First of all, it is important to define what it means exactly to have blood in the stool. In fact, there are mainly two possible situations susceptible to occur. In the first scenario, fresh red blood is coming out of the body in small quantity or in smear. It can come out while defecating and be present in the toilet afterwards or it can be present on the toilet paper after wiping. This fresh red blood is usually coming from the anus and can be caused by an anal fissure or coming from a lower digestive part affections, such as haemorrhoids or diverticulitis. Parasitic diseases can also cause it if the lower digestive tract is affected. The second scenario involved black and tarry stools meaning that the source of the blood comes from the higher digestive tract. This type of bleeding can be caused by a lot of different conditions, ranging from ulcers to cancer, and is though not restricted to parasitic diseases. It is then very important to consult a doctor to find what exactly is the cause of this internal bleeding, as it can range from a common mild cause to a life-threatening disease.
Even though they are not considered as a main cause of the presence of blood in the stool, some intestinal parasites are more susceptible to cause this symptom. For example, one of the known symptoms of amoebic dysentery, a severe form of the disease caused by the protozoan parasites Entamoeba histolytica, is indeed the presence of blood in the stool. Furthermore, the helminth Schistosoma is also known to cause the presence of blood in the stool or in the urine. In fact, all intestinal helminths are susceptible, if the worms damage the intestinal walls at some point, to cause the sporadic presence of blood in the stool. In certain cases, it is also possible that the bleeding is so small that it is impossible to detect it with the naked eyes. However, blood detection kit can be used to assess the presence of blood in the stool and reveal potential intestinal damages.
Aside from the presence of blood in the stool, some parasites are susceptible to specifically modify the stool. This is the case of the infection with the protozoan parasites Giardia intestinalis where it is common to experience pale or yellow diarrhoea, as well as the potential presence of blood in the stool.
Finally, it is interesting to note that some food can alter the color of your stool. For example, eating beetroots, blueberries and black pudding, can make you having black stool (or sometimes red in the case of beetroots) within the next 12 to 24 hours. Furthermore, taking iron supplements can also make you having black stool. Consequently, it is interesting to question ourselves about what we ate before jumping to quickly to the conclusion that reddish or black stool necessary involve the presence of blood in it.