Parasitic diseases can cause a wide range of symptoms. The severity of the symptoms varies a lot from a person to another. It can range from mild discomfort to imminent death by organ failures and it usually correlates with the gravity of the infection. However, fatigue and general malaise are common in parasitic diseases. Depending of the parasite species involved, infected person can also present a lot of nonspecific symptoms including for instance fever, coughing, headaches, joint pain, hair loss, teeth grinding while sleeping, and so on. These symptoms are one the reasons why parasitic diseases can be easily misdiagnosed as something else. Furthermore, especially in underdeveloped countries, parasitic diseases can coexist within the same patient with other infections that are also the cause of other symptoms.
More specific symptoms depend on the localisation of parasites within the body, meaning that these symptoms will be specific to the infected organs. As an example, intestinal parasites, including the majority of helminths, can cause a wide range of gastrointestinal symptoms, such as abdominal pain, bowel obstructions, diarrhoea and vomiting. These symptoms can be easily confused with other gastrointestinal illnesses or infections. However, these parasites generally cause a striking increase in appetite associated with weight loss. This phenomenon is caused by the parasites that feed themselves with the food ingested by the host meaning that the infected person eats a lot to feed the parasites, not himself. Consequently, a long-term untreated infection by these types of parasites can also cause symptoms of malnutrition by competing with the host for the ingested nutrients. For this reason, parasites are one of the main causes of physical and intellectual development impairments in underdeveloped countries leading a lot of children to learning problems.
Other symptoms of parasitic infections can be almost anything depending on the affected organs. Here are only few examples of these specific types of symptoms. For instance, some parasites, like Leishmania, can cause skin or mucosal (primarily mouth and nose) ulcers. The nematodes Onchocerca volvulus cause an inflammation of the cornea and is therefore the second most common cause of blindness due to an infectious agent. In some cases, such as infection with Leishmania or Plasmodium, hepatosplenomegaly, a condition referring to the simultaneous volume augmentation of the liver and the spleen, can occur. This symptom can also be seen in case of important uncontrolled infestation with the trematodes Schistosomia or some nematodes of the Filarioidea type. This is an example of symptoms that can occur at a latter stage in the illness. Some parasites, such as the protozoan. Trypanosoma brucei can eventually cross the blood-brain barrier, which usually prevent pathogens to infect the brain, and invade the central nervous system. This phenomenon cause a wide range of nervous system’s symptoms, such as confusion, limb’s paralysis, muscle weakness and sleep trouble, as well as psychiatric symptoms, such as aggressive behaviour or apathy.
It is interesting to note that some parasitic infections can be asymptomatic, meaning that the infected person has no symptom at all. Some scientists think that parasites’ asymptomatic carriers are more frequent than usually estimated. Some parasitic diseases can also alternate between symptomatic and asymptomatic phases. Asymptomatic people can still be infectious meaning that they can transmit the disease from their faeces or by being bitten by an insect vector . An active infection can also occur after a long asymptomatic period if the status of the human host changes, for example if the person’s immune system became unable anymore to counteract the parasite. This type of situation usually occurs when a person take immunosuppressive drugs or have cancer or AIDS. It is noteworthy that these people with challenged immune system are more at risk to become infected with a parasite and represent the majority of the infections in developed countries. Furthermore, for these people, along with the very young or very old people, the parasitic diseases’ symptoms can be worse or more unusual and these patients are more at risk to have complications.
Finally, as the symptoms of parasitic diseases can vary a lot and be almost anything, it is best to consult the symptoms’ list of each parasite in more detail.
Source: Centers for Disease Control and Prevention, Merck Manual