Elsevier

Veterinary Parasitology

Volume 265, January 2019, Pages 63-73
Veterinary Parasitology

Research paper
Infection dynamics of Theileria annulata over a disease season following cell line vaccination

https://doi.org/10.1016/j.vetpar.2018.11.012Get rights and content

Highlights

  • A genetically diverse Theileria annulata population causes tropical theileriosis.

  • Individual cattle experience a flux of parasite genotypes over a disease season.

  • Cell line vaccination has a transient effect on the parasite population.

  • Vaccine ‘breakthroughs’ are caused by field challenge.

Abstract

Tropical theileriosis is a tick-borne haemoparasitic disease of cattle caused by the protozoan parasite Theileria annulata. Globally, the economic impact of the disease is immense and enhanced control measures would improve livestock production in endemic regions. Immunisation with a live attenuated vaccine is an effective and widely used control method, however, the repeated use of live vaccines may have an impact on the field parasite population at a genetic level. Additionally, there has been an increasing number of reports of vaccine breakthrough cases in recent years. Thus, the present study was designed to evaluate the genetic composition of a parasite population over a disease season in a locality where live cell line vaccination is practised. A diverse range of parasite genotypes was identified and every T. annulata positive cattle blood sample harboured multiple parasite genotypes. An alteration in the major genotype and an increasing multiplicity of infection in individual animals was observed over the course of the disease season. Vaccination status was found not to effect within-host multiplicity of infection, while a significantly higher number of genotypes was detected in grazed cattle compared to non-grazed ones. A degree of genetic isolation was evident between parasite populations on a micro-geographic scale, which has not been reported previously for T. annulata. Analysis of parasite genotypes in vaccinated animals suggested only a transient effect of the vaccine genotype on the genetic diversity of the T. annulata population. The vaccine genotype was not detected among clones of two vaccine ‘breakthrough’ isolates and there is no suggestion that it was responsible for disease. The obtained data indicated that in the system studied there is no apparent risk of introducing the vaccine genotype into the population with only a transient effect on the genetic diversity of the parasite population during the disease season.

Introduction

Tropical theileriosis is caused by the protozoan parasite Theileria annulata and is transmitted by several species of Ixodid ticks of the genus Hyalomma. It is an economically important bovine disease, which is widespread between longitudes 30 °W-150 °E and latitudes 15 °N-60 °N. The parasite has a cattle-tick-cattle life cycle which, in the bovine host, involves two major asexual replicative phases. The first of these takes place within leukocytes and the second within erythrocytes. After piroplasm-containing erythrocytes are ingested by a feeding tick, a sexual cycle occurs within the tick (Schein and Friedhoff, 1978). Male and female gametes are formed which fuse to form zygotes, which in turn differentiate into kinetes that migrate to the salivary glands, ultimately generating bovine-infective sporozoites (Gauer et al., 1995; Schein and Friedhoff, 1978). Recent population genetic studies have provided further indirect evidence for the occurrence of a sexual phase in the parasite life cycle and these indicate that random mating is a feature of field populations of T. annulata (Al-Hamidhi et al., 2015; Gomes et al., 2016; Weir et al., 2007). Sexual recombination, together with a high transmission rate, is understood to play a significant role in generating T. annulata genetic diversity in different regions (Katzer et al., 2006; Pumpaibool et al., 2009).

Currently, prevention and control measures against tropical theileriosis comprise: (i) control of the tick vector, (ii) treatment of infected animals, (iii) use of disease-resistant breeds of cattle and (iv) vaccination with attenuated cell lines. Each of these methods, however, suffers from various drawbacks. Tick control using acaricides is unsustainable due to emerging resistance and food safety concerns (Graf et al., 2004; Khater et al., 2016). The drugs used for treatment, parvaquone and buparvaquone, have been in use since the 1980s and an increased rate of treatment failures has been observed in recent years, with buparvaquone-resistant parasites detected in Turkey (Hacilarlioglu, 2013), Tunisia (Mhadhbi et al., 2010) and Iran (Sharifiyazdi et al., 2012). A small number of indigenous cattle breeds from disease-endemic regions has been shown to possess innate disease-resistance, such as Sawihal and Kenana cattle in India (Glass et al., 2005). However, the ability of other breeds to resist or tolerate tropical theileriosis is largely unknown and a substantial amount of work is required to gauge the importance of breed resistance in combatting tropical theileriosis on a broad scale. Vaccinating cattle using attenuated T. annulata cell line vaccines has been shown to be an effective method for controlling disease (Darghouth et al., 1999; Seitzer and Ahmed, 2008) and this has been adopted in a number of countries, including Turkey. Attenuation of virulence of schizont-infected cell line cultures via long term in vitro passage has been associated with a reduction in the number of genotypes contained within the cell line (Darghouth et al., 1996; Pipano and Shkap, 2000). For example, the vaccine used in Turkey, based on the Pendik cell line, may comprise only a single haploid T. annulata genotype (Weir et al., 2011). The use of attenuated live cell line vaccines has been shown to provide solid immunity against homologous challenge and partial cross-protection against heterologous challenge (Darghouth et al., 1996; Gill et al., 1980; Hashemi-Fesharki, 1988). The protection provided by vaccination is not associated with the induction of sterile immunity and it may be hypothesised that vaccinating cattle exposed to field challenge with a single parasite genotype may perturb the parasite population harboured by these animals. It may be further hypothesised that vaccination could result in the positive selection of genotypes which are poorly protected against, thereby altering the genetic composition of the local parasite population.

The long-term effectiveness of current vaccines in endemic regions and the influence of vaccination on field parasite populations remain poorly understood. Clinical theileriosis has been observed in vaccinated cattle during the disease season in Turkey (Aysul et al., 2008). Recent field reports indicate an increasing number of ‘breakthrough’ cases in vaccinated animals (unpublished observation) and investigating the genetic basis of this phenomenon is now essential. Previous genetic analysis of T. annulata field populations using a panel of molecular markers has revealed a high level of genotypic diversity with large numbers of distinct parasite genotypes detected within limited geographical areas (Al-Hamidhi et al., 2015; Gomes et al., 2016; Weir et al., 2007; Yin et al., 2018). Furthermore, a previous study has indicated that the level of multiplicity of infection is influenced by vaccination status and that cell line vaccinated cattle tend to be infected with more genotypes than unvaccinated cattle (Weir et al., 2011). The influence of vaccination on field parasite population dynamics remains unknown. In the present study, we have investigated the dynamics of parasite infection over the course of a disease season following vaccination with a commercial cell line vaccine, measuring parasite genotypic diversity in the cattle population both pre- and post-immunisation together with investigating the genetic basis of ‘breakthrough’ cases in vaccinated animals.

Section snippets

Parasite material

The study was conducted at seven farms, with a history of tropical theileriosis, located within four different villages (one farm from Seferler, three from Centrum, one from Sarikoy and two from Kabalar) in the Akçaova district of Aydın province in Western Turkey where tropical theileriosis is endemic. A map illustrating the geographical location of sampling sites is shown in Supplementary Fig. S1. A total of 143 cattle from Seferler (n = 13), Centrum (n = 55), Sarıkoy (n = 20) and Kabalar

Screening for the presence of T. annulata

A sub-set of the cattle present at each of the premises was sampled before and after vaccination. The number of animals that could be sampled on each premises varied at different time-points during the disease season due to local management factors (Table 1), but where possible, the same animals were sampled on subsequent visits. A breakdown of the sampling schedule for individual animals over each time-point is provided in Supplementary Table S3. For comparative purposes, a number of

Discussion

Tropical theileriosis hampers livestock production in endemic countries and has a particularly strong impact on the most productive cattle breeds. It is essential to investigate and quantify the dynamics of parasite infections to allow informed development and deployment of novel control strategies (Auburn et al., 2012; Weir et al., 2007). Among the control measures available to limit losses incurred by disease, live attenuated vaccination remains an important, effective and widely used method

Conclusion

This study describes, for the first time, the dynamics of T. annulata infection and parasite genotypic diversity in animals vaccinated with a commercial cell line vaccine over the course of a disease season. The field parasite population was found to be highly diverse and an alteration in the major genotype and MOI was observed over the course of the season. Interestingly, vaccination status was shown not to affect within-host diversity in the middle of the disease season. From the results of

Competing interests

None of the authors of this study have any financial or personal relationships with other people or organisations that could have inappropriately influenced this work.

Authors contributions

HBB, SB and TK designed the study and interpreted the data. HBB, AA, AHU, OK, SH and SB carried out the experimental work. WW performed the data analysis. HBB, TK and WW wrote the manuscript. All authors read and approved the final manuscript.

Acknowledgment

Financial support for this study was provided by a grant from TUBITAK (Ref. TUBITAK-111O718).

References (56)

  • E.J. Glass et al.

    Bos taurus and Bos indicus (Sahiwal) calves respond differently to infection with Theileria annulata and produce markedly different levels of acute phase proteins

    Int. J. Parasitol.

    (2005)
  • J. Gomes et al.

    Population diversity of Theileria annulata in Portugal

    Infect. Genet. Evol.

    (2016)
  • R. Hashemi-Fesharki

    Control of Theileria annulata in Iran

    Parasitol. Today (Regul. Ed.)

    (1988)
  • M. Mhadhbi et al.

    In vivo evidence for the resistance of Theileria annulata to buparvaquone

    Vet. Parasitol.

    (2010)
  • C.A.L. Oura et al.

    Population genetic analysis and sub-structuring of Theileria parva in Uganda

    Mol Biochem Parasit

    (2005)
  • D.E. Radley et al.

    East coast fever: 3. Chemoprophylactic immunization of cattle using oxytetracycline and a combination of theilerial strains

    Vet. Parasitol.

    (1975)
  • F. Sayin et al.

    A comparison of susceptibilities to infection of four species of Hyalomma ticks with Theileria annulata

    Vet. Parasitol.

    (2003)
  • H. Sharifiyazdi et al.

    Point mutations in the Theileria annulata cytochrome b gene is associated with buparvaquone treatment failure

    Vet. Parasitol.

    (2012)
  • V. Shkap et al.

    Attenuated vaccines for tropical theileriosis, babesiosis and heartwater: the continuing necessity

    Trends Parasitol.

    (2007)
  • W. Weir et al.

    Genetic exchange and sub-structuring in Theileria annulata populations

    Mol Biochem Parasit

    (2007)
  • W. Weir et al.

    Population diversity and multiplicity of infection in Theileria annulata

    Int. J. Parasitol.

    (2011)
  • Salama Al-Hamidhi et al.

    Genetic diversity and population structure of Theileria annulata in Oman

    PLoS One

    (2015)
  • S. Auburn et al.

    Characterization of within-host Plasmodium falciparum diversity using next-generation sequence data

    PLoS One

    (2012)
  • N. Aysul et al.

    Prevalence of tropical theileriosis in cattle in the Aydin Region and determination of efficacy of attenuated Theileria annulata vaccine

    Turkiye Parasitol Derg.

    (2008)
  • L. Ben Miled et al.

    Genomic and phenotypic diversity of Tunisian Theileria annulata isolates

    Parasitology

    (1994)
  • H.B. Bilgic et al.

    Selection of genetic markers to determine diversity in Theileria annulata populations after recombination

    Turkiye Parasitol Derg.

    (2017)
  • C.G.D. Brown

    Theileriidae

  • M.A. Darghouth et al.

    A preliminary study on the attenuation of Tunisian schizont-infected cell lines of Theileria annulata

    Parasitol. Res.

    (1996)
  • 1

    First author.

    2

    These authors contributed equally

    View full text