4.1 Hot issues
The scientometric analysis developed allowed a better understanding of the knowledge of environmental pollutants on canines, helping to understand even better the relationships, patterns and trends that exist in this recent field of research (NCR 1991; Poma et al. 2020; Sonne et al. 2020). These components have had greater research interest in recent decades (Figure 1), probably due to the metabolic and adaptive similarities with human beings, and their advantage of showing the phenomena of affinity, efficacy and pathogenic action that these pollutants would have on a large part of the exposed organic systems (Gupta 2018; Pastorinho and Sousa 2020), in addition to having achieved validation as sentinel models in typical regions of the planet such as the Arctic as described by several authors (Kirkegaard 2009; Abbott 2017; Letcher et al. 2017; Sonne et al. 2020). Despite this, it should be mentioned that the study of environmental pollutants in canines has been a subject of recent appearance that has been slowly consolidating, becoming increasingly stronger by different disciplines. Therefore, scientific communities are invited to pay more attention and interest in the validation of sentinel organisms due to the advantages they provide in relation to studies in humans, for example, especially in regions of the planet where short-term results and greater precision are required.
In relation to the data set obtained, a low amount of bibliographic material was evidenced that was compiled with the key terms previously referenced in this document. Therefore, it is necessary to promote the integration of this type of research lines with new approaches that allow growth, development and consolidation for the following years, a situation that has already been happening with the collaborative efforts of multiple disciplines such as those proposed by One Health, where ecologists, veterinary and human medical sciences have a considerable role.
In the same way it must be taken into account that it is a topic that has been in research since 1969, but only the last 13 years have been more dedicated to the development and consolidation of the effects of environmental pollutants in canines (Figure 1), which currently turns out to be a research area that allows increasing the spectrum of the problems related to environmental pollution and climate change on planetary health (Pastorinho and Sousa 2020; Sonne et al. 2020).
The scientometric analysis developed in this study very surely allowed a better understanding of the knowledge of environmental pollutants on canines, helping to understand even better the relationships, patterns and trends that exist in this recent field of research (NCR 1991; Poma et al. 2020; Sonne et al. 2020). In this sense, the development of scientometric studies in relation to environmental pollutants, and lately climate change, in human beings have allowed the approach and evaluation of the current state of these research lines during the last decades, such as other scientometric analyzes developed by Dhital and Rupakheti (2019); Lie et al. (2018) and Sweileh (2020) in other similar subjects.
Studies between animals and environmental pollutants could have greater research interest in the last decade (Figure 1), probably due to new studies that have shown metabolic, genetic, and adaptive similarities with human beings, and the affinity, efficacy, and pathogenic action that these pollutants have on a large part of the exposed organic systems in canines (Gupta 2018; Pastorinho and Sousa 2020). In addition, canines as sentinel species for human health have been validated recently in specific regions of the planet such as the Arctic as described by several authors (Kirkegaard 2009; Abbott 2017; Letcher et al. 2017; Sonne et al. 2020).
Regarding the top of the analyzed journals in this analysis (Figure 2) these are considered by the scientific community as the bibliographic sources with the most information reported for this class of studies, therefore, researchers with a great interest in the development of this type of research, it is recommended to refer to these sources to make their pertinent updates on the subject. However, there is also a clear trend in the growth of other sources of scientific production in relation to air pollutants, suggesting new research questions regarding the issue of environmental air pollutants and canines (Figure 2).
Although the scientific production of this topic has had a recent appearance in the last five decades, authors such as Sonne, Dietz and Letcher, have managed to consolidate the role of canines as one of the study models that allows the evaluation of environmental pollutants and their effects in dogs, the above according to the impact factor or H index resulting from this scientometric analysis in the author´s section. It should be mentioned in this sense that the collaboration between these authors from 2005 (Figure 3) allowed an increase in the number of publications on this subject until 2018, one of the most productive years and the period of maximum consolidation with relation to these themes.
In this sense, the participation of these three authors has allowed the development of important scientific production, such as the reference text: Pets as Sentinels, Forecasters and Promoters of Human Health (Pastorinho and Sousa 2020). One of the most complex texts where the opinion of a large group of experts on the subject of environmental pollutants is compiled, they even touch on different associated topics such as the use of canines for the evaluation of comorbidities and various diseases such as obesity, cancer, immunological alterations, immunomodulation and neurodegenerative alterations that both species share from these types of themes.
Regarding the tripartite graph (Figure 4), there is a clear interconnection between the most cited authors regarding the keywords that were used as input for the development of this analysis (dog and environmental pollutants). However, words such as asthma, exposure and health are likely topics that are beginning to develop as new lines of research that could be related to this topic, as can be seen in the graph of the networks of associations between the words (Figure 7). Also, in the same graph (Figure 4) it is possible to observe that the Universities of Heme and Hokkaido, located in Japan, are leading the top of institutional affiliations that have generated the most contributions in the development and scientific production on issues related to canines and environmental pollutants. Which may be due to the fact that this country has been conducting research related to air pollutants in recent decades and new lines of research such as particulate matter and allergens (Zhang et al. 2019; Wang et al. 2010) due to the problems that are currently related worldwide (Calderon et al. 2001; Selin 2009).
In this same sense, the University of Aarhus located in Denmark continues to be one of the most representative institutional affiliations due to the relationship between the most prominent authors in relation to this topic such as Sonne and Dietz (Pastorinho and Sousa 2020; Sonne et al. 2020). A valid explanation for this phenomenon is the thematic interest shared by these authors, when generating scientific production in relation to this research topic, thus producing an increase in the number of collaborations. In this sense and for this scientometric analysis, these authors are undoubtedly the most prominent for the development of research related to canines and environmental pollutants, for which people interested in the development of this type of studies are advised to create collaboration networks with this important nucleus of experts and/or to consult their scientific publications, due to the experience they have gained from the development of their studies.
For this scientometric analysis, the United States, Denmark and Canada were considered the countries with the greatest impact on production and scientific collaboration in relation to the issue of environmental pollutants and their effects on canines. However, it should be clarified that eastern countries such as Japan and China have begun to play an important role in the generation of new scientific knowledge in relation to air pollution issues, particulate matter, and the role that canines would have as models of sentinel studies to these pollutants (Zhang et al. 2020). Similar results have been found in studies in Latin America (Calderon et al. 2001). However, the development of more research studies in relation to air pollutants, chemical pollutants and sentinel study models in Latin America is required, to allow a better development of this important research line, as can be seen for other latitudes of the world (Figure 6).
4.2 Topics of lasting impacts
For this scientometric analysis, and according with these results, great advances have been made on the study of environmental pollutants such as polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and heavy metals such as mercury and lead, nowadays pollutants high-emission´s in urban centers (Pastorinho and Sousa 2020). In this sense the development of these topics has allowed to recognize the use of canines as good candidates for the follow-up and monitoring of the impacts that these pollutants would have on the human being (Calderon et al. 2001). This is due to the short time it takes to show the adverse effects of a wide variety of pollutants and toxins in comparison to humans (Hoffman et al., 2018). This could also be useful for linearity studies that are commonly used in human medicine, due to the similarities that exist between these two species (Lindblad et al., 2005), but mainly to the routes of exposure that seem to be very similar between them. This analysis could also be considered as a first approach of different scientometric analyses that could be carried out in relation to environmental pollutants and canines.
In the keyword analysis, it was possible to identify three groups with different foci. The largest group focuses on air pollution and asthma, in addition to its relationship with the group of caniforms, which probably precedes the effect that air pollution would have on terrestrial mammals such as canines, since it represents the main route of exposure in these animals, being the biomagnification of the terrestrial food chain less marked (Pastorinho and Sousa 2020), contrary to what would happen in aquatic organisms (Seabert et al. 2014).
The following group includes the effects of halogenated organic pesticides and heavy metals in dogs and sled dogs on organs such as the kidney and liver, this being the most prominent group due to the fact that the effects that these pollutants would have on target organs have been very well documented (Parker et al. 2006; Dunlap et al. 2007; Serpe et al. 2013, Pastorinho and Sousa 2020). Regarding this issue, Sonne et al. 2020 reported in dogs the appearance of persistent pollutants such as PBDEs and PCBs, which tend to accumulate on the marine trophic chain, and found high concentrations of these compounds in arctic and circumpolar areas, the above due to the consumption of products based on marine animals. Marine animals would be the main source of exposure in this population, which requires the development of much more studies on different latitudes of the planet to determine if these same results are similar under other environmental conditions.
The third smallest group focuses on biological and chemical contaminants that could be occurring in parallel and additively, and that canine sentinel models could probably better clarify the complex relationship between these issues, as clearly proposed in the scientific production such as the stipulated by Pastorinho and Sousa (2020) and Calderon et al. (2001).
Even though in this analysis there has been a small number of documents related to this topic compared to much more consolidated topics (Lei et al. 2018), it should be noted that in recent years this topic has had an increase in the number of publications when compared to previous years. This is also interesting considering that the number of case reports in the clinical practice are increasing and that usually canines are the first animals to be reported as early-warning systems for humans and other animals in these case reports (Pastorinho et al., 2020).
The most cited references described in this document highlight the relevance of the work of each research team in this area of scientific knowledge. It should be emphasized in the development of this type of studies and the influential role of the respective research teams in the development of these topics. Among them, Sonne et al. 2010, 2011, 2014, 2015; Letcher et al. 2009, 2017; Mizukawa and Nomiyama 2020, Poma et al. 2020 and Pastorinho and Sousa, 2020 draw attention, because they focus their studies specifically on the effect and relationship that canines exposed to different emerging environmental pollutants such as heavy metals, halogenated organic compounds and aromatic hydrocarbons would have on different biomarkers, including on the response of immunological, hepatorenal biomarkers and some endocrine parameters (Hansmann et al. 2009; Chung et al. 2020). In the same way, the role of Asian research that has allowed important advances in recent years in relation to these issues stands out, such as the research contributed by Zhang, Wang and Chen which has addressed important issues such as water quality, biological pollutants, air pollution, gastric toxins, among others (Wang et al. 2010, 2020; Zhang et al. 2019; Chen et al. 2021), investigations that clarify much better the knowledge gaps on these issues as previously discussed, allowing broadening the spectrum of study of environmental pollutants in relation to canines, all of the above through the use of methodologies applied to omics sciences, a field of study less studied in developing countries (Álvarez et al. 2021) .
In the case of Latin America, a region highly affected by environmental pollutants (Canham et al. 2020), the production of scientific information related to the subject of canines seems to be much lower than the average of publications developed in other regions of the world, being Argentina (Garcia 1974), Chile (Muñoz et al. 2014), Brazil (Scaini et al. 2003) and Uruguay (Mañay et al. 2008) the only representatives identified for this class of studies, this is probably because they are mostly published in Spanish language and were not taken for this analysis (only one typology was found in Spanish for this review in the period covered by this study), being the German and Japanese languages the languages that have been gaining strength in relation to these publications, with a total of 10 and 4 publications for the last three decades correspondingly. Likewise, for this region of the planet the scarce number of collaboration and scientific production in relation to this issue is discussed, this probably due to the little interest or financing that they have had in relation to this problem, despite the fact that it is one of the regions of the world where phenomena related to climate change and pollutants prevail (Canham et al. 2021). However, more searches in other databases are required to confirm the hypotheses raised in this document.