Depending on the parasite species involved, it is very likely to experience relapses or reinfection after the treatment. Relapses occur when a parasite comes back after being only partially treated meaning that it has persisted within the body despite the treatment. It is also the case for parasites that remain within the body for life, such as Toxoplasma gondii, and for which only symptoms, if they occur, are periodically treated. Reinfection occurs when the parasitic disease was totally cured implying that the person gets a new infection with the same parasite.
As parasites could be very persistent within the human body and hide from the reach of the treatment and the activity of the immune system for a long period of time, relapses are quite common for this type of microorganisms. As such, some parasites still present in the body in an asymptomatic phase can reappear and cause a relapse. It is also interesting to note that people with weaken immune system, such as people living with AIDS, are more at risk to have relapses than healthy people.
However, in some cases, such as Leishmania, the presence of hidden parasites within the body seems to prevent the reinfection. In this case, it means that the presence of the parasites within the body eliminates the risk of being newly infected by another species of the same parasite. However, studies showed that as soon as this parasite is completely eliminated from the body, reinfection becomes possible.
Furthermore, it is also likely that a sub-population of parasites become resistant to the treatment and proliferates afterwards. In this case, the person could initially feel better with the treatment, but experience a quick relapse as the resistant parasites proliferate.
Trichomonas vaginalis, a common sexually transmitted parasite in developed countries, is one of the greatest examples of parasitic reinfection. In fact, the Centers for Disease Control and Prevention estimate that about 20% of the people treated for trichomoniasis get infected again within 3 months. It is the reason why it is very important that all the sexual partners are also treated and that sexual intercourses are avoided until all the symptoms are gone which takes about one week after the single-dose treatment. However, the fact that this infection is mostly asymptomatic in men complicates the situation.
Reinfections are also very frequent for people living in countries endemic for some parasites. Actually, these people are frequently in contact with many types of parasites. As such, these people can be cured from their parasitic disease and become infected again by another contact with the parasite. This situation is common because the source of the infection in the environment is not eliminated.
Reinfections are common with parasites because immunity against this type of microorganisms is very hard to achieve. It means that, as oppose to other type of infectious diseases such as the chicken pox for example, having one specific parasitic disease once in your life does not usually confer resistance against reinfection. This phenomenon is mostly linked to the mechanisms used by the parasites to evade the immune system that impair the achievement of immunity against parasites. It is also one of the reasons why vaccines against parasites are very hard to develop and are then not yet available.
Finally, the difference between relapses and reinfections is not well defined. Consequently, in some cases, it is not possible to distinguish between both without doing laboratory analysis confirming that it is genetically exactly the same parasite in the case of a relapse or not in the case of a reinfection. However, it is usually more pertinent to do this type of parasite characterisation in the context of scientific research rather than in the context of parasitic disease’s treatment.
Sources: Centers for Disease Control and Prevention
Yazdanbakhsh, M., & Sacks, D. L. (2010). Why does immunity to parasites take so long to develop? Nature Reviews Immunology, 10(2), 80-81.