East Coast fever carrier status and Theileria parva breakthrough strains in recently ITM vaccinated cattle in Iganga District, Eastern Uganda

Abstract

East Coast fever (ECF), a tick-borne disease caused by Theileria parva (T. parva) is the most important hindrance to the livestock industry in the eastern, central and southern Africa which is the major source of income and animal protein. The ‘Muguga cocktail’ live vaccine, delivered by an infection and treatment method (ITM) remains the only immunization strategy of controlling ECF. However, there are challenges of the live vaccine inducing ECF carrier status in immunized animals and the possibility of lack of protection and or re-infection from parasite strains antigenically different from the vaccine cocktail strains. In Uganda, there is insufficient data regarding the ECF carrier status and T. parva genetic diversity in vaccinated and associated non-vaccinated cattle and yet this is important in assessing the effectiveness of ITM vaccination. To address these challenges, blood was collected from recently ECF vaccinated (98) and non-vaccinated (73) cattle from Iganga district in eastern Uganda at 120 days post-vaccination. Genomic DNA was extracted using Qiagen extraction kit. Primers designed from p104 gene were used to screen for T. parva parasites and 11 minisatellite and 3 micro satellite markers were used for genotyping of T. parva parasites. A nested p104 PCR revealed a general prevalence of T. parva of 46.8% (80/171), where prevalence of 61.2% (60/98) (RR 2.234, 95%CI 1.49-3.35, p-value <0.001) was among recently ITM vaccinated cattle and 27.4% (20/73) (RR 1.00) among non-vaccinated cattle. Two minisatellite markers (MS7 and MS19) were used to determine whether ECF carrier status was due to the T. parva vaccine or local strains. The T. parva Muguga cocktail vaccine strains were responsible for carrier status in 58.8% (10/17) (MS7) and 64.7% (11/17) (MS19) of vaccinated cattle. Genotypes of T. parva with different-sized alleles to the vaccine alleles which could be potential ‘breakthroughs’ were observed in 11.8% (2) and 23.5% (4) of isolates from vaccinated cattle with MS7 and MS19 minisatellite markers respectively. Using eleven mini- and three micro-satellite markers, parasite diversity in terms of number of alleles and expected heterozygosity was higher in recently ITM vaccinated than non-vaccinated cattle. The principal component analysis (PCA) showed that the isolates from recently vaccinated cattle were closely related to alleles from non-vaccinated cattle and they were more close to alleles from Muguga cocktail vaccine. The analysis of molecular variance (AMOVA) also revealed high genetic variation within T. parva isolates from vaccinated and non-vaccinated cattle (96%) and less variation between isolates from within recently vaccinated and non-vaccinated cattle (4%). This study therefore reveals the role of ITM in inducing the carrier status and increased T. parva diversity in both vaccinated and non-vaccinated cattle. The increased genetic diversity of T. parva in carrier cattle may play an important role in restricting infections from local genotypes.