Iranian J Parasitol: Vol. 6, No.1, 2011, pp.66-72 Discussion For the long time it was assumed that T. lestoquardi is the only pathogenic parasite in small ruminants (6). Recently, however, a previously identified parasite has been de- scribed as the species of T. china 1 and china 2 , which are pathogenic for small ruminants causing fatal diseases (6-9). There are different methods to diagnose theileriosis. The Geimsa stained of blood smear is the common method for the identification and characterization of these piroplasms in Iran, which accompanied with some technical problems causing false morphological diagnosis, and in some cases needs special diagnostic knowledge. In certain cases, sero- logical methods such as the immuno fluores- cence antibody test (IFAT) or immunoperoxidase test have also been ap- plied (11-13). Detecting antibodies against Theileria using serological tests is a useful technique in epidemiological surveys, but false positive and negative results due to cross-reactions or weak specific immune re- sponse are some disadvantages of these tests (15, 16). The gold standard method for the characterization of Theileria species is the method of polymerase chain reaction (PCR).PCR or PCR-RFLP was used for the identification of piroplasms in blood by many investigator (8, 14, 17-22). Shayan and Rahbari (2005) used the stained blood smears as the sources for DNA extraction and applied it for the characterization of Theileria spp. and Babesia spp. simultane- ously (14). According to the framework of “integrated control of ticks and tick borne diseases in globalized world managed by EU-ICTTD-3 project” the members try to expand their activity as network in the boundary countries. In order to such cooperation activity, sam- ples from liver, spleen, lymph node and lung were sent from Sultan Qaboos University, 70 College of Agricultural and Marine Sciences, Department of Animal and Veterinary Sci- ences to the laboratory of Iranian Research Center for Ticks and Tick-borne Diseases (IRCTTD). Eleven sheep from complete 53 sheep at Ru- mais (Barka) region in the Sultanate of Oman were died. One of these sheep died suddenly without any clinical signs, whereas the other ten showed anorexia and anemia before they were died. In accordance to the pathology of theileriosis, the dead animal showed icterus, splenomegaly, hepa- tomegaly, mottled and yellowish color in the liver and congestion in lung. Although the blood smears did not show any piroplasmic or schizont stage in the corresponding cells, the schizont as structures were detectable in the impression smears prepared from lymph node, liver, and lung. The PCR analysis of DNA extracted from liver, lung, and lymph node revealed the Theileria specific ampli- con of approximately 428 bp product. Tageldin et al. (2005) reported that Oman sheep are highly susceptible for Theileria infection and the infected sheep mostly die before the microschizonts or erythrocytic form of Theileria appear in the nucleated or erytrocytic cells respectively. The same phe- nomenon was observed with the 11 out of 53 sheep (23). The schizonts were but observed in the different examined organ and there- fore it is to speculate that the sheep could be died under toxic shock caused by induction of cytokine production such as TNF-α, IL-1 and IL-6 in infected cells (24, 25). The nucleotide sequence analysis of 18S rRNA gene of Oman Theileria showed 99 percent homology to the T. annulata and T. lestoquardi. The nucleotide sequence of Oman Theileria differed only in one nucleo- tide at position 254 with the corresponding nucleotide sequence of T. lestoquardi and