Genetic diversity and population structure of Theileria parva in South Sudan

East Coast Fever Carrier Status and Theileria parva Breakthrough Strains in Recently ITM Vaccinated and Non-Vaccinated Cattle in Iganga District, Eastern Uganda

East Coast fever (ECF) is a tick-borne disease of cattle that hinders the development of the livestock industry in eastern, central and southern Africa. The 'Muguga cocktail' live vaccine, delivered by an infection and treatment method (ITM), remains the only immunisation strategy of controlling ECF. However, there are challenges of the live vaccine inducing ECF carrier status in immunised animals and the possibility of lack of protection from parasite strains that are antigenically different from the vaccine strains. In Uganda, there are insufficient data regarding the ECF carrier status and T. parva genetic diversity in vaccinated and associated non-vaccinated cattle to assess the effectiveness of ITM vaccination. Blood was collected from recently ECF vaccinated (98) and non-vaccinated (73) cattle from Iganga district in Eastern Uganda at 120 days post-vaccination. The p104 gene nested PCR was used to screen for T. parva DNA, 11 minisatellite and 3 microsatellite markers (SSR) were used for genotyping. 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 prevalence of T. parva based on p104 nPCR was 61.2% (60/98) (RR 2.234, 95% CI 1.49-3.35, p-value < 0.001) among recently vaccinated cattle and 27.4% (20/73) (RR 1.00) among associated non-vaccinated cattle. The Muguga cocktail vaccine strains were responsible for carrier status in 10 (58.8%) by MS7 and 11 (64.7%) by MS19 in vaccinated cattle. Genotypes of T. parva with different-sized alleles to the vaccine strains that could be potential 'breakthroughs' were detected in 2 (11.8%)) and 4 (23.5%) isolates from vaccinated cattle based on MS7 and MS19 minisatellite markers, respectively. Using 14 SSR markers, T. parva diversity was higher in vaccinated (Na = 2.214, Ne = 1.978, He = 0.465) than associated non-vaccinated (Na = 1.071, Ne = 1.048, He = 0.259) cattle. The principal component analysis (PCA) showed isolates from vaccinated cattle were closely related to those from non-vaccinated cattle. The analysis of molecular variance (AMOVA) revealed high genetic variation (96%) within T. parva isolates from vaccinated and non-vaccinated cattle but low variation (4%) between vaccinated and non-vaccinated cattle. This study reveals the role of ITM in inducing the carrier status and higher T. parva genetic diversity in vaccinated cattle. The low genetic variation between T. parva isolates in both vaccinated and non-vaccinated cattle may be suggestive of the protective role of vaccine strains against genetically related local strains in the study area.

Analysis of genetic diversity and population structure of Babesia gibsoni

Babesia gibsoni is a tick-borne apicomplexan protozoan causing canine babesiosis. This parasite has diploid sexual reproduction in ticks, during which genetic exchanges can occur leading to increased genetic diversity, which is an important factor in adapting to environmental changes. Exploring the genetic variation of B. gibsoni population can provide a foundation for understanding the patterns of disease transmission and developing babesiosis control strategies. Partial 18S rRNA fragment sequences were obtained from 11 B. gibsoni isolates collected from different regions in China and 117 publicly available sequences were from 12 geographical areas including China. The genetic variation, demographic expansion and population structure were examined. A total of 34 haplotypes were identified among B. gibsoni populations. Analysis of molecular variance, pairwise Fst and structure analysis showed that high genetic variation within populations, low genetic differentiation and obvious mixture haplotype were apparent in a single continent, but higher genetic differentiation was detected across different continents. Neutrality tests implied that B. gibsoni populations had experienced population extension. These findings will contribute to understand the genetics and evolution of B. gibsoni and will be useful for formulating effective management strategies to prevent and control this parasite.

Genotyping of Theileria parva populations in vaccinated and non-vaccinated cattle in Malawi

Theileria parva is an apicomplexan protozoan parasite that causes bovine theileriosis (East Coast Fever; ECF) in central, eastern and southern Africa. In Malawi, ECF is endemic in the northern and central regions where it has negatively affected the development of dairy industry. Despite its endemic status the genetic population structure of T. parva in Malawi is currently unknown. To obtain an understanding of T. parva in Malawi, we performed population genetics analysis of T. parva populations in cattle vaccinated with the Muguga cocktail live vaccine and non-vaccinated cattle using mini- and microsatellite markers covering all the four T. parva chromosomes. The T. parva Muguga strain was included in this study as a reference strain. Linkage disequilibrium was observed when all samples were treated as a single population. There was sub-structuring among the samples as shown by the principal coordinate analysis. Majority of the samples clustered with the T. parva Muguga reference strain suggesting that the isolates in Malawi are closely related to the vaccine component, which support the current use of Muguga cocktail vaccine to control ECF. The clustering of samples from non-endemic southern region with those from endemic central region suggests expansion of the distribution of T. parva in Malawi.

Population Genetic Analysis and Sub-Structuring of Theileria annulata in Sudan

Theileria annulata, which causes tropical theileriosis, is a major impediment to improving cattle production in Sudan. Tropical theileriosis disease is prevalent in the north and central regions of Sudan. Outbreaks of the disease have been observed outside the known endemic areas, in east and west regions of the country, due to changes in tick vector distribution and animal movement. A live schizont attenuated vaccination based on tissue culture technology has been developed to control the disease. The parasite in the field as well as the vaccine strain need to be genotyped before the vaccinations are practiced, in order to be able to monitor any breakthrough or breakdown, if any, after the deployment of the vaccine in the field. Nine microsatellite markers were used to genotype 246 field samples positive for T. annulata DNA and the vaccine strain. North and central populations have a higher multiplicity of infection than east and west populations. The examination of principal components showed two sub-structures with a mix of all four populations in both clusters and the vaccine strain used being aligned with left-lower cluster. Only the north population was in linkage equilibrium, while the other populations were in linkage disequilibrium, and linkage equilibrium was found when all samples were regarded as single population. The genetic identity of the vaccine and field samples was 0.62 with the north population and 0.39 with west population. Overall, genetic investigations of four T. annulata populations in Sudan revealed substantial intermixing, with only two groups exhibiting regional origin independence. In the four geographically distant regions analyzed, there was a high level of genetic variation within each population. The findings show that the live schizont attenuated vaccine, Atbara strain may be acceptable for use in all Sudanese regions where tropical theileriosis occurs.

A review of recent research on Theileria parva: Implications for the infection and treatment vaccination method for control of East Coast fever

The infection and treatment (ITM) live vaccination method for control of Theileria parva infection in cattle is increasingly being adopted, particularly in Maasai pastoralist systems. Several studies indicate positive impacts on human livelihoods. Importantly, the first detailed protocol for live vaccine production at scale has recently been published. However, quality control and delivery issues constrain vaccination sustainability and deployment. There is evidence that the distribution of T. parva is spreading from endemic areas in East Africa, North into Southern Sudan and West into Cameroon, probably as a result of anthropogenic movement of cattle. It has also recently been demonstrated that in Kenya, T. parva derived from cape buffalo can 'breakthrough' the immunity induced by ITM. However, in Tanzania, breakthrough has not been reported in areas where cattle co-graze with buffalo. It has been confirmed that buffalo in northern Uganda national parks are not infected with T. parva and R. appendiculatus appears to be absent, raising issues regarding vector distribution. Recently, there have been multiple field population genetic studies using variable number tandem repeat (VNTR) sequences and sequencing of antigen genes encoding targets of CD8+ T-cell responses. The VNTR markers generally reveal high levels of diversity. The antigen gene sequences present within the trivalent Muguga cocktail are relatively conserved among cattle transmissible T. parva populations. By contrast, greater genetic diversity is present in antigen genes from T. parva of buffalo origin. There is also evidence from several studies for transmission of components of stocks present within the Muguga cocktail, into field ticks and cattle following induction of a carrier state by immunization. In the short term, this may increase live vaccine effectiveness, through a more homogeneous challenge, but the long-term consequences are unknown.

Monitoring vaccinated cattle for induction and longevity of persistent tick-transmissible infection: Implications for wider deployment of live vaccination against East Coast fever in Tanzania

The infection and treatment (ITM) procedure remains the only available method of immunization against Theileria parva infection. One constraint to deployment is the perception that the carrier state induced by ITM could result in enhanced disease problems. More than one million cattle have been ITM vaccinated in pastoralist systems in Tanzania over the last 2 decades. We present the results of a longitudinal study of six groups of cattle in Maasai villages in northern Tanzania exposed to natural tick challenge for between 2 weeks and 14 years post-vaccination. The p104 nested PCR revealed a higher frequency of T. parva carriers among vaccinates (30%) compared with controls (8%) (OR = 4.89, p = .000), with the highest frequency of carriers found in calves vaccinated 6 months previously, although carrier state was also detected in cattle vaccinated >10 years prior to the study. Variable number tandem repeat genotype analysis revealed 6 MS7 alleles with sizes ranging from 150 bp to 500 bp, but only two alleles were detected in cattle vaccinated >4 years earlier, relative to five alleles detected in recently vaccinated cattle and controls. In terms of heterozygosity, diversity was maximal in calves vaccinated within the last 2 weeks (h = 0.776) but lowest in cattle vaccinated 4 years earlier (h = 0.375). The analysis suggested close genetic relatedness of parasites in vaccinated and unvaccinated groups and up to 96% of variation was within rather than between the groups. These results confirm that ITM leads to a long-term T. parva carrier state in cattle and the detection of vaccine component VNTR in co-grazing unvaccinated cattle suggests potential vaccine transmission by ticks. However, vaccination stocks did not totally replace local genotypes, at least in cattle populations. These findings should mitigate concerns that ITM modifies T. parva field populations in a way that enhances disease in the medium term.

Microsatellite and minisatellite genotyping of Theileria parva population from southern Africa reveals possible discriminatory allele profiles with parasites from eastern Africa

The control of Theileria parva, a protozoan parasite that threatens almost 50% of the cattle population in Africa, is still a challenge in many affected countries. Theileria parva field parasites from eastern Africa, and parasites comprising the current live T. parva vaccine widely deployed in the same region have been reported to be genotypically diverse. However, similar reports on T. parva parasites from southern Africa are limited, especially in Corridor disease designated areas. Establishing the extent of genetic exchange in T. parva populations is necessary for effective control of the parasite infection. Twelve polymorphic microsatellite and minisatellite loci were targeted for genotypic and population genetics analysis of T. parva parasites from South Africa, Mozambique, Kenya and Uganda using genomic DNA prepared from cattle and buffalo blood samples. The results revealed genotypic similarities among parasites from the two regions of Africa, with possible distinguishing allelic profiles on three loci (MS8, MS19 and MS33) for parasites associated with Corridor disease in South Africa, and East Coast fever in eastern Africa. Individual populations were in linkage equilibrium (V_D<L), but when considered as one combined population, linkage disequilibrium (V_D>L) was observed. Genetic divergence was observed to be more within (AMOVA = 74%) than between (AMOVA = 26%) populations. Principal coordinate analysis showed clustering that separated buffalo-derived from cattle-derived T. parva parasites, although parasites from cattle showed a close genetic relationship. The results also demonstrated geographic sub-structuring of T. parva parasites based on the disease syndromes caused in cattle in the two regions of Africa. These findings provide additional information on the genotypic diversity of T. parva parasites from South Africa, and reveal possible differences based on three loci (MS8, MS19 and MS33) and similarities between buffalo-derived T. parva parasites from southern and eastern Africa.

Theileria parva: a parasite of African buffalo, which has adapted to infect and undergo transmission in cattle

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Theileria parva parasites show extensive genotypic diversity and undergo frequent genetic recombination during tick transmission.

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Theileria parva maintained in cattle is much less genotypically diverse than the buffalo-maintained population.

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Theileria parva transmitted from buffalo to cattle usually fails to differentiate to the tick-transmissible stages in cattle.

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These differences have resulted in the parasites in the two hosts being maintained largely as separate populations.

The tick-borne protozoan parasite Theileria parva causes an acute, often fatal disease in cattle throughout a large part of eastern and southern Africa. Infection of African buffalo (Syncerus caffer) is also widespread in this region but does not cause clinical disease in this species. This difference most likely reflects the evolutionary history of the parasites in these species, in that cattle were only introduced into Africa within the last 8000 years. In both hosts, T. parva establishes a carrier state, involving persistence of small numbers of parasites for many months following the acute phase of infection. This persistence is considered important for maintaining the parasite populations. Although cattle and buffalo parasites both produce severe disease when transmitted to cattle, the buffalo-derived parasites are usually not transmissible from infected cattle. Recent studies of the molecular and antigenic composition of T. parva, in addition to demonstrating heterogeneity in the populations in both host species, have revealed that infections in individual animals are genotypically mixed. The results of these studies have also shown that buffalo T. parva exhibit much greater genotypic diversity than the cattle population and indicate that cattle parasites represent a subpopulation of T. parva that has adapted to maintenance in cattle. The parasites in cattle and buffalo appear to be maintained largely as separate populations. This insight into the genotypic composition of T. parva populations has raised important questions on how host adaptation of the parasite has evolved and whether there is scope for further adaptation of buffalo-maintained populations to cattle.

Genetic and antigenic variation of the bovine tick-borne pathogen Theileria parva in the Great Lakes region of Central Africa

Background

Theileria parva causes East Coast fever (ECF), one of the most economically important tick-borne diseases of cattle in sub-Saharan Africa. A live immunisation approach using the infection and treatment method (ITM) provides a strong long-term strain-restricted immunity. However, it typically induces a tick-transmissible carrier state in cattle and may lead to spread of antigenically distinct parasites. Thus, understanding the genetic composition of T. parva is needed prior to the use of the ITM vaccine in new areas. This study examined the sequence diversity and the evolutionary and biogeographical dynamics of T. parva within the African Great Lakes region to better understand the epidemiology of ECF and to assure vaccine safety. Genetic analyses were performed using sequences of two antigen-coding genes, Tp1 and Tp2, generated among 119 T. parva samples collected from cattle in four agro-ecological zones of DRC and Burundi.

Results

The results provided evidence of nucleotide and amino acid polymorphisms in both antigens, resulting in 11 and 10 distinct nucleotide alleles, that predicted 6 and 9 protein variants in Tp1 and Tp2, respectively. Theileria parva samples showed high variation within populations and a moderate biogeographical sub-structuring due to the widespread major genotypes. The diversity was greater in samples from lowlands and midlands areas compared to those from highlands and other African countries. The evolutionary dynamics modelling revealed a signal of selective evolution which was not preferentially detected within the epitope-coding regions, suggesting that the observed polymorphism could be more related to gene flow rather than recent host immune-based selection. Most alleles isolated in the Great Lakes region were closely related to the components of the trivalent Muguga vaccine.

Conclusions

Our findings suggest that the extensive sequence diversity of T. parva and its biogeographical distribution mainly depend on host migration and agro-ecological conditions driving tick population dynamics. Such patterns are likely to contribute to the epidemic and unstable endemic situations of ECF in the region. However, the fact that ubiquitous alleles are genetically similar to the components of the Muguga vaccine together with the limited geographical clustering may justify testing the existing trivalent vaccine for cross-immunity in the region.

Detection of carrier state and genetic diversity of Theileria parva in ECF-vaccinated and naturally exposed cattle in Tanzania

Infection and Treatment Method (ITM) has been practiced in Tanzania for over 20 years as a prevention measure against East Coast Fever disease. It is known that ITM, like natural ECF infection, leads to a carrier state, whereby vaccinated cattle become asymptomatic carriers of the parasite. It is expected that ECF vaccination using ITM also leads to generation of combinations of vaccine specific Theileria parva and local strains that circulate in the field what contributes to an unknown level of parasite diversity. Moreover, the long term impact of ITM on carrier state and parasite diversity in cattle are largely unknown. To address this question blood was collected from ECF-vaccinated (n = 239) and unvaccinated (n = 97) cattle from Loiborsoit, Emboreet, Esilalei, Manyara ranch and Mswakini villages in the Maasai steppe of northern Tanzania, as well as Mruazi and Leila farms in Tanga in eastern Tanzania. Screening for T. parva using nested PCR revealed an overall prevalence of T. parva to be 34.5%, with a significant higher prevalence among ECF-vaccinated cattle. Using three VNTR markers (ms2, ms5 and MS7) higher parasite genetic diversity in terms of higher number of alleles and expected heterozygosity was shown in vaccinated than unvaccinated cattle. These parameters were highest in cattle from Manyara ranch. Nevertheless, the principle component analysis (PCoA) showed no distinct clustering patterns as most T. parva alleles clustered together throughout the four quadrants implying parasite homogeneity among the sampled populations. However, some of the parasite alleles closely clustered with Muguga vaccine alleles in two of the quadrants, consistent with closer genetic relatedness between the vaccine strains and the T. parva populations from the Maasai steppe. Likewise analysis of molecular variance (AMOVA) revealed most of the genetic variation (93%) being contained within populations with only 7% being among populations. This study therefore confirms the role of ECF vaccination in enhancing carrier state and T. parva diversity in vaccinated cattle populations. Higher T. parva diversity may play an important role in carrier cattle by way of restricting breakthrough infections from field parasite strains.