Many species of freshwater snails have been reported as intermediate hosts of highly infective fluke (trematode) larvae of the genus Schistosoma which cause schistosomiasis also known as bilharziasis. Some ecological factors like water quality, temperature, microphytes, human settlements contribute significantly to the distribution of these intermediate hosts of Schistosoma. Freshwater snails are considered to be intermediate hosts because the sexual stages of the parasites are found in man while the snails harbour the asexual stage 1234. Humans serve as vectors by contaminating the environment. Transfer of the infection requires no direct contact between snails and humans, but rather, the disease is acquired by repeated contacts with freshwater snails 567. Therefore, researchers have shown that the prevalence and distribution of schistosomiasis are dependent on the presence and distribution of the intermediate snail host in natural freshwater bodies 891011.

Urinary schistosomiasis, caused by Schistosoma haematobium which has a suitable snail host species of aquatic freshwater Bulinus snails, is one of the neglected tropical diseases very common in Nigeria, with some States having very high prevalence especially in the southeastern part of the country 1213. Urinary schistosomiasis constitutes a major infection of public health importance in Nigeria and affects mainly the rural poor and some disadvantaged urban populations 141516. It is characterized by haematuria, dysuria, bladder wall pathology, hydronephrosis (swelling of kidney due to build-up of urine) , It can also present with squamous cell carcinoma 171819. In adults, the infection can result to genital ulcers and other types of lesions that can cause poor reproductive health with sexual dysfunction and infertility. These pathological effects occur mostly among school-age children, adolescent and young adults 202122232425.

Apart from reports on the prevalence of schistosomiasis, there is scarcity of research on the risk factors associated with this infection especially in the South East region of the country including Imo State 29303132. This lack of information on risk factors have been shown to constitute limitations in providing intervention and control programmes. This is because availability of such information is crucial in identifying and implementing effective control measures 333435. However, studies have observed that the complex schistosome life cycle and the intricate interactions that are encountered with the host immune system make it difficult for neither drugs nor vaccines to adequately control schistosomiasis without first identifying the parasite load. Studies have shown that in Nigeria, the main disease control measures are the provision of potable water, population-based chemotherapy and health education , but that most of these control measures are affected by lack of baseline data on the distribution of the host and its infectivity 3637383940. This study therefore, examined the distribution of snail species and their infectivity rates along freshwater canals in the rural poor areas of Imo State Nigeria.

This study is of interest because most individuals including school children living close to the study area use water from the canals for several domestic activities including swimming. Also, the snails from these canals serve as the main source of protein as well as means of raising money for sustenance. It is as a result of the activities of people living close to these canals that the researchers deemed it necessary to identify the snail species and their infectivity rates so as to protect the individuals resident in this rural area from likely infection that could result to disease conditions. This study is unique because it helped to reconcile the public health objective of eradicating deadly pathogens so as to protect their environmental biodiversity.

This study also aimed to contribute a quota in helping to achieve the World Health Assembly (WHA) endorsed resolution which has the vision of having a world free from schistosomiasis so as to eliminate schistosomiasis as a public health problem by the year 2025. If this goal is realized, millions will be spared from future sufferings from the pathological effects of this disease condition.

A total of one thousand four hundred and nine (1,409) snails was collected from drainage systems around the residential areas close to the Iyiechu Stream Okigwe. The snails so collected were identified as Lymneae and Bulinus genera.

Table 1. Shows the monthly distribution and the relative percentage of the freshwater snail species collected from Iyiechu Stream Okigwe, the area of study. From this table, there is a marked variation in the number of snails collected. In the first month of snail collection, (May), 619 snails were collected, in June 421 snails were collected while in July, 369 were collected. In May, the snails collected consisted of 318 (53.1%) Lymnaea, and 301(48.63%) Bulinus. This is the largest collection made during the period of study. For the second month, June, the snails collected were made up of 309 (73.4%) Lymnaea, 112(26.6%) Bulinus, while in July, being the third month, the snails collected were 249(67.5%) Lymnaea, and 120(32.5%). For all the collections made during the period of study, 876(62.2%) were of the Lymnaea and 533(37.8%) Bulinus genera respectively.

Table 2 contains the proportion of fresh water snails that shed cercariea showing that they were infected. From the table, out of 619 fresh water snails collected in May, 428 of them representing 69.2% were infected. In June, out of 421 snails collected, 211(50.1%) shed cercariea, while in July, out of 369 snails collected 124(33.6%) were also infected giving a total of 764(54.2%) that were infected.

Table 3 contains physico-chemical parameters of the analysis of the water canals from Iyiechu Stream where the snails studied were collected. From the physico-chemical analysis on this table, dissolved oxygen (DO), Biological oxygen demand (BOD) and P^H significantly influenced snail distribution in the area studied (p<0.05).

The fact that the snails collected were more in May and June when the rains are not yet heavy than the ones collected in July when rainfalls are heavy, shows that snails breed less during heavy rains than other periods. The result obtained from the study showed the abundance and diversity of freshwater snail intermediate hosts in the water canals studied. This abundance and diversity of freshwater snail hosts are of public health importance because of the risks to human infections. This finding corroborates with that of 118 that the presence of freshwater snails is of public health importance because of the indication for parasitic transmission.

The high proportion of cercariae shed in both Lymneae and Bulinus genera reflect the extent to which consumers of these snail species and those with regular contacts with the freshwater canals including school children are exposed to infections. The findings that individuals living in this vicinity use water from the canals for domestic activities means that they are likely to have regular contacts with the freshwater from the canals and therefore, human and animal excreta may be common in the freshwater where the snails grow. This finding agrees with those of 192033 in which they observed that freshwater body is usually polluted with human and animal excreta and that snails that grow in freshwater body polluted with human and animal excreta produce some organic matters that increase the concentration of detritus and the proliferation of algae which form the diets for planorbid and prosobranch snails.

The fact that some of the infected freshwater snails studied were of Bullins specie which serves as intermediate host to highly infective larval trematode of the genus Schistosoma shows that Schistosomiasis infection may be common in the study area especially among school children who were seen swimming in the canals near their school premises as well as picking snails for food. This regular human contacts with the canals presupposes the extent to which the surface water must have been contaminated by excreta and others.

The finding that majority of the freshwater snails that were infected were of the Lymmeae specie which according to studies by 28 is the chief intermediate host of Fasciola which predisposes human and animals to liver disease (fascioliasis), shows that some individuals resident in this area may be infected with fascioliasis.

Realizing the fact that snail is the main source of protein and avenue for generating revenue for sustenance among some residents in the area of study, presupposes that a significant proportion of the residents may have continuous contacts with the definitive host which would influence the occurrence of disease conditions like fascioliasis, schistosomiasis and others. Other human activities near the canal such as farming, bathing, swimming, washing of clothes, cassava and vehicles that were observed during the study, may be what contributed to the poor environmental conditions noted around the freshwater canals. These numerous human activities suggest constant contacts with parasites which occur on aquatic plants that serve as foods and shelter to freshwater snails thereby, increase their number and diversity. Also the fact that there is increase in the number and diversity of the freshwater snails make them available even on semidried soils where they are easily picked by individuals. One striking observation in the study is that a good number of individuals picking these snails move barefooted, thereby add to the predisposing factors to schistosoma and Fasciola infections as also pointed out in the researches done by 610.

The finding that dissolved oxygen (DO), Biological oxygen demand (BOD) and P^H significantly influenced snail distribution in the area shows that the presence of snails increased the biomass of filamentous green algae and decrease the biomass of periphyton thereby reducing the DO of the water which is needed by aquatic organisms for existence. This finding agrees with that of 1125.

This shows that the more the presence of snails, the more the biomass of periphyton is reduced.

Freshwater snail intermediate hosts of trematodes were common in the freshwater canal studied. Inhabitants in the area depended largely on water from the canal for several domestic activities probably because water from most of the bore holes were sold and as a result, it may not be easy considering the economic status of most individuals in the rural areas, to have the resources to buy water for drinking as well as for other domestic activities. As a result, a good number of those who cannot afford the money, resort to using water from the canals. It is evident that the numerous activities that occurred near the canals contributed to the poor environmental sanitation which could facilitate infection of snails and the continuation of their life cycles.

Since most people in this area consume some of these snails as a source of protein as well as market them for sustenance, and realizing that the infective larvae (cercariae) develop within freshwater snails which when released penetrate the skin of a human/mammalian body in contact with the water, it may be right to conclude that a good number of the individuals who have contacts with the freshwater canal and the snails that grow here may be infected with schistomiasis and fascioliasis. Therefore, there is the need to health educate residents in this area on the need for hygienic practices, provision of potable water, less contact with freshwater canals, and to avoid the consumption of freshwater snails so as to prevent infections associated with freshwater snails and their attendant pathological effects on humans.

Therefore, the findings in this study will contribute a quota to support the WHA resolution to maintain a world free of schistosomiasis by controlling its morbidity and eliminating schistosomiasis as a public health problem by 2025.