Theileria parva

Location and host

• This parasite is found in RBCs, lymphocytes and histocytes of cattle and buffalo.

Morphology

• Trophozoite forms in erythrocyte are rod-shaped but may also be round, oval and comma-shaped.
• Koch bodies are present in lymphocytes and endothelial cells of spleen or lymph nodes. Average size is 8µm but can range upto 12 µm or more.
• Schizont is circular or irregular shaped and measures about 2µm-12µm in diameter. Two forms of schizonts are recognized ; macro schizonts contain large chromatin granules and produces macro merozoites. Micro schizonts contain smaller chromatin granules and produce micro merozoites.

Lifecycle

In Cattle:

• Cattle are infected by infected tick vector. Tick releases large number of sporozoites in blood.
• Sporozoites don’t transmit immediately upon feeding. First, they develop in salivary gland during the first two to four days of engorgement.
• After infection, first visible stages occur in lymph nodes after 5-8 days of infection. These stages are small-round bodies with round nucleus.
• Schizogony occurs in lymphoid and reticulo-endothelial tissues. Macro schizont develops synchronously with lymphoblast to produce two infected cells. This macro schizont in lymphocyte is called Koch’s blue bodies.
• About 12 days of infection, micro schizonts develop from macro schizonts.
• Micro merozoites are produced from micro schizonts which enters erythrocyte.
• Later on, both forms form micro and macrogamont inside RBCs after few binary fissions.

In tick

• After ingestion of intra-erythrocytic stage of parasite (merozoites) by tick host, RBCs lyses and merozoites are liberated.
• Liberated merozoites differentiate into sexual stages. Macrogamonts and microgamonts develops.
• Gametes from these gamonts fuse to form zygote. Zygote leaves gut cell and reaches to hemolymph to become ookinete.
• Ookinetes further develops into sporoblast.
• Inside sporoblast, sporonts are formed through process of sporogony.
• Sporozoites are produced and released into saliva of tick. These then enter new host when tick feeds on new host.

• For completion of lifecycle, piroplasm require to be ingested by larvae or nymphal stages. Sexual phase occurs in gut of tick followed by formation of sporoblast in salivary glands. No further development occurs until next stage of ticks starts to feed.
• Incubation period: 8-24 days
• No transovarian transmission.

Transmission

• Bite of vector tick; Rhipicephalous appendiculatus

Pathogenesis

• Sequence of events in typical acute and fatal infection progress through three phases:

1. 1^st phase: Incubation period of about 1 week. Neither parasite nor lesion detected.
2. 2^nd phase: It is followed by marked hyperplasia and expansion of infected lymphoblast population, initially in regional lymph nodes draining site of tick bite and ultimately throughout body. It occurs during 2^nd
3. 3^rd phase: There is lymphoid depletion and disorganization associated with massive lymphocytolysis and depressed leucopoiesis.

• Disease caused by this parasite is called East coast fever.
• Higher mortality in susceptible stock.

Clinical signs

• Parotid lymph node becomes enlarged and animal become pyroxic (40-41.7 ⁰C, 104-107 ⁰F).
• There is generalized swelling of superficial lymph nodes , eyes, ears and sub mandibular region.
• Animal become anorexic, shows decreased milk production and rapid loss of condition.
• Cessation of rumination
• Animal becomes weak with rapid heartbeat.
• Detechial hemorrhages may occur under tongue and on vulva.
• Blood-stained diarrhea and dyspnoea.

PM findings

• Enlargement of superficial lymph nodes.
• Spleen is usually enlarged with soft pulp and prominent Malpighian corpuscles.
• Liver is enlarged, friable, brownish-yellow.
• Kidneys either congested or pale brown.
• Flabby heart with petechiae on epicardium and endocardium.
• Lungs are congested and edematous.
• Hydrothorax and hydro pericardium and kidney capsule contain large amount of serous fluid.
• Petechiae in visceral and parietal pleura, adrenal cortex, urinary bladder and mediastinum.
• Characteristics ulcers of 2.5 mm in abomasum, small and large intestine.
• Swollen payers patches and yellowish intestinal contents.

Diagnosis

• On the basis of necroscopy findings, clinical signs
• Demonstration of schizonts in biopsy smears of lymph nodes.
• Giemsa-stained blood smears show piroplasm in red cells.
• Serological test: Indirect fluorescent antibody test.

Treatment, prevention and control: Same as in Theileria annulata