Parasitic infections, especially helminthic diseases, have adapted to their host during long-term infections, which usually lead to chronic disease with reduced mortality. During this evolutionary adaptation to their hosts, including vertebrates, parasites have helped modulate several molecular and physiological mechanisms, for example, the host immune system. Thus, the parasitic helminths elicit a modulated Th2 responses in their vertebrate hosts, resulting in immune response with a well-controlled inflammatory component, including inhibition of pro-inflammatory cytokines and induction of hyporesponsive state by IL-10 and produces Treg cells . In addition, the health hypothesis suggests that the absence of helminthic infections in the population of developed countries and the absence of parasitic immune stimulants increase autoimmune diseases with an exacerbated inflammatory component such as asthma and allergies . Therefore, it can be concluded that infection with helminthic diseases such as Fasciola hepatica, from which various immunomodulatory molecules have been isolated , can lead to a milder course of COVID-19 disease. We speculate that Fasciola hepatica, can lead to decreased immune responses, thereby reducing the severity and symptoms of COVID-19 disease and, consequently, the hospitalization of COVID-19 patients. Therefore, it can be hypothesized that the prevalence of fascioliasis in COVID-19 patients who had admitted to the hospital is lower than the normal population.
In the current study, the frequency of Fasciola hepatica infection in COVID-19 patients was 2.2%, significantly higher than the healthy control (without COVID-19 symptoms) (4.3%). However, as the reported prevalence of fascioliasis in the general population in Iran up to 50.0%, the relatively low frequency of Fasciola infection in our control group required cautious interpretation.
Of the 4 COVID-19 patients seropositive to Fasciola, 3 (75.0%) were female. However, this was higher than the overall proportion of the female patients (47.3%). Fascioliasis is seen more frequently among middle-aged adults probably due to exposure to the infectious agent. The relatively gradual increase in seroprevalence associated with age suggested that the ingestion of metacercariae in water or leafy vegetables that grow near water, which is greatest during the adulthood years, may not be the principal mechanism by which persons are exposed to Fasciola hepatica.
In this study, COVID-19 consisted of various age groups from young adults to elderly, age ranging between 14 and 72 years. The seropositivity rate was not affected by age, though there were significantly more seropositive COVID-19 patients who were housekeeper.
A main finding of the study was the significant different between seroprevalence in the rural population compared with the urban population to Fasciola. Of the four patients classified as having anti-Fasciola antibodies, 75.0 percent were resided in the rural area. This observations suggested that population from rural area were more likely to be exposed to the parasite.
This study has some limitations. First, this study is a case-control study at a particular time, so the causal relationship between the research variables cannot be fully inferred. Second, because of the limited studies of Fasciola infection in patients with COVID-19, the results of this study cannot be consistent or inconsistent with other studies.