Host-pathogen genetics and the epidemiology of lentivirus infection in German and Iranian sheep

Abstract

Small ruminant lentiviruses (SRLVs) cause maedi-visna disease (MV) in sheep and are widespread throughout the world, including Germany and Iran. There is no vaccine or cure available for SRLVs. Generally, there are two strategies for controlling the spread of SRLVs: (1) Diagnosis of an SRLV infection followed by culling, segregation of infected animals and feeding their offspring with virus-free colostrum/milk. SRLVs have been classified into four genotypes: A-C and E. Genotype A is found mostly in sheep and has been subdivided into 20 subtypes. Because SRLVs have high variability, to design suitable diagnostic tests, it is critical to know the genetic characteristics of the circulating virus. There is no information on the genetic composition of German and Iranian SRLVs, making SRLV diagnosis in those countries difficult. (2) Breeding sheep to lower their susceptibility to SRLV infection. Recently, an amino acid substitution (E/K) at position 35 of the transmembrane protein 154 (TMEM154) was found in sheep in the United States, along with a deletion in the chemokine (C-C motif) receptor type 5 gene (CCR5), which, reportedly, affected the serological MV status or SRLV provirus concentration. Before present studies, no efforts have been made to evaluate the association of TMEM154 or CCR5 variants with SRLV infections in German or Iranian sheep.The overall objective of the present studies was to extend our knowledge of host-pathogen genetics and the epidemiology of lentivirus infection in German and Iranian sheep. The research project was divided into two main parts for simplicity: the host and the pathogen genetic. The host genetic, together with the epidemiology of SRLV infection, was examined in studies 1 and 2. The pathogen genetic was explored in study 3. In study 1, the association of the two candidate genes TMEM154 and CCR5 with SRLV infection was evaluated over 500 samples in four sample sets. The association of TMEM154 with SRLV infection status was significant in Texel and Milk sheep. The deletion of the CCR5 promoter did not show a consistent association with SRLV infection status. In study 2, we aimed at evaluating the association of TMEM154 with SRLV infection in Iranian and some more German sheep flocks/breeds. The following results were obtained in study 2: (1) The association of TMEM154 E35K with SRLV infection status was tested in four sheep breeds and found to be significant in Kermani, Merinoland, and Brown Hair; (2) regression analysis showed that SRLV susceptibility in a flock/breed could be predicted based on the frequency of allele E with a variation of approximately +/ - 20%; and (3) for the first time, SRLV positive sheep flocks were found in the Iranian province of Western Azarbaijan through a PCR-based test. The results of studies 1 and 2 showed that the amino acid substitution at position 35 of TMEM154 could be a promising marker for breeding a lower number of serologically MV positive sheep in German and Iranian flocks. Concerning the epidemiology of SRLV infection in German sheep flocks, the range of infection varies from 10-100% and corresponds to positive flocks. Among Iranian sheep flocks, one-third of flocks (10 out of 30 flocks) were SRLV positive and had at least one sample that was SRLV positive. The range of infection within samples of individual flocks varied from 0-89%. The initial aim of study 3 was the phylogenetic classification of German and Iranian SRLVs. Based on 54 SRLV gag sequences from German (n=48) and Iranian (n=6) sheep, the results revealed: SRLV subtypes A4, A5, A11, A16 and A21 (new) are found in German sheep, and, SRLV subtype A22 (new) are found in Iranian sheep. Notably, in study 3, further analyses provided additional conclusions: (1) It was shown that the genotype A likely has two ancestors; one is related to Turkish SRLVs, and the other is related to Iranian SRLVs; and (2) the transmission routes of some SRLVs are likely in line with the domestication pathways of sheep.