East coast fever
Bovine theileriosis is a tick-borne protozoan parasite disease caused by six species of Theileria spp.
Two of these species, Theileria parva and Theileria annulata, are pathogenic and responsible for significant economic and production losses in affected ungulates. T. parva', the causative agent of East Coast fever, occurs in Africa between southern Sudan, South Africa, and Zaire. T. parva can infect cattle, Indian water buffalo, African buffalo and waterbucks, however cattle and Indian water buffalo are the only species that develop symptomatic infections. Those breeds not native to areas where East Coast fever occurs are particularly susceptible.
The primary vector for East Coast fever is Rhipicephalus appendiculatus but R. zembeziensis and R. duttoni also are capable of transmitting the disease. Protozoal sporozoites are produced in the salivary glands of nymph or adult ticks, and subsequently inoculated into a susceptible animal during feeding. Typically, a tick must be attached for 48-72 hours before it transmits the sporozoites. However, if environmental temperatures are high, the organism can be transmitted in only a few hours. Once inside the host, the organisms target lymphocytes. Schizonts may be found in regional lymph nodes within 5-8 days after inoculation. The schizont, also referred to as a macroschizont or Koch’s blue body, causes blast-transformation of the infected lymphocyte (Figure 1) which produces two daughter cells, each of which contains a schizont. Every 3 days, the number of schizonts increases ten-fold. Approximately 10-25 days post-infection (average 14 days), schizonts develop into merozoites which are also known as microschizonts. These merozoites are initially found in the cytoplasm of lymphocytes, reticular cells, and macrophages. They subsequently invade erythrocytes where they become piroplasms. The piroplasm-containing erythrocytes are ingested by larval or nymphal ticks during feeding. Once inside the gut of the tick, the protozoa undergo a sexual cycle which results in the formation of a motile stage of the organism which allows it to reach the salivary glands of the tick so the entire life cycle can begin again. Figure 1: Wright’s-stained impression smear from an enlarged lymph node of a cow with East Coast fever. Smear is composed of numerous large lymphoblasts with irregular nuclear outlines and prominent nucleoli.
The incubation period for East Coast fever is 7-25 days. Initial signs include anorexia, pyrexia (up to 106° F), and enlargement of the draining lymph nodes. The tick vector commonly feeds around the host’s ears, thus the parotid node is most commonly involved. Later, a generalized lymphadenopathy occurs and the affected animal continues to lose body condition. Other signs that may be seen include lacrimation, nasal discharge, diarrhea, corneal opacity, and dyspnea. Although East Coast fever can cause a chronic wasting disease, most infections are fatal within 18-30 days. Prior to death, the animal is recumbent and there is a drop in body temperature. Anemia and leukopenia may also be present. Dyspnea intensifies and a frothy nasal discharge due to pulmonary edema may be observed. Some animals also develop neurologic disease just before death. This condition is called "turning sickness" and occurs when affected cells block capillaries within the central nervous system causing ischemia (Figure 2). The few animals that survive East Coast fever become lifelong immune carriers (due to the persistence of the schizont stage of the parasite within the host) and fail to thrive. Figure 2: hematoxylin and eosin-stained-section of brain and meningeal vessels from a cow with East Coast fever. Note the vessel is filled with large lymphoblasts, macrophages containing schizonts, and fibrin thrombi.
Animals that die from East Coast fever will have severe pulmonary edema with froth in the trachea and nares. In some animals, interlobular emphysema is also noted. Lymph nodes are typically enlarged and may be hyperemic, hemorrhagic, and/ or edematous. This change does not occur with chronic East Coast fever in which the lymph nodes tend to be shrunken and necrotic. Ulceration and hemorrhage of the entire gastrointestinal tract is common, but the most severe lesions occur in the small intestine and abomasum where necrosis of Peyer’s patches is frequently observed. Hepatomegaly, splenomegaly, and pseudoinfarcts (white foci due to lymphoid cellular infiltration) of the liver and kidney are also typically present. In the heart, hemorrhagic myocardial degeneration is seen. Small hemorrhages may be noted on the serosa of all internal organs and body cavities may contain serous fluid.
East Coast fever should be suspected in a R. appendiculatus, R. zembeziensis and/or R. duttoni infested, febrile animal with enlarged lymph nodes. In endemic areas, high mortality rates may occur among calves because most of the surviving adult cattle are immune carriers.
A definitive diagnosis of theileriosis can be made in the field by demonstrating Theileria sp. parasites within erythrocytes on the Romanowsky-stained blood smear (Figure 3). Giemsa stain can be used to evaluated blood smears or smears from lymph node biopsies for the presence of schizonts. Piroplasms, which appear as signet rings or comma-shaped organisms, can be seen within erythrocytes using Romanowsky-stained blood smears. The particular species of Theileria cannot be differentiated solely on the basis of peripheral blood smear morphology. Figure 3: peripheral blood smear from a white-tailed deer with Theileria cervi. Arrowheads denote piroplasms in erythrocytes, while the arrow indicates basophilic stippling.
To confirm a case of East Coast fever in a live animal, blood smears, buffy coat smears, lymph node biopsy imprints, and/ or liver biopsies should be submitted to the laboratory. Specimens that should be collected at necropsy include lymph nodes, spleen, lung, liver, and kidney. When possible, serum also should be collected and lymph node imprints should be prepared. The imprints should be air-dried and fixed in methanol.
There are several methods used to diagnose East Coast fever. Piroplasms occasionally are seen in blood smears, but schizonts must be detected to make a definitive diagnosis. Schizonts can be found in tissue sections taken during necropsy but the best way to detect them is by making cytologic imprints of the lymph nodes or other internal organs. Antibiodies to T. parva can be detected with either an enzyme-linked immunosorbent assay (ELISA) or an indirect fluorescent antibody (IFA) test, but the most widely used antibody test is the IFA. More recently, polymerase chain reaction (PCR) tests and DNA probes have been used to detect East Coast fever in both susceptible ruminants and ticks.
Treatment Note: Treatment of animals should only be performed by a licensed veterinarian. Veterinarians should consult the current literature and current pharmacological formularies before initiating any treatment protocol.
Several drugs (parvaquone, buparvaquone and halofuginone lactate) are available for the treatment of East Coast fever. However, treatment must be initiated at the onset of clinical signs to be effective. These drugs are expensive and may be cost-prohibitive. Tetracyclines also can be used to treat animals with East Coast fever, but often the parasites are resistance to these antimicrobials. As with the other drugs, treatment must be initiated at the onset of clinical signs.
The most common method used to prevent East Coast fever is to infect and treat susceptible animals. Animals are inoculated with an extremely high dose of sporozoites harvested from ticks and then treated with one of the drugs mentioned previously. Since cross protection does not occur, the inoculum must contain multiple Theileria species/ strains. Immunity conferred from this form of immunization lasts for approximately 3½ years.
Control of East Coast fever is based on a multifaceted approach including pasture management, herd-selection of resistant animals, tick (acarid) control, and immunization. In addition, the transfer of blood between animals should be avoided.
As a foreign animal disease, suspected or confirmed cases of East Coast fever should be reported to state or federal veterinary officials. The web address to contact the proper Federal Area Veterinarian in Charge is www.aphis.usda.gov/vs/area_offices.htm. The web address to contact a State Veterinarian is www.aphis.usda.gov/vs/sregs/official.html.
The Office of International des Epizooties (OIE) is an intergovernmental organization created by international agreement in 1924 to insure full disclosure of the animal disease status within each member country (currently there are 167 member countries). The OIE also has collaborated with the World Trade Organization to publish health standards for international trade of animals and animal products. The standard developed for East Coast fever is outlined below:
When importing cattle or buffalo from countries considered to be infected with East Coast fever, the requisite International Veterinary Certificate must attest to the following:
* Animals did not have clinical sign of theileriosis on the day of shipment
* Since birth, animals have been kept in a zone known to be free of theileriosis for the previous 2 years
* Animals did not have clinical sign of theileriosis on the day of shipment
* Animals were subjected to a diagnostic test for theileriosis with negative results during the 30 days prior to shipment
* Animals were negative for parasitemia following microscopic examination of blood smears
-AND, with either of the above conditions-
* Animals were treated with an acaricide prior to shipment and were completely free of ticks