Global distribution, host range and prevalence of Trypanosoma vivax: a systematic review and meta-analysis

Background Trypanosomosis caused by Trypanosoma vivax is one of the diseases threatening the health and productivity of livestock in Africa and Latin America. Trypanosoma vivax is mainly transmitted by tsetse flies; however, the parasite has also acquired the ability to be transmitted mechanically by hematophagous dipterans. Understanding its distribution, host range and prevalence is a key step in local and global efforts to control the disease. Methods The study was conducted according to the methodological recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist. A systematic literature search was conducted on three search engines, namely PubMed, Scopus and CAB Direct, to identify all publications reporting natural infection of T. vivax across the world. All the three search engines were screened using the search term Trypanosoma vivax without time and language restrictions. Publications on T. vivax that met our inclusion criteria were considered for systematic review and meta-analysis. Result The study provides a global database of T. vivax, consisting of 899 records from 245 peer-reviewed articles in 41 countries. A total of 232, 6277 tests were performed on 97 different mammalian hosts, including a wide range of wild animals. Natural infections of T. vivax were recorded in 39 different African and Latin American countries and 47 mammalian host species. All the 245 articles were included into the qualitative analysis, while information from 186 cross-sectional studies was used in the quantitative analysis mainly to estimate the pooled prevalence. Pooled prevalence estimates of T. vivax in domestic buffalo, cattle, dog, dromedary camel, equine, pig, small ruminant and wild animals were 30.6%, 6.4%, 2.6%, 8.4%, 3.7%, 5.5%, 3.8% and 12.9%, respectively. Stratified according to the diagnostic method, the highest pooled prevalences were found with serological techniques in domesticated buffalo (57.6%) followed by equine (50.0%) and wild animals (49.3%). Conclusion The study provides a comprehensive dataset on the geographical distribution and host range of T. vivax and demonstrates the potential of this parasite to invade other countries out of Africa and Latin America.


Introduction
Trypanosomes are protozoan parasites belonging to the family of Trypanosomatidae and the genus Trypanosoma (T.). The genus Trypanosoma comprises many species such as T. brucei, T. congolense, T. equiperdum, T. evansi, T. simiae, T. suis and T. vivax, that cause diseases called trypanosomoses in different mammalian hosts including humans [1]. Trypanosomoses are widely distributed in Africa, Latin America and Asia [2,3].
T. vivax is one of the most important Trypanosoma species known to infect both domestic and wild ungulates [4,5]. T. vivax is reported from cattle, dromedary camel, [6], goat, sheep, pig, dog [7], horse, donkey [8], buffalo, warthog, hippopotamus, reedbuck, waterbuck [9], antelope [10], giraffe [11], rhinoceros [12], rodents, pangolin, primates, reptiles and different wild ungulates and carnivores [13]. In Sub-Saharan Africa, T. vivax is mainly transmitted by tsetse ies (Diptera: Glossinidae) in which the parasite can multiply and remain infective throughout the insect's life [14]. The parasite has acquired the ability to be also transmitted mechanically by hematophagous ies like Tabanus spp., Stomoxys calcitrans and Haematobia irritans, which are responsible for the spread of T. vivax in tsetse free areas of Africa and in Latin America [4,[15][16][17][18] T. vivax infection can be suspected by clinical and/or serological evidence and can be con rmed by parasitological or molecular methods [19]. Reported prevalences of T. vivax show considerable variation with geography, abundance of tsetse or blood sucking ies, and host species. In tsetse infested areas of tropical Africa, the T. vivax prevalence is typically reported between 5-15% and often accounts for up to half of the total trypanosome prevalence. Outside of the tsetse belt, T. vivax prevalence is lower between 2-10% and it is related to local and seasonal variation in biting y abundance [20].
Trypanosomosis caused by T. vivax is an important cause of economic losses related to morbidity, mortality, reproductive issues and decreased production [4]. For example, economic losses associated with bovine trypanosomosis have been estimated to be around US$5 billion a year in Africa and the continent spend at least $30 million every year to control bovine trypanosomosis in terms of curative and prophylactic treatments [21]. Estimates outside Africa indicate that more than 11 million head of cattle with a value of more than US$ 3 billion are at risk from T. vivax infection in the Brazilian Pantanal and Bolivian lowlands, with potential losses in excess of US$ 160 million [16].
Large numbers of studies have been conducted on T. vivax over the past 100 years. Studies before 1950's focused more on the morphology and taxonomy [22,23], pathogenicity [24] and treatment [25,26]. However, since the 1950's, a considerable number of epidemiological studies have been conducted.
Notwithstanding the excellent review on livestock trypanosomoses and their vectors in Latin America [18] and a recent general review on T. vivax [20], a systematic literature review on the global distribution, prevalence and host range of T. vivax is lacking. Moreover, no information on the global distribution of T. vivax is available at the World Animal Health Information System of the World Health Organisation (https://www.oie.int/wahis_2/public/wahid.php/Diseaseinformation/Diseasedistributionmap). Thus, this study was conducted to provide the global distribution of T. vivax and to estimate the pooled prevalence of trypanosomosis caused by T. vivax in naturally infected domestic and wild animals.

Methods
The systematic review and meta-analysis were conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist [27]. Screening and data extraction were performed by two authors (SL and EF) independently. All disagreements were discussed and resolved by consensus. A third author (PB) was also involved in the search for full text papers to ensure that all relevant publications were included.

Literature search
All the studies conducted in the 27 African countries reported the presence of T. vivax in at least one host species; natural T. vivax infections were found in 12 of the 13 studied Latin American countries ( Figure 2 and Table 1). In Martinique, Alonso and co-workers didn't nd clinical nor serological evidence of T. vivax in cattle on this island [50]. One article mentions a cross-sectional study on 300 equines in Pakistan, but all animals were negative in molecular tests for T. vivax [231]. We couldn't nd any other reports on the presence of T. vivax in Asia, Antarctica, Australia, Europe and North America.
Host range A total of 232,627 tests were performed and 24,420 of them were positive for natural infection of T. vivax. Trypanosoma vivax was reported from 9 domestic animal species: buffalo, cattle, dog, donkey, dromedary camel, goat, horse, pig and sheep. Among them, cattle were the most studied species with 198,593 tests performed on cattle in 36 countries and two territories (192 publications) and 20,964 were positive for T. vivax. Next to cattle, goat, sheep, pigs and donkey were the most frequently studied species. The protozoal parasite was also reported from wild animals including diverse species of antelopes, Cape buffalo, hippopotamus, black rhinoceros, pangolin and warthog. Trypanosoma vivax was reported from 39 wild fauna species, including many antelope species and Cape buffalo ( Table 2 and 3).
Pooled prevalence estimates according to host species and type of diagnostic test Pooled prevalence estimates by test methods for different hosts are presented in Table 4 and funnel plots of the meta-analysis and the subgroup analyses can be found in Supplementary les S2 and S3. Substantial heterogeneity was observed in the pooled estimate except for dog, which remained signi cant even after sub-group analysis.
Cattle: a total of 145 cross-sectional studies from 32 countries were included in estimation of natural infection of T. vivax in cattle. The random effect model indicates the pooled prevalence to be 6.4% (5.7-7.2, 95% CI). High in between studies heterogeneity was observed with 96 Subgroup pooled prevalences estimated according to the type of diagnostic test, as represented in Table 4, were lowest with parasitological techniques (from 1.1% in pigs to 13.2% in wild animals) and highest with serological techniques (from 13.8% in small ruminants to 57.6% in buffalo).

Publication bias
The presence of publication bias was analyzed only in ve species since there were not enough publications to discuss its possible in uence in camel, buffalo and dogs. Possible publication bias was demonstrated by visualization of asymmetry in funnel plots for cattle ( Figure 3A), small ruminants ( Figure 3B), equines ( Figure 3C), pigs ( Figure 3D) and wild animals ( Figure 3E). It was further con rmed by 'metabias' test (Egger's test) with p value less than 0.05. Thè trim ll` method imputed 170, 43, 30, 27 and 11 studies to obtain symmetry in funnel plots in cattle, wild animals, equines, small ruminants and pigs respectively. The new estimated prevalence equals to 14.8% for cattle, 26.8% for wild animals, 21.6% for equines, 9.5% for small ruminants and 24.5% for pigs.

Discussion
This study presents the rst systematic review of published literature since the 1950s describing global distribution, host range and prevalence of trypanosomosis caused by T. vivax. Not surprisingly, most publications report on T. vivax infections in domestic mammalian species, in particular in cattle and small ruminants, while few publications describe natural infections in wildlife.
Looking at the T. vivax distribution map (Fig. 1), there is an evident data gap for some sub-Saharan African countries where tsetse ies are present and therefore T. vivax may be endemic. Although our formal search strategy couldn't retrieve any publication on these "missing" countries, conventional Google search con rms the presence of T. vivax in South Sudan and Zimbabwe [285,286] and Genevieve et al [287] reported on the presence of potential vectors in Central African Republic. Since Angola, the Central African Republic and the Republic of Congo are endemic for human African trypanosomosis, the presence of T. vivax in these countries is likely [288]. Due to its adaptation to mechanical transmission, T. vivax is also present outside the tsetse belt in Africa, e.g. in Ethiopia and Sudan [114,289]. As a consequence, the trypanosomosis control efforts with focus on tsetse eradication might have little effect on T. vivax. Also, economic impact assessments that are solely based on tsetse distribution alone could seriously underestimate the problem of trypanosomosis due to T. vivax.
Colombia, French Guiana, Guadeloupe, Guyana, Martinique, Panama, Suriname and Venezuela [18]. Stephen [290] reviewed the presence of the parasite in Costa Rica, Ecuador, El Salvador, Paraguay, and Peru and according to Gardiner et al [15], T. vivax was present in the Caribbean thus posing a threat to the livestock industries. From our literature search we can only con rm T. vivax to be endemic in 12 Latin American countries of which 7 (Argentina, Bolivia, Brazil, Colombia, Guyana, Peru, Venezuela) are also endemic for T. evansi [291]. Apparently, T. vivax has never spread into Asia, unlike T. evansi although similar to the latter; it can be mechanically transmitted by bloodsucking ies. Unless there is a particular biological or environmental factor preventing T. vivax to invade the Middle East and Asia, as well Northern Africa, North America and Europe, we must remain alert about the risk of importing T. vivax into non-endemic countries as it happened with Latin America. This review suggests that T. vivax has a very diverse host range, including 9 domestic mammals and almost 40 wild fauna species. Regarding the latter however, data should be interpreted with caution. Diagnostic tests, whether parasitological, serological or even molecular, have their limitations. For examples, by sequencing of PCR amplicons, Auty and co-workers [11] clearly demonstrate that wildlife may harbor a diversity of trypanosomes, including taxonomically unde ned species. Therefore, it is likely that many reports on T. vivax infection in wildlife and tsetse in fact deal with other trypanosome species that are not necessarily pathogenic for domestic animals.
The pooled prevalence of trypanosomosis in different hosts varies signi cantly depending on the detection methods; signi cantly higher estimates were reported in publications using serological techniques. Higher estimates using serological technique could be due to the persistence of antibody over several months after curative treatment and the possibility of low undetectable parasitemia in parasitological techniques [20,93,292,293]. Moti et al [187] compared the percentage positivity obtained with different diagnostic techniques and showed that relative to the microhaematocrit centrifugation technique, the percent positivity increased by 50 and 250% when using PCR-RFLP. Also Garcia et al [118] reported that the detection of trypanosomes, PCR-based assays are twice as sensitive as parasitological techniques such as the microhaematocrit centrifugation.
The study has the following limitations. The literature search was almost exclusively based on electronic databases whereby some older literature must have been missed. The data showed a large degree of heterogeneity among studies, which remain signi cant after sub-group analysis. There is a signi cant publication bias which could be due to incomplete or inaccurate information provided in the publications. In addition, studies were conducted between 1956 and 2017 and the result may not accurately re ect the current epidemiological situation and therefore could limit interpretation of the result to some degree. Furthermore, we suspect numerous data gaps mainly because of two reasons. Firstly, due to lack of a country level monitoring and reporting system for trypanosomosis, most of the data included in this analysis is from research activities. Secondly, trypanosomosis diagnosis in most endemic countries relied to a great extent on low-sensitivity parasitological methods, while more sensitive molecular tools are rarely used. Thus, prevalences presented here may signi cantly underestimate the real situation and the economic impact of T. vivax trypanosomosis in Africa and Latin America.

Conclusion
With this study, we intended to provide comprehensive information on the geographical distribution, host range and prevalence of trypanosomosis caused by T. vivax worldwide. The results con rm the wide geographical distribution and a diverse host range of T. vivax. The parasite parasitizes almost all domestic mammals and many wild animal species thus suggesting the potential to get established in other countries with favorable environmental conditions, e.g. in the Middle East, Asia and Australia. The meta-analysis showed a high degree of variability in estimated prevalence values. The variability can be attributed to diagnostic tests used and the species of the animal infected.

Declarations Funding
No funding was available for this study Availability of data and materials All data analyzed in this paper are provided as supplementary le.
Authorsʼ contributions P.B. and F.R. conceived the research and S.L. designed the research, and E.F. and S.L. drafted the manuscript. E.F., S.L. and P.B. extracted and compiled the data. All authors read and approved the nal version.
Ethics approval and consent to participate Not applicable.

Consent for publication
Not applicable.