against the parasite or other tick-borne diseases if treatment is interrupted. Regular application of the acaricides has its drawbacks- it is environmentally unfriendly and can lead to emergence of insecticide resistant ticks.

Treatment of theileriosis is possible with drugs and vaccines.