1. Introduction
Schistosomiasis is a parasitic disease caused by blood flukes (trematode) of the genus
Schistosoma. Schistosomiasis one of the neglected tropical diseases (NTDs) targeted for elimination by World Health Organization [
1]. Today, at least 251.4 million people worldwide required preventive chemotherapy (or mass drug administration: MDA) in 2021 [
2].
Schistosoma mekongi is endemic along the Mekong River basin in Champasak Province, southern part of the Lao People’s Democratic Republic (Lao PDR) and Kratié Province, northern part of Cambodia. In the Lao PDR, two Districts: Khong (152 villages: 86,095 people) and Mounlapamok (50 villages: 37,063 people), are the endemic areas of
S. mekongi [
3,
4]. Significant progress has been made in the past decades to reduce the prevalence of schistosomiasis mekongi through preventive chemotherapy using praziquantel and community awareness programs or health educations [
5]. According to the results of the recent reports on schistosomiasis mekongi, the prevalence of human schistosomiasis ranged from 0.0% to 5.0% without heavy infection in sentinel site villages and endemic communities using Kato-Katz thick smear method for parasite egg detection in stool samples [
5,
6]. Although the prevalence of human schistosomiasis has been relatively well monitored and gradually declining over the past decades in the Lao PDR, the prevalence among potential domestic reservoir animals in the endemic areas has not been well studied. A few studies with limited sample size showed that 14.7% (10/68) of dogs in Don Khone and Don Som in 2011-2012 [
7] and 12.2% (12/98) of pigs in Had Xay Khoun village in 1999 were infected with
S. mekongi [
8]. Previous studies report that cats, buffaloes, and cattle were not infected with
S. mekongi in Don Khone and Don Som. However, it is important to note that these studies used solely the traditional diagnostic method, e.g., microscopy. Therefore, it is possible that light intensity infections could be missed due to the relatively low sensitivity of the tests.
In 2017, WHO adopted a new strategy that accelerates the elimination of Asian schistosomiasis in the Western Pacific Region, i.e., transmission interruption by 2025 and verifying elimination by 2030 [
1]. One of the criteria of transmission interruption is “no new case of animal infection.” Toward this goal, we conducted a cross-sectional study to determine the infection status among the potential domestic reservoir animals with adequate sample size and high sensitivity diagnostic methods in the endemic villages in Khong and Mounlapamok Districts, Champasak Province, southern Lao PDR.
4. Discussions
In the present study, only one dog and one pig stool sample detected S. mekongi DNA by the LAMP test. These dog and pig stool samples were considered confirmed and suspected positive, respectively, due to the varied results for the pig sample LAMP test. Microscopic examination using the FECT method showed negative results with the same stool samples. These different results suggested that the LAMP test is more sensitive than the FECT. All stool samples from other domestic animals (cattle, buffalo, and goat) were negative for S. mekongi by both the LAMP test and the FECT.
Our previous study between October 2011 and August 2012 showed that 14.7% of dogs (10/68) were positive for
S. mekongi eggs using the FECT method in two islands (Donkhone, 15.9%, and Donsom, 12.5%). The prevalence of schistosomiasis among the villagers was as high as 23.6% (112/475) in Donkhon and 21.0% (109/519) in Donsom, with heavy-intensity infections of 1.8% (2/112) and 5.5% (6/109), respectively [
7]. It is important to note that the settings of our present study geographically overlap with a previous study conducted in 2011 and 2012. Khone village is in Donkhone (Khone island), and Thamakhep and Somvenok villages are in Donsom (Som Island). In the previous study, the prevalence of schistosomiasis among the villagers was as high as 23.6% (112/475) in Donkhon and 21.0% (109/519) in Donsom, with heavy-intensity infections of 1.8% (2/112) and 5.5% (6/109), respectively[
7]. The most recent study showed a much lower prevalence of schistosomiasis among villagers in Khone, Thamakhep, and Somven-Ork at 7.3%, 6.0%, and 4.5%, respectively, without a heavy intensity [
5]. The low prevalence of
S. mekongi infection in animals observed in this study may be associated with the significant reduction of schistosomiasis in these endemic communities.
In addition, 88.2% of dogs (127/144) in the present study were less than 3 years old (average 2.2 years old), whereas the S. mekongi positive dog was 4 years old. Although the age of the dogs may not be accurate because there is no official age documentation, this result suggests that S. mekongi transmission among dogs has either not occurred or has been limited since 2015 in the study areas.
Our previous study conducted in 2011–2012, the stool samples of cats (n = 64), pigs (n = 105), and buffaloes (n = 94) were also examined using the FECT method and were all negative for
S. mekongi eggs. However, another study conducted in Hadxaykhoun village, Khong District, in 1999 found a positive rate for
S. mekongi eggs in 12.2% (12/98) of pigs [
8]. Strandgaard et al. suspected that the most likely route of pig infection was ingestion of cercariae-infested drinking water that the owners brought directly from the Mekong River to feed their pigs daily. At that time (1999), most pigs were normally tied up or kept in pens, but occasionally they were allowed to roam freely. During our study, we observed a similar manner of keeping pigs to 1999 in Khong district. However, the extensive intervention conducted over the past decades has reduced the prevalence of
S. mekongi in the endemic areas to a very low level, with no severe cases observed in the community [
5]. In addition, we observed that most of the households in the endemic communities pumped the water from the Mekong River and stored it in a tank or well for hours before using it. Previous studies suggested that
Schistosoma cercariae can remain infective in freshwater for one to three days [
12], depending on the water temperature [
13,
14]. If the owner of the pigs gave the storage water (stored for more than three days) to their pigs, the risk of pig infection with
S. mekongi would be lower compared with previous practices. Moreover, the age of the pigs in this study was relatively young, with an average of 5 months (ranging from 1 month to 2 years old). In fact, the study team did not see any older pigs (>2 years old) in this study. This relatively high turnover of the pig population would decrease the chance of
S. mekongi infection in the study areas.
In the present surveys, we observed that many dogs took baths on the Mekong River side (
Figure 3), while no pigs came to the Mekong River as they were normally kept in pens. Buffaloes like to stay in the water for a long time. Cattle and goats normally come to the Mekong River's riverside only when they want to drink water.
S. mekongi is genetically and morphologically close to
S. japonicum, which has a wide variety of mammal hosts, such as cattle, buffalo, pigs, dogs, rats, and so on [
3,
15]. In contrast,
S. mekongi has only two mammalian hosts as natural reservoirs, namely, dogs and pigs in the endemic areas [
7,
8]. Rats are a mammalian host in a laboratory setting [
16] and buffaloes are suspected as a potential reservoir animal [
3], but this has yet to be proven. The present study and the accumulated data from previous studies suggest that only dogs and pigs are the natural reservoir animals for
S. mekongi. Rats were not examined in this study because they are not domestic animals and it is challenging to collect them. Therefore, it remains unknown whether rats would contribute to the transmission of
S. mekongi in the endemic areas.
Author Contributions
Conceptualization, investigation, methodology, formal analysis, validation, writing – original draft, S.S, P.K and M.I; investigation, methodology, S.K, P.P, PS, S.S (Sonesimmaly Sannikone), TK, SP, PI, and BH; writing – review & editing, P.T.B; funding acquisition S.K (Shigeyuki Kano), M.I, Writing – review & editing, S.K (Shigeyuki Kano).
Acknowledgments
The authors sincerely thank the staffs of the Center of Malariology, Parasitology, and Entomology, Ministry of Health, Vientiane Capital, Lao PDR, the Champasak Provincial Health Department, the Khong District Health Department, and the Mounlapamok District Health Department. We also thank the staff of the Animal Health Service Center, the Khong District Agriculture and Forestry Office, the Mounlapamok District Agriculture and Forestry Office, and the Champasack Provincial Department of Agriculture and Forestry. We are also grateful to the chiefs of the study villages, village health volunteers, village animal health volunteers, and all the owners of the animals for their support and contributions to this study. We are grateful for the English correction by Ms. Miley Sinantha-Hu, a native English speaker and research assistant at the Lao Tropical and Public Health Institute.