Research paperInfection dynamics of Theileria annulata over a disease season following cell line vaccination
Graphical abstract
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)
- et al.
Haemoparasite infection kinetics and the population structure of Theileria parva on a single farm in Uganda
Vet. Parasitol.
(2013) - et al.
Babesia bovis biological clones and the inter-strain allelic diversity of the Bv80 gene support subpopulation selection as a mechanism involved in the attenuation of two virulent isolates
Vet. Parasitol.
(2012) - et al.
Evaluation of cytochrome b as a sensitive target for PCR based detection of T. annulata carrier animals
Vet. Parasitol.
(2010) - et al.
Immunity and vaccine development in the bovine theilerioses
Adv. Parasitol.
(1999) - et al.
Molecular, epidemiological, haematological and biochemical evaluation in asymptomatic Theileria annulata infected cattle from an endemic region in Spain
Ticks Tick-Borne Dis
(2017) - et al.
Genotypic diversity in Babesia bovis field isolates and vaccine strains from South Africa
Vet. Parasitol.
(2014) Review on the experience with live attenuated vaccines against tropical theileriosis in Tunisia: considerations for the present and implications for the future
Vaccine
(2008)- et al.
Vaccination of calves with an attenuated cell line of Theileria annulata and the sporozoite antigen SPAG-1 produces a synergistic effect
Vet. Parasitol.
(2006) - et al.
Live immunization against East Coast fever - current status
Trends Parasitol.
(2009) - et al.
Immunological relationship between strains of Theileria annulata Dschunkowsky and Luhs 1904
Res. Vet. Sci.
(1980)
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.
Population diversity of Theileria annulata in Portugal
Infect. Genet. Evol.
Control of Theileria annulata in Iran
Parasitol. Today (Regul. Ed.)
In vivo evidence for the resistance of Theileria annulata to buparvaquone
Vet. Parasitol.
Population genetic analysis and sub-structuring of Theileria parva in Uganda
Mol Biochem Parasit
East coast fever: 3. Chemoprophylactic immunization of cattle using oxytetracycline and a combination of theilerial strains
Vet. Parasitol.
A comparison of susceptibilities to infection of four species of Hyalomma ticks with Theileria annulata
Vet. Parasitol.
Point mutations in the Theileria annulata cytochrome b gene is associated with buparvaquone treatment failure
Vet. Parasitol.
Attenuated vaccines for tropical theileriosis, babesiosis and heartwater: the continuing necessity
Trends Parasitol.
Genetic exchange and sub-structuring in Theileria annulata populations
Mol Biochem Parasit
Population diversity and multiplicity of infection in Theileria annulata
Int. J. Parasitol.
Genetic diversity and population structure of Theileria annulata in Oman
PLoS One
Characterization of within-host Plasmodium falciparum diversity using next-generation sequence data
PLoS One
Prevalence of tropical theileriosis in cattle in the Aydin Region and determination of efficacy of attenuated Theileria annulata vaccine
Turkiye Parasitol Derg.
Genomic and phenotypic diversity of Tunisian Theileria annulata isolates
Parasitology
Selection of genetic markers to determine diversity in Theileria annulata populations after recombination
Turkiye Parasitol Derg.
Theileriidae
A preliminary study on the attenuation of Tunisian schizont-infected cell lines of Theileria annulata
Parasitol. Res.
Cited by (7)
Detection of the cytochrome B (cytb) insecticide resistance gene in Theileria annulata isolated from cattle in Hilla city, Babylon governorate, Iraq
2023, Iraqi Journal of Veterinary SciencesCross-sectional survey of cattle haemopathogens in Constantine, Northeast Algeria
2021, Veterinary Medicine and Science