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 Table of Contents  
ORIGINAL ARTICLE
Year : 2018  |  Volume : 8  |  Issue : 3  |  Page : 145-149

Spatio-Temporal analysis of brucellosis in Hamadan Province, West of Iran: 2009–2015


1 Department of Epidemiology, School of Public Health, Hamadan University of Medical Sciences, Hamadan, Iran
2 Modeling of Non Communicable Diseases Research Center, Hamadan University of Medical Sciences, Hamadan, Iran
3 Neurology and Neurosciences Research Center, Qom University of Medical Sciences, Qom, Iran
4 Department of Community Medicine, School of Medicine, Zahedan University of Medical Sciences, Zahedan, Iran
5 Department of Epidemiology, Deputy of Health, Hamadan University of Medical Sciences, Hamadan, Iran
6 Department of Public Health, School of Public Health, Dezful University of Medical Sciences, Dezful, Iran
7 Centers for Communicable Disease Control and Prevention, Ministry of Health and Medical Education, Tehran, Iran
8 Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran

Date of Web Publication24-Sep-2018

Correspondence Address:
Shahrzad Nematollahi
Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/AIHB.AIHB_12_18

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  Abstract 


Background: Regarding the geographical variations of the disease and recent changes of ecosystems, this study aimed to investigate the geographic patterns and trends of human brucellosis in Hamadan province, west of Iran. Methods: This is a cross-sectional study on brucellosis cases recorded by provincial authorities of brucellosis national surveillance system at Hamadan University of Medical Sciences from March 2009 to March 2015. Cochran–Armitage test and Poisson regression models were used to obtain standardised rates for the disease. Results: Of all 9318 recorded cases, 62.83% were female and 81% were rural residents. The observed number of cases exceeded expected in almost all of the cities, except for Hamadan, Bahar and Asadabad. Conclusion: The clusters of disease occurrence were more evident in areas of the province with better agriculture coverage. This finding provides more in-depth clues to detect special transmission routes and would help mobilise educational and preventive measures for specific occupational groups.

Keywords: Hamadan province, human brucellosis, Iran, spatial analysis


How to cite this article:
Khazaei S, Karami M, Mohammadbeigi A, Ayubi E, Shojaeian M, Mansouri K, Amiri B, Nematollahi S. Spatio-Temporal analysis of brucellosis in Hamadan Province, West of Iran: 2009–2015. Adv Hum Biol 2018;8:145-9

How to cite this URL:
Khazaei S, Karami M, Mohammadbeigi A, Ayubi E, Shojaeian M, Mansouri K, Amiri B, Nematollahi S. Spatio-Temporal analysis of brucellosis in Hamadan Province, West of Iran: 2009–2015. Adv Hum Biol [serial online] 2018 [cited 2020 Oct 23];8:145-9. Available from: https://www.aihbonline.com/text.asp?2018/8/3/145/241920




  Introduction Top


 Brucellosis More Details is a re-emerging zoonotic disease regarded as a threat to public health and agriculture worldwide.[1] The disease is considered by the Food and Agriculture Organization, the World Health Organization and the Office International des Epizooties as one of the most widely spread zoonoses in the world.[2] Brucellosis is transmitted to humans through contact with animals or their products (e.g., consumption of unpasteurised dairy products);[3] it is also defined as an occupational hazard to persons engaged in certain professions (e.g., veterinarians, slaughterhouse workers and farmers).

Alongside being an important global health problem, brucellosis exists principally, and in particular, in the countries of Mediterranean region, including Iran. Despite the successful vaccination campaigns for animal brucellosis throughout the country, Iran has been an endemic area for human brucellosis for decades. Several studies in Iran have shown that the consumption of fresh cheese, animal husbandry, working in a laboratory and veterinary professions are the main risk factors for brucellosis infection.[4],[5],[6] Hamadan province located in the western part of the country is well known for the extensive prairies and livestock markets. With the production of 598,000 tons of livestock during 2016, the province ranked first in the western part and ninth throughout the country in terms of animal husbandry and livestock.[7] The transmission of human brucellosis in Hamadan province is reported to be mostly due to consumption of unpasteurised dairy products and contact with infected animals.[8]

Nonetheless, in the few past decades, the epidemiology of human brucellosis has evolved reflecting alterations in socioeconomic conditions, improved recognition and notification systems and establishment of on-going eradication programmes for animal brucellosis. New endemic foci are emerging, warranting eradication programmes beyond national boundaries.[9]

The idea of using geospatial analytical techniques and a geographic information system to study the spatial and temporal distribution of health events have been successful in uncovering factors related to the occurrence of disease.[10],[11] These methods are also used for human brucellosis recently. One recent study revealed the potential relationships between the incidence of human brucellosis cases in China and some environmental factors, including temperature, rainfall, hours of sunshine, relative humidity and average wind velocity.[12]

Published literature on brucellosis incorporating spatial analyses has identified areas of high human cases reporting associated with specific ethnic populations and the consumption of unpasteurised food products in USA,[13] Germany [14] and Azerbaijan.[15] Identifying the major risk factors for human brucellosis is crucial for reaching a comprehensive understanding of the nature of the disease and its transmission routes for eradication of human brucellosis. Therefore, the aim of the present study is to explore geographical factors related to the occurrence of human brucellosis in Hamadan province during 7 years of follow-up.


  Methods Top


Registered data on brucellosis cases by provincial authorities of brucellosis national surveillance system at Hamadan University of Medical Sciences from March 2009 to March 2015 were included. Hamadan province is located in the west of Iran, and based on the National Population and Housing Census, it had a population of 1,758,268 in 2011. The city of Hamadan is the centre for Hamadan province. It also consists of nine cities namely: Asadabad, Bahar, Hamadan, Famenin, Kabudarahang, Malayer, Nahavand, Tuyserkan and Razan.

The data for the study including gender, age group, location (urban/rural), city, time of diagnosis (year) and disease type (new vs. recurrent case) were extracted from patient's assessment forms filled out by health experts in health centres through interview as a routine task in the flow of data gathering by staff of brucellosis national surveillance system.

Based on the national guidelines for brucellosis,[16] a case of brucellosis is defined as the presence of confirmed clinical diagnosis based on compatible signs and symptoms, standard tube agglutination test ≥1:80 and 2-mercaptoethanol agglutination ≥40.

Cochran–Armitage test was used to investigate the changes in trend of disease according to aforementioned demographical and clinical characteristics. Poisson model was used to obtain standardised rates (per cities) for brucellosis. In any given population i, observed number of events depends on (1) area-specific relative risk (RRi) and (2) number of events with a Poisson distribution with mean of Ei RRi. Expected events are calculated as follows:



Where ni was the population of city i and yi was the observed number of events in city i. RRi was estimated as dividing the observed number of event in county by the expected event in that county. The Kulldorff scan statistic with a Poisson model was used to detect purely spatial clusters. These were done by gradually scanning windows across spaces and comparing the number of observed and expected observations inside the window at each location with outside the window. The maximum spatial cluster size was defined equal to 50% of the at-risk population. For each detected cluster, the likelihood ratio test (LRT) is defined as follows:



Where C is the total number of cases, c is the observed number of cases within window, and E[c] is crude expected number of cases within the window under the null hypothesis and C − E[c] is expected number of cases outside the window. The cluster with highest LRT and statistically significant will be defined as most likely cluster. Statistical significance of cluster is determined with Monte–Carlo hypothesis testing because the exact distribution of LRT is unknown.

For conducting the analyses, data were obtained on spatial coordinates (longitude and latitude) of cities, number of brucellosis cases and population size at that city.

Spatial analysis and temporal analysis were done by SaTScan ver 9.4 and STATA ver 12 (StataCorp, College Station, TX, USA), respectively. P ≤ 0.05 was considered as statistically significant.


  Results Top


During 7 years of the study (2009–2015), a total of 9318 cases of human brucellosis were registered in Hamadan province. [Table 1] shows the baseline characteristics of study participants. Among all the cases, 62.83% were female, while the relative frequency of females was constantly and significantly higher than males throughout the study period, with the lowest of 55% in 2009 to highest of 65% in 2013 (P for trend: <0.001).
Table 1: Background characteristics of brucellosis in Hamadan province, 2009-2015

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In addition, the highest proportion of the cases significantly belonged to the age group of 25–44 years, while this proportion was lowest in 2009 and highest in 2014 (29.4% and 38.3%, respectively, P for trend: <0.001). Overall, 81% of cases were rural residents and 93.5% were new cases. However, the proportion of the new cases changed significantly during the study period with a decrease to 84.9% in 2009 and an increase to 95.9% in 2015 (P for trend: <0.001).

A geographical disparity was observed across the province because of the variation of the spatial distribution of brucellosis at city level. Specifically, the ratio of observed to expected cases ranged from 0.17 in Hamadan city to 2.34 in Kaboudarahang city. In other words, the observed cases exceeded expected ones in almost all of the cities, except for Hamadan, Bahar and Asadabad [Figure 1]. We further found two distinct clusters in areas with high risk of the disease. First cluster was observed in cities such as Famenin, Kaboudarahang and Razan; with a overall population of 302,093, where the clusters of the cases were primary, the annual reported cases of brucellosis was 143.2 (per 100,000 population) and the ratio of observed versus expected cases was 1.94. The second pattern was observed in cities such as Malayer, Nahavand and Tuyserkan, where the overall population was 573,479 and the type of the clusters was secondary. The annual reported cases of the disease reached 2540.8 (per 100,000 population), and the ratio of observed versus expected cases was 1.41 [Table 2].
Figure 1: The observed by expected number (rate ratio) of brucellosis in Hamadan province (2009–2015).

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Table 2: High- and low-risk cluster for brucellosis by purely spatial scan statistics in Hamadan province (2009-2015)

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[Table 2] also shows three distinct clusters in the low-risk areas of brucellosis. First cluster was observed in Hamadan city with a total population of 651,821 and with primary clusters, the ratio of observed versus expected of 0.75, and the annual cases of 18.4 (per 100,000 population). Second cluster was observed in Asadabad with a population of 107,006 and all of the clusters being secondary. In the second cluster, the annual reported cases were 54.4 (per 100,000 population) and the ratio of observed versus expected was 0.74. Finally, the third cluster was observed in Bahar, with a population of 123,869, and the clusters being tertiary. The annual cases were 71.6, and the ratio of observed versus expected cases was 0.97.


  Discussion Top


Brucellosis is one of the most common global zoonotic diseases and is considered as an important public health problem in many developing countries.[17],[18] The spatial patterns of human brucellosis in different parts of the world have been explored abundantly;[15],[19] nevertheless, this study represents the first attempt to describe local (district level) patterns of the disease in Hamadan province.

Despite the paucity of evidence indicating higher risk of brucellosis in males,[8],[20],[21] we found females as more prone to the disease. We also found that the highest proportion of cases belonged to the productive ages of 22–44 years. Other studies consistently have shown that productive ages such as age range of 30–59 years have a higher risk of brucellosis, likely due to the participation in particular occupations or lifestyles.[22]

We also found that human brucellosis persists annually throughout much of the province, particularly the northern and south-eastern parts. This finding is affirming the notion of endemicity of the disease in the province.

Using spatial analysis, we were able to identify the spatial clusters of cumulative incidence in each geographic area, which illustrated the localised areas of high caseloads. Accordingly, primary clusters were predominantly reported from cities such as Famenin, Kabudarahang, Razan and the capital city Hamadan. On the other hand, secondary clusters were predominantly observed in cities such as Malayer, Nahavand, Tuyserkan and Asadabad. Since direct transmission of human brucellosis is highly unlikely, clustering of secondary cases could be as a result of common-source outbreaks or aggregation of time-space risk factors for the disease.[13] This notion is further supported by the results of a previous study in Hamadan province, indicating that the transmission of human brucellosis is mostly related to the consumption of unpasteurised dairy products and contact with infected animals.[8]

Cities with the highest adjusted incidence during the study period were those with a high degree of agricultural activity, suggesting the importance of traditional exposures, for instance, occupation.[13] Description of occupational features of human brucellosis would help explain why more cases are reported in the north (humid, grassland-dominant) regions near to Qazvin and Zanjan provinces than south (arid and semiarid, desert-dominant) regions near to Lorestan province.

We also found a lower ratio of observed versus expected cases in low-risk areas compared to high-risk ones. In other words, the number of reported cases in low-risk areas was lower than what the surveillance system had predicted. This finding can be explained by the nature of human brucellosis featured with a substantial portion of cases being misdiagnosed and underreported,[23] where low sensitivity of traditional serological tests such as Rose Bengal determines the extent of underreporting and misdiagnosis.[24] Noteworthy, those locations which predict high probability of the disease presence do not necessarily have a large number of reported cases in the past. They may just have suitable conditions and consequently the potential for future occurrences. Pilot surveillance programmes, therefore, should be launched to determine if any under reporting or reporting biases exists at those locations. Moreover, additional educational programmes specifically for the purpose of enhancing the awareness of workplace transmission of brucellosis for certain groups will be warranted.

We recommend that financial and scientific supports from international organisations should target the new endemic areas and become a global health priority. Enhanced surveillance of human brucellosis may be responsible for the alteration in the endemicity of the disease in certain countries, but this fact serves only to underline the magnitude of the problem and the possibility of it further inflating in the near future. The fact that the continuing existence of human (and animal) brucellosis extends beyond medical and veterinary duties and encompasses political factors goes to show that the eradication of the disease will not be an easy task for the future.[9]


  Conclusion Top


We found that human brucellosis was persistent during 7 years of study period as endemic disease in Hamadan province. The clusters of disease occurrence were more evident in areas of the province with better agriculture coverage. This finding provides more in-depth clues to detect special transmission routes and would help mobilise educational and preventive measures for specific occupational groups.

Acknowledgement

We would like to thank the Health staffs of Hamadan province for their kind collaboration.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Abdullayev R, Kracalik I, Ismayilova R, Ustun N, Talibzade A, Blackburn JK, et al. Analyzing the spatial and temporal distribution of human brucellosis in Azerbaijan (1995 – 2009) using spatial and spatio-temporal statistics. BMC Infect Dis 2012;12:185.  Back to cited text no. 15
    
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