34.4%. Similar to roundworm, the rate of whipworm infection is very high in the North, ranging from 38.1-50.5% but very low in the South, the rate is only from 0.1-3.8%. In contrast, the Central and Southern regions have a very high rate of hookworm infection/beak. In the Central region, the average hookworm infection/beak is 47.8% and in the South from 35.9%- 41.1% [1], [7].
Recent survey data on children under 60 months in the North also showed a similar infection rate structure to our results.
According to Nguyen Thu Huong (2015), in Ha Giang, in children aged 12-60 months, the rate of roundworm infection was 26.3%, whipworm infection was 16.1% and only 1.0% was infected with hookworm. In Thanh Hoa, roundworm infection was 11.3%, whipworm infection was 10.9% and there were no cases of hookworm infection [9].
According to Nguyen Phuong Huyen's (2016) investigation in Hanoi, the worm infection rate in two districts of Phuc Tho and Ba Vi was 8.15%, of which roundworm infection was 5.16%, hookworm infection was 2.16% and whipworm infection was 1.5% [43].
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Nguyen Thu Huong (2015), the infection rates of roundworm, whipworm, and hookworm in children 12-60 months old in Lai Chau were 20.8%, 3.6%, and 0.7%, respectively [44].
According to Nguyen Hoang Linh Chi (2011), in children aged 12-23 months in Quang Tri, the overall infection rate was 26.0%, roundworm, whipworm and hookworm were 20.5%, 6.5% and 4.5%, respectively [45].
According to Chu Trong Trang (2014), the infection rate of roundworm, whipworm, hookworm/beakworm in children under 5 years old in Nghe An is very high, respectively 87.3%, 21.4% and 6.9% [49].
Nguyen Luong Tinh (2018) tested 304 children aged 12-60 months in Lao Cai and found that the infection rate of roundworm was 39.1%, whipworm was 41.8% and hookworm was 3.0% [91].
On the contrary, investigations in the Central region showed that the rate of hookworm infection in children was higher, as in the investigation by Le Huu Tho (2014), in primary school students in Ninh Hoa district, Khanh Hoa province, the rate of roundworm infection was 15.1%, hookworm infection was 12.9%, and there were no children infected with whipworm [94].
According to Bui Khac Hung (2018), the rate of worm infection in primary school students in Dak Lak is 25.1%, mainly hookworm infection 22.8%, followed by roundworm 2.0% and the lowest is whipworm 0.3% [95].
Our study showed that there was no difference in the rate of worm infection between the two sexes. The rate of worm infection between boys was 22.6% and that of girls was 25.0%. This is also the general opinion of many authors at home and abroad.
According to Van der Hoek (2009), a synthesis of studies on GTQD in Vietnam also showed that the infection rates of roundworms, whipworms, and hookworms are similar in both sexes [7].
According to Nguyen Thu Huong (2015), in the group of children aged 12-60 months in Lai Chau, the rate of worm infection in boys was 21.8%, in girls 25.0%. There was no difference in this rate between the two groups [44].
According to Le Huu Tho (2014), the rate of worm infection in boys is 13.9%, in girls is 11.7%. Thus, the rate of GTQD infection in both sexes is the same [94].
In the study of Nguyen Thu Huong in Ha Giang and Thanh Hoa, the study of Nguyen Luong Tinh in Lao Cai also showed that the rate of worm infection in boys and girls was not different [9], [91]. Author Bui Khac Hung studied the situation of GTQD infection in primary school students in Dak Lak in 2016 and also found no difference in the rate of worm infection in the two sexes [95]. In the study of Lahiru Galgamuwa (2018) in children aged 1-12 in Sri Lanka, the infection rate in boys and girls was similar [97].
Our study results show that the rate of infection with worm in children aged 12-17 months is 19.6%, in children aged 18-23 months is 27.5%. The difference is statistically significant with p<0.05. This can be completely explained because the older the age, the more active the child is, the more likely they are to be exposed and have more behaviors or activities that are at risk of infection with worms.
According to Van der Hoek (2009), a synthesis of studies on worm infection in Vietnam also shows that the infection rate increases with age. Accordingly, the infection rate of worms
Chopsticks, hair, hooks/beaks in children under 5 years old were 43.7%, 16.1% and 5.7%, respectively. These infection rates in the 6-15 year old group were 62.6%, 26% and 24.6%, and in people > 15 years old were 57.1%, 55.3% and 34.5% [7].
In Ha Giang, Nguyen Thu Huong showed that the highest rate of GTQD infection was in the group of children over 60 months and the lowest was in the group of 24-36 months (47.1% and 20.0%) [9]. Also according to Nguyen Thu Huong in Tam Duong district (Lai Chau), the group of 12-month-old children had the lowest rate of worm infection at 8.9%, the group of 13-24-month-old children had the rate of infection at 20.2%. The infection rate increased gradually with age and the group from 49-60 months of age had the highest rate of worm infection at 31.7% [44].
Nguyen Hoang Linh Chi (2011), tested 692 children and showed that the 24-36 month old group had a 35.5% rate of STD infection while the rate in the 12-23 month group was 26.0% [45].
In 2013, Tran Thi Lan in Quang Tri tested stool using the Kato-Katz method for 680 children aged 12-36 months. The rate of worm infection in children aged 12-23 months was 27.0% while the rate of worm infection in the group of children aged 24-36 months was 36.3% [47], [48].
In a study by Joseph (2015) in Peru, 1760 children aged 12-14 months were tested for a prevalence of 14.5%. After 18 months of retesting these children, the prevalence of worm infection had increased to 28.5% [33].
Ojja in Uganda (2018) investigated 562 children aged 1-5 years, of which the 1-2 year old group had a worm infection rate of 17.3-18%. The infection rate and intensity of worm infection increased with age, of which the 5 year old group had the highest infection rate, 36.4% (p<0.05) [98].
4.2.1.2. Intensity of soil-transmitted helminth infection
This study showed that children aged 12-23 months were mainly infected with one type of worm with low infection intensity.
The prevalence of mild roundworm infection was 74.7%, moderate infection was 21.4% and only 3.9% had severe intensity.
Up to 98.6% of cases of trichinosis had mild intensity and 100% of cases of hookworm/thrush had mild intensity.
This assessment also coincides with the general assessment of many domestic and foreign authors. However, depending on the characteristics of each locality, there are certain differences.
According to Nguyen Phuong Huyen's (2016) investigation in Hanoi, the worm infection rate in two districts of Phuc Tho and Ba Vi was 8.15%, of which roundworm infection was 5.16%, hookworm infection was 2.16% and whipworm infection was 1.5%. All children were infected with worms at a mild level [43].
In Ha Giang, the rate of roundworm infection in children aged 12-60 months was 26.3%, whipworm infection was 16.1% and only 1.0% was infected with hookworm. Up to 55% of cases of roundworm infection were moderate and 2.5% were severely infected. There were 91.8% of cases of whipworm infection with mild intensity and 8.2% of cases of moderate intensity. In Thanh Hoa, the author also reported up to 97% of cases of roundworm infection with mild intensity and 2.9% of cases of roundworm infection with moderate intensity. All cases of whipworm and hookworm infection were mild intensity. [9]. Similarly, the rates of roundworm, whipworm and hookworm infection in children aged 12-60 months in Lai Chau were 20.8%, 3.6% and 0.7%, respectively. Among the roundworm infections, 62.5% were mild and 90.9% were mild, with no cases of severe infection [44].
According to Nguyen Hoang Linh Chi (2011), in children aged 12-23 months in Quang Tri, the overall infection rate was 26.0%. The infection rates of roundworm, whipworm and hookworm/beak were 20.5%, 6.5% and 4.5% respectively. In this study, the author showed that the infection rate of hookworm/beak was higher than in our study. At the same time, the rate of moderate and severe roundworm infections was 53.9%, much higher than in our study [45].
Nguyen Luong Tinh (2018) tested 304 children aged 12-60 months in Lao Cai and found that the average rate of roundworm infection was 24.4%, and severe infection was 2.5%. All cases of whipworm and hookworm infection were at a mild intensity level [91].
In Indonesia, Novianty (2018) investigated children aged 1-5 years and found that the rate of roundworm infection was 28.8%, of which 84.7% were mildly infected and 15.3% were moderately infected [96].
In the study of Lahiru Galgamuwa (2018), the rate of roundworm infection in children aged 1-6 years was 38.8%, of which 50% of cases had mild infection and 50% had moderate or severe infection [97].
Among the three districts, Tuan Giao is the district with the highest infection rate and density of all three types of worms.
The average density of roundworm eggs in the three districts ranged from 111-4508 eggs/gram of feces, with Tuan Giao being the highest and Van Yen being the lowest.
The density of whipworm eggs at all three research sites was quite low, ranging from 1.0 - 19.1 eggs/gram of feces, of which Tuan Giao was the highest. The density of hookworm eggs/beak was very low in both Tuan Giao and Meo Vac.
4.2.1.3 Mono- and poly-worm infections in children 12-23 months
Our study found that children aged 12-23 months are still young and are often infected with one type of worm, and rarely with a combination of two or more types of worms. Of the 294 cases of worm infection, 85.4% of children were infected with one type of worm. The number of children infected with a combination of two or more types of worms accounted for 13.6%. Of these, only 3 cases were infected with all three types of worms at the same time, accounting for 1.0%.
According to Tran Thi Lan in Quang Tri in 2013, the investigation of children 12-36 months old showed that the rate of single infection was 82.3%, the rate of multiple infection was 17.7% [47], [48].
Novianty (2018) investigated 1-5 year old children in Indonesia and found that the rate of mono-infection was 90%, and the rate of mixed infection with 2 types of worms was 10% [94].
Compared with some other authors, the rate of multiple infections in our study was lower due to our study subjects.
is a young age group. According to Nguyen Thu Huong in Ha Giang in 2015, the number of cases of mono-infection with helminthiasis in children aged 12-60 months accounted for 60.6%, the rate of infection with two types of worms was 38.3% and there was only 1 case of infection with three types of worms, accounting for 1.1% [9]. Also in children aged 12-60 months, Nguyen Luong Tinh also reported the rate of infection with at least one type of worm in Lao Cai was 68.6%, the rate of infection with two or more types was 31.4% [91]. This may be due to differences in the age groups of the subjects. The subjects in our study were children aged 12-23 months, while other authors were in the older age group of 12-60 months. In addition, the difference in research locations is also an important factor leading to differences in the rate and structure of helminthiasis infection. In this study, we found that the density of infection with different types of worms varied depending on the research location. The points with high infection rates also had high infection densities. The average roundworm egg density in the three districts ranged from 111-4508 eggs/gram. The whipworm egg density in all three study points was quite low, ranging from 1.0 - 19.1 eggs/gram of feces, of which the highest was in Tuan Giao corresponding to the highest infection rate of 32% and the lowest was in Van Yen corresponding to the rate of
lowest worm infection 16%.
Nguyen Phuong Huyen (2016) in a survey in the suburbs of Noi reported that the average number of eggs of roundworm, whipworm, and hookworm in Phuc Tho was 24, 249, and 48 eggs/gram, respectively; in Ba Vi it was 213, 375, and 135 eggs/gram. This difference can be completely explained because our research location is in the northern mountainous region where GTQD is still prevalent with a very high infection rate [43].
According to a survey by the Central Institute of Malaria - Parasitology - Entomology in Quang Ninh in 2018-2019, the average egg density of roundworms in primary school students was 201,562 eggs/gram, and that of whipworms was 13,737 eggs/gram [83].
In Lao Cai in 2018, Nguyen Luong Tinh reported the average infection density in children aged 12-60 months for roundworms was 7604 eggs/gram of stool, for whipworms was 330 eggs/gram of stool and for hookworms was 544 eggs/gram of stool [91].
In Peru, Bluin (2018) evaluated 880 children aged 12-14 months with a 12.4% infection rate of at least one type of Ascaris, with an average egg density of 329.9 eggs/gram. After 12 months of testing, the average number of eggs increased to 2303.2 eggs/gram of stool, and after 5 years, the number was 6157.3 eggs/gram of stool [99].
Thus, it can be seen that the infection rate, infection intensity and worm infection density vary in each epidemiological area and tend to increase with age because children are exposed to risk factors as well as have many behaviors that lead to increased risk of infection with soil-transmitted helminths.
4.1.3. Some factors related to soil-transmitted helminth infection in children 12-23 months old at the study site
Because the group of children in the study was young, we determined factors related to worm infection in children through interviews with parents and caregivers.
Factors related to STD infection depend on the subject, study location and type of STD.
Children aged 18-23 months have a 1.65 times higher risk of worm infection than children aged 12-17 months. Our research results are also consistent with reports by domestic and foreign authors showing that the older the child, the higher the rate of worm infection tends to be because children are more active and have a higher risk of exposure to sources of worm infection.
According to Teha Shumbej (2015), in Ethiopia, the rate of worm infection in the 12-23 month group was 14%, while the rate of infection in the 24-35 month group was 21.7%, and the 36-47 month group was 33.0%. Compared to the 12-23 month group, the 24-35 month group had a 1.3 times higher risk of worm infection, while the 36-47 month group had a 2.5 times higher risk of worm infection [34].
According to Kirwan, the group of children from 12-17 months had a 2.18 times higher risk of worm infection, the group of children from 18-24 months had a 2.52 times higher risk of worm infection than the group of children from 7-11 months [35].
Do Thuy Trang (2007) in a study in a suburban commune of Hanoi, 620 adults (>15 years old) and 187 children from 0-72 months were tested for stool and interviewed. The author pointed out that the age group >15 years old had a 2.92 times higher risk of infection with GTQD than the group of children under 6 years old, in particular, the risk of hookworm infection in people >15 years old was 22.26 times higher than in children <6 years old [54].
Similarly, Susanna Nery (2019) showed that the older the age group, the higher the risk of hookworm/helminth infection, while factors that reduce the overall risk of hookworm infection include hand washing before eating (OR= 0.8) and clean water source (OR= 0.74) [100]
Aya (2007) in a study in Hoa Binh showed that the infection rate and hookworm infection density/beak were closely related to age group. The older the age, the higher the hookworm infection rate/beak. In addition, the author also showed that farmers had a 2 times higher risk of hookworm infection, and the intensity of infection was 3 times higher than that of non-farmers [53]. This is also a reason to explain why the hookworm infection rate/beak in children 12-23 months old in our study was so low.
The risk of worm infection in children in the group of parents with inadequate knowledge about STDs is 1.72 times higher than that of the group of parents with adequate knowledge. When parents have knowledge about STDs and how to prevent them, they will follow hygiene rules for themselves, their families and their children. Therefore, the risk of worm infection will be reduced.
Our research results are consistent with Nguyen Luong Tinh's 2018 study in Lao Cai, the author also showed that the group of children with mothers with poor knowledge had a 2.07 times higher risk of worm infection than the group of children with mothers with average or higher knowledge [91].