S. japonicum mainly parasitizes the mesenteric vein and portal vein of the host. Reproductive success was investigated approximately 24 days after the host was infected with cercariae. Schistosoma eggs are mainly deposited in the liver and intestines, with only a small amount deposited in the mesenteric lymph nodes, pulmonary system, spleen and central nervous system [15, 16]. The S. japonicum eggs deposited in the intestinal wall can cause inflammation in the tissues around the eggs, leading to ulceration and rupture of the intestinal wall. The Schistosoma eggs enter the intestinal cavity from the ruptured part of the intestinal wall and are excreted in the host’s faeces. Therefore, eggs in the faeces can be detected to determine whether the host is infected with S. japonicum. The aetiology of livestock schistosomiasis is determined mainly by examining the miracidia hatched from the eggs in faecal samples. A certain number of faecal samples are collected and processed with the conventional incubation method; then, the samples are screened to remove factors that influence the results of the hatching method, such as related residues or pigments in the faecal samples. After hatching under specific conditions, the presence of infection and the degree of infection can be determined by observing the hatched miracidia.
Aetiological diagnosis of S. japonicum in domestic animals is important in the diagnosis of schistosomiasis, and the faecal hatching method is the basic and common method for aetiological diagnosis[17, 18]. Some faecal samples from domestic animals, especially herbivorous livestock such as rabbits and goats, have special characteristics. The small intestine of domestic animals exhibits peristaltic movement, namely, segmented movement, which is mainly accomplished by circular muscle relaxation and contraction. After the absorption of water by the colon and rectum, a spherical faecal sample is formed. In the field of pathogen detection, there are differing opinions on the distribution of S. japonicum eggs in faeces[19–23]. Therefore, the challenge of how to analyse spherical faecal pellets has also become a controversial problem. The controversy revolves around whether it is necessary to crush the faecal pellet and incubate it. Some people think that crushing the pellet is required because eggs need to fully contact the hatching water to successfully hatch; however, others do not recommend crushing goat faeces, as there are many disadvantages after soaking. First, it increases the operation time. Second, it causes some eggs to be lost or hatch before bottling. Third, it increases the influence of faecal impurities in the hatching water, and fourth, it increases the turbidity of the hatching water, which affects the observation of miracidia.
The results showed that there was no significant difference in the hatching rate among 10 to 80 direct-incubated faecal pellets and the same amount of crushed faeces. However, the number of miracidia detected by the direct incubation method was higher than that detected by the conventional faecal hatching method.
An artificially infected S. japonicum animal model were employed to investigate the possible causes. First, we compared the distribution of deposited eggs of the Chinese mainland S. japonicum strain in the intestinal wall in rabbits at 42 and 60 days post-infection. The data showed that although the deposited eggs were distributed throughout the intestinal wall of the host, there were certain changes over time. The density of the eggs in the rectum was the highest, followed by the terminal tissues in the lower colon and caecum. Taking into account the size of each tissue segment, we calculated the proportion of eggs in each section of the intestinal wall and the egg distribution percentages according to the EPG results. The number of eggs deposited in the small intestine remained low during the course of the disease. The rectum and caecum accounted for more than 60% of the total eggs in the intestinal wall, especially the total number of eggs in the rectum, which accounted for 65% of the total eggs in the intestinal wall at 60 d post-infection. The faecal pellet is formed in the rectum, and the function of the rectum is mainly to absorb water. The eggs in the intestinal wall are rarely transferred to the faecal ball. It can be inferred that the eggs in the intestinal wall enter the faecal sample mainly in the large intestine, and the eggs in the rectum may enter the intestinal cavity when the intestinal wall is ruptured. They are mainly distributed in the periphery of the faecal sphere during the process of peristalsis and movement through the intestinal tract. During the necropsy of rabbits at 60 d post-infection, we also found that injuries to the rectal wall were more serious than those to the small intestine wall. Erosion, ulceration, irregular haemorrhage in the rectum, paving stone-like changes in the rectum and typical yellowish-grey nodules of S. japonicum eggs were observed. The faecal pellet can also cause mechanical friction, which leads to bleeding and allows eggs to enter the intestinal cavity.
According to the investigation of the distribution of schistosome eggs in the intestines of experimental rabbits, it was found that the egg density in the large intestine, especially the rectum, was the highest and the pathological damage was the most severe. The eggs in each segment of the intestine enter the faeces via the effect of intestinal peristalsis on chyme and digestive juice mixing. However, the spherical shape of the faeces is formed in the large intestine, especially the rectum; therefore, the eggs in the blood vessels or nodules are unlikely to be transferred to the inside of the faecal pellet; rather, they remain on the periphery of the faecal pellet. Considering the substantial total number of eggs in the rectal segment, it is concluded that it is feasible to hatch eggs in the faeces directly in water.
To observe the distributions of eggs in the faecal pellets, we used the section technique. The results showed that the distribution of eggs in a faecal pellet was consistent with that the distribution of eggs in the intestinal wall. There was no significant difference between the number of eggs the periphery and interior of the faecal pellet. Section observation revealed that although the surface of the spherical faecal pellet was smooth and uniform, the interior of the faecal pellet was loose and porous. It is speculated that during the incubation process, water easily permeates the interior of the faecal pellet so that eggs inside the faecal pellet can hatch successfully and the miracidia can emerge into the water.
It was demonstrated that the direct hatching method can be used for aetiological detection of schistosomiasis in spherical faecal samples. This method reduces the number of operational steps, such as mixing and crushing the faecal samples, and avoids the loss of eggs in the operational process. Moreover, the contamination of hatching water by pigments or toxins released after crushing the faecal samples can also be avoided with this method; this improves the observation of miracidia and thus has a prominent effect on detection. Therefore, this method is suitable for the qualitative detection of goat schistosomiasis on-site.