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Abstract [Objectives]This study was conducted to understand the current status of duck related research. [Methods]CiteSpace knowledge graph analysis tool， VOSviewer visual analysis software and HistCite citation graph analysis tool were used to analyze the number of publications and subject distribution of duck-related research fields in the Web of Science core collection database. The main countries （regions） and institutions， the main academic journals， the main research scholars， the main research hotspots and their changing trends were used for quantitative analysis. [Results]① Duck-related research is receiving more and more attention， which further supports ducks' important significance in the livestock and poultry industry. ② Countries such as China， the United States， Canada， and France had published more articles in this field， and there was close cooperation between countries. ③ The top two journals with local citations in the duck-related research fields were PLOS ONE and JOURNAL OF VIROLOGY， and the journal with the highest impact factor in 2019 was JOURNAL OF VIROLOGY. ④ High-yield scholars in duck related research fields included Cheng Anchun， Wang Mingshu， Jia Renyong， etc. ⑤ Infection was a research hotspot in duck-related research. Typical pathogenic microorganisms included Newcastle disease， Avian influenza， and campylobacter. ⑥ The future trend of duck-related research mainly focuses on breeds， living environment， diseases， and molecular mechanisms. [Conclusions]Based on data from the Web of Science core collection database， this paper provides a complete bibliometric analysis of the current research status of ducks.
Key words Duck; Bibliometric analysis; Infection; CiteSpace; VOSviewer; HistCite
Duck occupies an important position in the world's animal husbandry. According to FAO data， China's duck farming accounts for approximately 74% of the world's total. This makes China's duck industry the world's largest producer of ducks， and China has been a major consumer of ducks. The duck industry has become an important part of China's animal husbandry and agricultural economy， as well as an important source of farmers' income， which shows the importance of ducks[1-3]. However， related research on ducks has always been more in terms of diseases and molecular mechanisms， such as duck plague[4]， avian influenza[5]， parvovirus[6]， adenovirus[7]， etc.， molecular mechanisms such as the difference between long non-coding RNA and mRNA in duck reproduction[8]， comparing transcriptomes to reveal duck egg-yolk ratio genes [9]， etc. However， there are few reports on the quantitative analysis of the number of papers published in these fields， the distribution of disciplines， the source of the publications， the published academic journals， research scholars or teams， research hotspots and their changing trends. CiteSpace， as an efficient and accurate web citation analysis tool， has been widely used in many fields， such as education， agronomy， medicine， and management[10]. Similarly， VOSviewer， as another bibliometric software， is also widely used in measurement research in many fields[11]. The core collection of Web of Science （WOS） contains high-impact academic journals around the world. It is an important database for obtaining global academic information， and the content covers many fields and disciplines. The number of published articles retrieved through this database can reflect the attention paid by the scientific community to a field and the degree of development of the field[12]. Through CiteSpace， we can get the proportions of all subjects in a field， reflecting the research degrees of these subjects in the field， but this can only reflect the degree of intersection of disciplines and fields， not the degree of integration of disciplines and disciplines. At this time， another indicator is needed， that is， betweenness centrality. Key words can briefly express the theme of the full text， and can also reflect research hotspots in a field through the VOSviewer keyword co-occurrence network[13]， and the research and development trend of an indicator can be analyzed and revealed through keyword burstness[14]. The research degree and cooperation intensity of different countries/regions and institutions， and major research scholars （teams） in a certain field can be obtained by performing quantitative analysis on the major countries/regions and institutions and authors related to article publication using the bibliometric tool VOSviewer[15]. Although both CiteSpace and VOSviewer can perform author， institution， document co-citation， and journal co-citation analysis， the analysis results of the two are different， and the results of VOSviewer are closer to the data of Web of Science （WOS）[16]. In this study， we combined the respective advantages of CiteSpace and VOSviewer software， used CiteSpace to analyze the discipline distribution of duck-related research fields and keyword burstness， used VOSviewer software to analyze the cooperative relationships among main publishing countries/regions， institutions and authors， and analyzed the research hotspots in this field based on VOSviewer's keyword co-occurrence network. Meanwhile， the important parameters of the HistCite citation map analysis tool， i.e.， the total local citation score （TLCS） and the total global citation score （TGCS）， were used to locate major academic journals and other information[17]. Through quantitative analysis， duck-related research and the research of its scholars in this field were analyzed， so as to comprehensively understand the development status and frontier trends of duck-related research fields and to provide a theoretical reference for future duck-related research and prevention and control.
Materials and Methods
Data sources
The data of this bibliometric analysis came from the WOS core collection database of Thomson Reuters. The subject term duck for the literature search was set to be （duck） OR （mallard duck）OR（mallard ducks）， and the retrieval time was from 2000 to 2020. The retrieval date of all documents was June 1， 2020， and the selected document types were article and review. Through the search， the number of documents obtained was 8 661.
Research methods
This bibliometric analysis used the analysis tools that came with the WOS core collection， CiteSpace software， VOSviewer software， and HistCite software to analyze the 8 661 articles published between 2000 and 2020， from the number of publications and subject distribution， issuing countries/regions and institutions， journal sources， research team， research hotspots and development trends. Category was selected as the node type of CiteSpace analysis for analyzing the distribution of disciplines， and the keyword burstness after 2015 was selected to describe the research development trend in the past five years. The VOSviewer scientific research cooperation network was used to analyze the cooperation relationships among the main countries/regions， organizations， and authors， and analyze the total link strength （TLS） between nodes. Based on the co-occurrence analysis of VOSviewer， the option of all keywords was selected， and a threshold （30） was set to count keywords with more occurrences， which reflected the research hotspots in this field. In addition， the two important parameters， the total local citation score （TLCS） and the total global citation score （TGCS） in HistCite， were used to determine the major academic journals in the duck-related research field. Results and Analysis
Number of publications and subject distribution
The search results showed that the number of duck-related research articles published was very small from 2000 to 2007， but increased rapidly after 2007 and then increased steadily year by year （Fig. 1A）， that is， duck-related research has become more and more popular after 2007. Duck-related research involves multiple disciplines and fields. The analysis of WOS's own software showed that there were more articles published in the WOS category of duck-related research such as veterinary science and ecological environment science （Fig. 1B）. Using CiteSpace to conduct field co-occurrence analysis， the results showed that duck research involved multiple research fields and has attracted the attention of multiple disciplines （Table 1）. The top three disciplines in terms of the number of papers were veterinary science， environmental science and ecology， and agriculture， but biochemistry and molecular biology had the highest betweeness centrality.
Main countries/regions and institutions that issue the articles
The VOSviewer visualization software was used to analyze the number of articles published by various countries in the duck-related research field and the cooperation relationships between countries. The result is shown in Fig. 2. The size of the circle in the figure reflects the paper issuing status of the country/region， and the line indicates the closeness of the mutual connection. According to statistics on the number of papers， China had the most （2 553 papers）， followed by the United States （2 077 papers）. In addition， Canada， France， Britain， Germany， the Netherlands， Australia， Italy， Belgium， etc.， had also published many excellent papers in duck-related research fields. The main publishing organizations in China included the Chinese Academy of Agricultural Sciences （issued 365 articles）， Sichuan Agricultural University （issued 329 articles）， China Agricultural University （issued 192 articles）， Chinese Academy of Sciences （issued 152 articles）， Nanjing Agricultural University （issued 148 articles）， Yangzhou University （issued 136 articles）， Huazhong Agricultural University （issued 124 articles）， Shandong Agricultural University （issued 112 articles）， South China Agricultural University （issued 110 articles）， etc.
According to the total link strength （TLS） parameter value of VOSviewer， the United States （TLS value 1 298）， China （TLS value 650）， United Kingdom （TLS value 578） and France （TLS value 526） had close international cooperation in duck-related research fields. From the connection in Fig. 2， we can see that the United States， Canada， France and Australia cooperated closely with China in duck related research. In addition， it can be seen from the institution cooperation analysis network that the Chinese Academy of Agricultural Sciences （Chinese acad agr sci） had a close cooperation relationship with the major institutions that issue papers （Fig. 3）. The total link strength value was 497 for US geol survey， 363 for Chinese Academy of Agricultural Sciences， 349 for US fish & wildlife serv， and 205 for univ Georgia. As can be seen from the connection in Fig. 3， the Chinese Academy of Agricultural Sciences had close cooperation with Sichuan Agricultural University， China Agricultural University， Illinois State University， Chinese Academy of Sciences and other institutions. Major international periodicals publishing the articles
According to the analysis results of the HistCite software， the top three journals with the number of articles are POULTRY SCIENCE （321 articles）， PLOS ONE （304 articles） were AVIAN DISEASES （221 articles）. The journals with a high local citation score （TLCS） and total global citation score （TGCS） included PLOS ONE， JOURNAL OF VIROLOGY， etc. Most of these journals were classified into virology， and there were also typical veterinary journals such as AVIAN DISEASES and AVIAN PATHOLOGY， etc.， but the number of articles published in these journals was relatively small （Table 2）. The major periodicals with a higher impact factor in duck-related research fields were： JOURNAL OF VIROLOGY， VETERINARY MICROBIOLOGY and PLOS ONE.
Leading research scholars
As shown in Fig. 4， the size of the circle indicates the number of articles， and the figure only shows the connections between authors with lose cooperation relationships. In terms of the number of articles， Cheng Anchun （167 articles）， Wang Mingshu （163 articles）， Jia Renyong （151 articles）， Zhu Dekang （146 articles）， Che Shun （133 articles） and Liu Mafeng （105 articles） were highly productive authors in duck-related research fields. According to the analysis of the total link strength parameter of VOSviewer， Cheng Anchun （TLS parameter 1573）， Wang Mingshu （TLS parameter 1565）， Jia Renyong （TLS parameter 1522） and Zhu Dekang （TLS parameter 1522） had close cooperations with other authors in the field.
Duck-related research hotspots
Keywords
Keywords briefly express the main theme of the full text and represent the main research content of an article. The greater the frequency of a keyword， the higher the degree of attention related to the topic. Infection was the most frequently occurring keyword in addition to Duck/Ducks， with a total of 603 occurrences， indicating that the current hot spot for duck-related research was infection （Fig. 5）. In the key word co-occurrence network， virus， Newcastle disease， Avian influenza， H5N1 influenza virus （H5N1）， protein， identification， replication， expression， growth， chicken， and pathogenicity were closely related to infection.
Pathogenic microorganisms attracting special attention
Among these high-frequency key words， the top three duck pathogenic microorganisms with frequency of occurrence were typical pathogenic microorganisms such as Avian influenza， Newcastle disease and Campylobacter. The top two diseases according to the occurrence frequencies were all typical virus diseases， which also reflected the predominance of viruses among duck pathogenic microorganisms. Duck-related research hotspot clustering
According to the keyword co-occurrence network （Fig. 5）， keywords in the duck-related research fields were divided into four clusters， and different colors in the figure indicate different clusters. Keyword cluster one was mainly duck diseases， including keywords Avian infiuenza， Newcastle disease， H5N1， evolution， transmission， and arbovirus. Key word cluster two was mainly based on the classification and living environment of ducks， including keywords waterfowl （waterbirds）， birds， diversity， Anas platyrhynchos， conservation， habitatus， and ecology. Key word cluster three was mainly based on the meat quality and performance of ducks， including key words growth， meat， performance， muscle， quality， and growth performance. Key word cluster four mainly focused on duck biotechnology and prevention and control technology， including key words identification， gene， protein， expression， sequence， vaccine， and PCR assay.
The development trend of duck-related research fields
Based on CiteSpace's analysis on key word burstness in the duck research field， words burst after 2015 were selected to describe the research development trend in the past five years. These 43 key words burst can be used to reflect the research development trend in the past five years （Table 3）.
Conclusions and Discussion
Through the quantitative analysis this time， we can have a more comprehensive understanding of the development status and frontier trends of duck-related research fields. From the results of the literature search， the overall number of duck-related research literature had increased， but the number of articles published in veterinary journals was limited to less than 20%， which is different from the results of Zhang[18]， possibly because of the differences in search time and search themes. What's more noteworthy is that when calculating betweenness centrality， it was found that the betweenness centrality of all disciplines was less than 0.1， while this indicator was used to discover and measure the importance of literatures and reveal the adjustment and control abilities among these disciplines. The betweenness centrality was lower than 0.1， indicating that these disciplines still lack strong influence in this field， and their research strength needs to be further improved. There may also be an important reason that is the existence of certain academic barriers between these disciplines， which resulting in the condition that information exchange and sharing cannot be achieved[19]. Analyzing the total link strength （TLS） between nodes in the VOSviewer scientific research cooperation network， it was found that although China was the country with the most published documents （2 553 articles） and the United States was the second （2 077 articles）， we could find that the TLS value of the United States is 1 298， but the TLS value of China was only 650， which might indicate that we should strengthen international exchanges between disciplines in this field and increase the influence of literatures. Most of the journals published in duck-related research fields had low impact factors， which might also suggest why there was so little research in this field before 2007， that is they were willing to study the fields or disciplines which journals with relatively high impact factors pay attention to. Keywords can not only express the main theme of an article but also highlight the main research content. For example， the greater the frequency of a key word， the higher the degree of attention related to the topic. In this analysis， infection was the keyword with the most occurrences in addition to duck/ducks， with a total of 603 occurrences， indicating that the current hot spot for duck-related research was infection， and the keywords in the keyword co-occurrence network closely related to infection included virus， Newcastle disease， Avian influenza， arbovirus， etc. Newcastle disease was once considered to be infective or invisible to waterfowl， but after 1997， Wang et al. isolated a virulent NDV strain from diseased geese， breaking the claim that Newcastle disease does not cause disease to waterfowl， and then later studies on duck-source Newcastle disease in Shandong Province showed that it was probably derived from the early epidemic of goose-source Newcastle disease[20]. The hot spot analysis found that the main epidemic areas were China and South Korea. The virus was often mixed with H5N1 and the hot gene was the F gene.
Studies have shown that ducks， as a natural virus reservoir， usually carry influenza A virus asymptomatically. Sometimes even if they are infected with influenza A virus that is highly pathogenic to humans， ducks may only show mild symptoms. Later researchers even showed at the beginning， it was a highly pathogenic influenza virus to ducks， but its pathogenicity to ducks decreased rapidly over time， although it is still lethal to mice and ferrets[21]. People tried to find out why this happened. It may be the reason why people have always been keen to study influenza viruses on ducks. Through the key word analysis of previous studies， it was found that the most commonly studied genotypes of avian influenza are H1N1， H5N1， H5N2， H5N6， H5N8， H9N2， and the commonly used detection methods are ELISA and RT-PCR， while the pb1 and pb2 peptides are hot spots in terms of genes. Duck Tembusu disease is a new acute infectious disease caused by Tembusu virus （TMUV）， which is a typical mosquito-borne virus. However， some scholars have proposed that the virus does not rely on insects for transmission among ducks[22]. The arbovirus that appeared in the key word search in this article was mainly the tambusu virus. After the virus was introduced into China in 2010， it showed a relatively wide spectrum of infection during the epidemic period， and the host range expanded rapidly. However， the virus is sensitive to mammalian interferons， so it will not be pathogenic to primates under normal circumstances， and has a low probability of causing public health safety hazards. Through in-depth research， it was found that host monocytes/macrophages are important target cells for TMUV infection[23]. ELISA RT-PCR and RT-LAMP are commonly used detection methods， and pathogenesis and evolution analysis are the research hotspots. Of course， in addition to viral diseases， there is also an important bacterial disease， Campylobacter spp. SPP， which is a zoonotic disease and can cause Guillain-Barre syndrome （GBS） in humans， and ducks are the most important reservoir of Campylobacter. The main reason for the infection of Campylobacter is related to direct contact with ducks carrying the bacteria or eating under-processed duck meat. Once the human body is infected， diarrheal enteritis， diarrhea， abdominal pain， fever and other symptoms will occur. The incidence of the bacteria has surpassed Salmonella， Shigella and Listeria， and the bacteria's drug resistance is significantly enhanced[24]. Through CiteSpace's analysis of keyword burstness in duck-related research fields， it was found that in vitro had a high burstness （Table 3）. This keyword also appeared more frequently in the keyword co-occurrence network， indicating that the trend of duck related research is in vitro research. For example， Li et al. established a reliable method for fluorescent labeling of live Muscovy duck lymphocytes in vitro to further clarify the pathogenic mechanism of intestinal tissues infected by reovirus. However， nuclear factor-kappa B （NF-κB） was later discovered in mouse B lymphocytes by Ranjan Sen et al. and named in 1986. It was later found that it is expressed in almost all cells， and the cell signaling pathway with NF-κB as the core is called the NF-κB signaling pathway. This signaling pathway is known to be the main pathway for the development of inflammation[26]， and studies have shown that it interacts with duck plague virus infection[4]. Through the summary analysis of highlighted key words， it was found that most of them were research on duck breeds， living environment， diseases and mechanisms， which might indicate that the future research directions were mainly these four.
Based on the data from the core collection of Web of Science， this paper provides a relatively complete bibliometric analysis of duck-related research status using various bibliometric softwares.
References
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Key words Duck; Bibliometric analysis; Infection; CiteSpace; VOSviewer; HistCite
Duck occupies an important position in the world's animal husbandry. According to FAO data， China's duck farming accounts for approximately 74% of the world's total. This makes China's duck industry the world's largest producer of ducks， and China has been a major consumer of ducks. The duck industry has become an important part of China's animal husbandry and agricultural economy， as well as an important source of farmers' income， which shows the importance of ducks[1-3]. However， related research on ducks has always been more in terms of diseases and molecular mechanisms， such as duck plague[4]， avian influenza[5]， parvovirus[6]， adenovirus[7]， etc.， molecular mechanisms such as the difference between long non-coding RNA and mRNA in duck reproduction[8]， comparing transcriptomes to reveal duck egg-yolk ratio genes [9]， etc. However， there are few reports on the quantitative analysis of the number of papers published in these fields， the distribution of disciplines， the source of the publications， the published academic journals， research scholars or teams， research hotspots and their changing trends. CiteSpace， as an efficient and accurate web citation analysis tool， has been widely used in many fields， such as education， agronomy， medicine， and management[10]. Similarly， VOSviewer， as another bibliometric software， is also widely used in measurement research in many fields[11]. The core collection of Web of Science （WOS） contains high-impact academic journals around the world. It is an important database for obtaining global academic information， and the content covers many fields and disciplines. The number of published articles retrieved through this database can reflect the attention paid by the scientific community to a field and the degree of development of the field[12]. Through CiteSpace， we can get the proportions of all subjects in a field， reflecting the research degrees of these subjects in the field， but this can only reflect the degree of intersection of disciplines and fields， not the degree of integration of disciplines and disciplines. At this time， another indicator is needed， that is， betweenness centrality. Key words can briefly express the theme of the full text， and can also reflect research hotspots in a field through the VOSviewer keyword co-occurrence network[13]， and the research and development trend of an indicator can be analyzed and revealed through keyword burstness[14]. The research degree and cooperation intensity of different countries/regions and institutions， and major research scholars （teams） in a certain field can be obtained by performing quantitative analysis on the major countries/regions and institutions and authors related to article publication using the bibliometric tool VOSviewer[15]. Although both CiteSpace and VOSviewer can perform author， institution， document co-citation， and journal co-citation analysis， the analysis results of the two are different， and the results of VOSviewer are closer to the data of Web of Science （WOS）[16]. In this study， we combined the respective advantages of CiteSpace and VOSviewer software， used CiteSpace to analyze the discipline distribution of duck-related research fields and keyword burstness， used VOSviewer software to analyze the cooperative relationships among main publishing countries/regions， institutions and authors， and analyzed the research hotspots in this field based on VOSviewer's keyword co-occurrence network. Meanwhile， the important parameters of the HistCite citation map analysis tool， i.e.， the total local citation score （TLCS） and the total global citation score （TGCS）， were used to locate major academic journals and other information[17]. Through quantitative analysis， duck-related research and the research of its scholars in this field were analyzed， so as to comprehensively understand the development status and frontier trends of duck-related research fields and to provide a theoretical reference for future duck-related research and prevention and control.
Materials and Methods
Data sources
The data of this bibliometric analysis came from the WOS core collection database of Thomson Reuters. The subject term duck for the literature search was set to be （duck） OR （mallard duck）OR（mallard ducks）， and the retrieval time was from 2000 to 2020. The retrieval date of all documents was June 1， 2020， and the selected document types were article and review. Through the search， the number of documents obtained was 8 661.
Research methods
This bibliometric analysis used the analysis tools that came with the WOS core collection， CiteSpace software， VOSviewer software， and HistCite software to analyze the 8 661 articles published between 2000 and 2020， from the number of publications and subject distribution， issuing countries/regions and institutions， journal sources， research team， research hotspots and development trends. Category was selected as the node type of CiteSpace analysis for analyzing the distribution of disciplines， and the keyword burstness after 2015 was selected to describe the research development trend in the past five years. The VOSviewer scientific research cooperation network was used to analyze the cooperation relationships among the main countries/regions， organizations， and authors， and analyze the total link strength （TLS） between nodes. Based on the co-occurrence analysis of VOSviewer， the option of all keywords was selected， and a threshold （30） was set to count keywords with more occurrences， which reflected the research hotspots in this field. In addition， the two important parameters， the total local citation score （TLCS） and the total global citation score （TGCS） in HistCite， were used to determine the major academic journals in the duck-related research field. Results and Analysis
Number of publications and subject distribution
The search results showed that the number of duck-related research articles published was very small from 2000 to 2007， but increased rapidly after 2007 and then increased steadily year by year （Fig. 1A）， that is， duck-related research has become more and more popular after 2007. Duck-related research involves multiple disciplines and fields. The analysis of WOS's own software showed that there were more articles published in the WOS category of duck-related research such as veterinary science and ecological environment science （Fig. 1B）. Using CiteSpace to conduct field co-occurrence analysis， the results showed that duck research involved multiple research fields and has attracted the attention of multiple disciplines （Table 1）. The top three disciplines in terms of the number of papers were veterinary science， environmental science and ecology， and agriculture， but biochemistry and molecular biology had the highest betweeness centrality.
Main countries/regions and institutions that issue the articles
The VOSviewer visualization software was used to analyze the number of articles published by various countries in the duck-related research field and the cooperation relationships between countries. The result is shown in Fig. 2. The size of the circle in the figure reflects the paper issuing status of the country/region， and the line indicates the closeness of the mutual connection. According to statistics on the number of papers， China had the most （2 553 papers）， followed by the United States （2 077 papers）. In addition， Canada， France， Britain， Germany， the Netherlands， Australia， Italy， Belgium， etc.， had also published many excellent papers in duck-related research fields. The main publishing organizations in China included the Chinese Academy of Agricultural Sciences （issued 365 articles）， Sichuan Agricultural University （issued 329 articles）， China Agricultural University （issued 192 articles）， Chinese Academy of Sciences （issued 152 articles）， Nanjing Agricultural University （issued 148 articles）， Yangzhou University （issued 136 articles）， Huazhong Agricultural University （issued 124 articles）， Shandong Agricultural University （issued 112 articles）， South China Agricultural University （issued 110 articles）， etc.
According to the total link strength （TLS） parameter value of VOSviewer， the United States （TLS value 1 298）， China （TLS value 650）， United Kingdom （TLS value 578） and France （TLS value 526） had close international cooperation in duck-related research fields. From the connection in Fig. 2， we can see that the United States， Canada， France and Australia cooperated closely with China in duck related research. In addition， it can be seen from the institution cooperation analysis network that the Chinese Academy of Agricultural Sciences （Chinese acad agr sci） had a close cooperation relationship with the major institutions that issue papers （Fig. 3）. The total link strength value was 497 for US geol survey， 363 for Chinese Academy of Agricultural Sciences， 349 for US fish & wildlife serv， and 205 for univ Georgia. As can be seen from the connection in Fig. 3， the Chinese Academy of Agricultural Sciences had close cooperation with Sichuan Agricultural University， China Agricultural University， Illinois State University， Chinese Academy of Sciences and other institutions. Major international periodicals publishing the articles
According to the analysis results of the HistCite software， the top three journals with the number of articles are POULTRY SCIENCE （321 articles）， PLOS ONE （304 articles） were AVIAN DISEASES （221 articles）. The journals with a high local citation score （TLCS） and total global citation score （TGCS） included PLOS ONE， JOURNAL OF VIROLOGY， etc. Most of these journals were classified into virology， and there were also typical veterinary journals such as AVIAN DISEASES and AVIAN PATHOLOGY， etc.， but the number of articles published in these journals was relatively small （Table 2）. The major periodicals with a higher impact factor in duck-related research fields were： JOURNAL OF VIROLOGY， VETERINARY MICROBIOLOGY and PLOS ONE.
Leading research scholars
As shown in Fig. 4， the size of the circle indicates the number of articles， and the figure only shows the connections between authors with lose cooperation relationships. In terms of the number of articles， Cheng Anchun （167 articles）， Wang Mingshu （163 articles）， Jia Renyong （151 articles）， Zhu Dekang （146 articles）， Che Shun （133 articles） and Liu Mafeng （105 articles） were highly productive authors in duck-related research fields. According to the analysis of the total link strength parameter of VOSviewer， Cheng Anchun （TLS parameter 1573）， Wang Mingshu （TLS parameter 1565）， Jia Renyong （TLS parameter 1522） and Zhu Dekang （TLS parameter 1522） had close cooperations with other authors in the field.
Duck-related research hotspots
Keywords
Keywords briefly express the main theme of the full text and represent the main research content of an article. The greater the frequency of a keyword， the higher the degree of attention related to the topic. Infection was the most frequently occurring keyword in addition to Duck/Ducks， with a total of 603 occurrences， indicating that the current hot spot for duck-related research was infection （Fig. 5）. In the key word co-occurrence network， virus， Newcastle disease， Avian influenza， H5N1 influenza virus （H5N1）， protein， identification， replication， expression， growth， chicken， and pathogenicity were closely related to infection.
Pathogenic microorganisms attracting special attention
Among these high-frequency key words， the top three duck pathogenic microorganisms with frequency of occurrence were typical pathogenic microorganisms such as Avian influenza， Newcastle disease and Campylobacter. The top two diseases according to the occurrence frequencies were all typical virus diseases， which also reflected the predominance of viruses among duck pathogenic microorganisms. Duck-related research hotspot clustering
According to the keyword co-occurrence network （Fig. 5）， keywords in the duck-related research fields were divided into four clusters， and different colors in the figure indicate different clusters. Keyword cluster one was mainly duck diseases， including keywords Avian infiuenza， Newcastle disease， H5N1， evolution， transmission， and arbovirus. Key word cluster two was mainly based on the classification and living environment of ducks， including keywords waterfowl （waterbirds）， birds， diversity， Anas platyrhynchos， conservation， habitatus， and ecology. Key word cluster three was mainly based on the meat quality and performance of ducks， including key words growth， meat， performance， muscle， quality， and growth performance. Key word cluster four mainly focused on duck biotechnology and prevention and control technology， including key words identification， gene， protein， expression， sequence， vaccine， and PCR assay.
The development trend of duck-related research fields
Based on CiteSpace's analysis on key word burstness in the duck research field， words burst after 2015 were selected to describe the research development trend in the past five years. These 43 key words burst can be used to reflect the research development trend in the past five years （Table 3）.
Conclusions and Discussion
Through the quantitative analysis this time， we can have a more comprehensive understanding of the development status and frontier trends of duck-related research fields. From the results of the literature search， the overall number of duck-related research literature had increased， but the number of articles published in veterinary journals was limited to less than 20%， which is different from the results of Zhang[18]， possibly because of the differences in search time and search themes. What's more noteworthy is that when calculating betweenness centrality， it was found that the betweenness centrality of all disciplines was less than 0.1， while this indicator was used to discover and measure the importance of literatures and reveal the adjustment and control abilities among these disciplines. The betweenness centrality was lower than 0.1， indicating that these disciplines still lack strong influence in this field， and their research strength needs to be further improved. There may also be an important reason that is the existence of certain academic barriers between these disciplines， which resulting in the condition that information exchange and sharing cannot be achieved[19]. Analyzing the total link strength （TLS） between nodes in the VOSviewer scientific research cooperation network， it was found that although China was the country with the most published documents （2 553 articles） and the United States was the second （2 077 articles）， we could find that the TLS value of the United States is 1 298， but the TLS value of China was only 650， which might indicate that we should strengthen international exchanges between disciplines in this field and increase the influence of literatures. Most of the journals published in duck-related research fields had low impact factors， which might also suggest why there was so little research in this field before 2007， that is they were willing to study the fields or disciplines which journals with relatively high impact factors pay attention to. Keywords can not only express the main theme of an article but also highlight the main research content. For example， the greater the frequency of a key word， the higher the degree of attention related to the topic. In this analysis， infection was the keyword with the most occurrences in addition to duck/ducks， with a total of 603 occurrences， indicating that the current hot spot for duck-related research was infection， and the keywords in the keyword co-occurrence network closely related to infection included virus， Newcastle disease， Avian influenza， arbovirus， etc. Newcastle disease was once considered to be infective or invisible to waterfowl， but after 1997， Wang et al. isolated a virulent NDV strain from diseased geese， breaking the claim that Newcastle disease does not cause disease to waterfowl， and then later studies on duck-source Newcastle disease in Shandong Province showed that it was probably derived from the early epidemic of goose-source Newcastle disease[20]. The hot spot analysis found that the main epidemic areas were China and South Korea. The virus was often mixed with H5N1 and the hot gene was the F gene.
Studies have shown that ducks， as a natural virus reservoir， usually carry influenza A virus asymptomatically. Sometimes even if they are infected with influenza A virus that is highly pathogenic to humans， ducks may only show mild symptoms. Later researchers even showed at the beginning， it was a highly pathogenic influenza virus to ducks， but its pathogenicity to ducks decreased rapidly over time， although it is still lethal to mice and ferrets[21]. People tried to find out why this happened. It may be the reason why people have always been keen to study influenza viruses on ducks. Through the key word analysis of previous studies， it was found that the most commonly studied genotypes of avian influenza are H1N1， H5N1， H5N2， H5N6， H5N8， H9N2， and the commonly used detection methods are ELISA and RT-PCR， while the pb1 and pb2 peptides are hot spots in terms of genes. Duck Tembusu disease is a new acute infectious disease caused by Tembusu virus （TMUV）， which is a typical mosquito-borne virus. However， some scholars have proposed that the virus does not rely on insects for transmission among ducks[22]. The arbovirus that appeared in the key word search in this article was mainly the tambusu virus. After the virus was introduced into China in 2010， it showed a relatively wide spectrum of infection during the epidemic period， and the host range expanded rapidly. However， the virus is sensitive to mammalian interferons， so it will not be pathogenic to primates under normal circumstances， and has a low probability of causing public health safety hazards. Through in-depth research， it was found that host monocytes/macrophages are important target cells for TMUV infection[23]. ELISA RT-PCR and RT-LAMP are commonly used detection methods， and pathogenesis and evolution analysis are the research hotspots. Of course， in addition to viral diseases， there is also an important bacterial disease， Campylobacter spp. SPP， which is a zoonotic disease and can cause Guillain-Barre syndrome （GBS） in humans， and ducks are the most important reservoir of Campylobacter. The main reason for the infection of Campylobacter is related to direct contact with ducks carrying the bacteria or eating under-processed duck meat. Once the human body is infected， diarrheal enteritis， diarrhea， abdominal pain， fever and other symptoms will occur. The incidence of the bacteria has surpassed Salmonella， Shigella and Listeria， and the bacteria's drug resistance is significantly enhanced[24]. Through CiteSpace's analysis of keyword burstness in duck-related research fields， it was found that in vitro had a high burstness （Table 3）. This keyword also appeared more frequently in the keyword co-occurrence network， indicating that the trend of duck related research is in vitro research. For example， Li et al. established a reliable method for fluorescent labeling of live Muscovy duck lymphocytes in vitro to further clarify the pathogenic mechanism of intestinal tissues infected by reovirus. However， nuclear factor-kappa B （NF-κB） was later discovered in mouse B lymphocytes by Ranjan Sen et al. and named in 1986. It was later found that it is expressed in almost all cells， and the cell signaling pathway with NF-κB as the core is called the NF-κB signaling pathway. This signaling pathway is known to be the main pathway for the development of inflammation[26]， and studies have shown that it interacts with duck plague virus infection[4]. Through the summary analysis of highlighted key words， it was found that most of them were research on duck breeds， living environment， diseases and mechanisms， which might indicate that the future research directions were mainly these four.
Based on the data from the core collection of Web of Science， this paper provides a relatively complete bibliometric analysis of duck-related research status using various bibliometric softwares.
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