In the present study carried out in the southern part of Côte d'Ivoire, West Africa, no Paragonimus infections were detected, despite our emphasis to search for this lung fluke among two high-risk groups. Indeed, we screened 278 patients in two tuberculosis centres in the economic capital of Abidjan who suffered from chronic cough. Moreover, we examined 166 children in two schools where consumption of crab meat has been reported. On the other hand, a prevalence of 22.3% of pulmonary tuberculosis was found among patients in the two tuberculosis centres. Helminth infections other than Paragonimus, as well as intestinal protozoon infections were common, particularly in schoolchildren. The results on those patients in the tuberculosis centres without pulmonary tuberculosis (n = 216, 77.7%) call for more in-depth studies to elucidate the aetiology of their chronic cough, which requires differential diagnosis.
The absence of paragonimiasis in the two study groups reported here might be explained by culinary practices. While we found trematode infections in crabs purchased from local markets in Abidjan and Dabou, and hence there is a risk of Paragonimus infection through consumption of undercooked crab meat, with very few exceptions, study participants reported that they thoroughly boil shellfish and cook pig meat prior to consumption. Indeed, boiling and cooking of shellfish and pig meat destroys metacercariae of Paragonimus. Previous research has shown that metacercariae harboured in crabs are destroyed after boiling of shellfish for at least 10 min at a temperature of 55 °C .
However one also needs to bear in mind the relatively low sensitivity of the diagnostic techniques employed in the current study [30, 31]. To overcome this methodological limitation, we examined three sputum specimens (rather than a single one) from all patients in the tuberculosis centres. None of the sputum specimen was found positive for Paragonimus eggs. Nevertheless, paragonimiasis cases might still have been missed, because of the irregular nature of egg shedding by adult parasites, as shown by Miyazaki (1999) . Previous research has shown that Paragonimus infections were detected after repeated analysis of sputum and stool samples collected at regular time intervals in the same individual.
With regard to stool examinations, we employed two diagnostic approaches in the school survey: Kato-Katz and an ether-concentration method using SAF-fixed stool samples. Combination of techniques are known to enhance the accuracy of helminth diagnosis [33, 34]. Yet, in the present study, no Paragonimus infections were found.
Paragonimiasis is difficult to diagnose through stool and sputum examinations using direct diagnostic techniques . Serological techniques such as enzyme-linked immunosorbent assays (ELISA) and immunoblot, produced promising results for paragonimiasis diagnosis . Moreover, the diagnosis is refined with polymerase chain reaction (PCR) using either stool or sputum samples of patients. For example, a study conducted in Cameroon aimed to identify Paragonimus species, comparing the sensitivities of serodiagnostic and microscopic methods and evaluated a copro-DNA test for detection of eggs in faeces. Results suggest that serology is considerably more sensitive than sputum examination for the diagnosis of paragonimiasis and a copro-DNA test may be more sensitive than a microscopic search for eggs in faeces . Unfortunately, these diagnostic techniques are not currently available in Côte d'Ivoire and elsewhere in the developing world.
A number of large-scale studies have been carried out in different parts of Côte d'Ivoire, both in schools and entire communities (see Figure 1). Interestingly, in none of these studies were Paragonimus eggs detected in stool samples. Although this parasite was not the primary interest in these prior studies, Paragonimus eggs are very large, and hence, they would have been detected and noted by the teams of experienced laboratory technicians without any doubt. Parasite eggs (larvae, cyst) of similar and smaller sizes were diagnosed, at times with high prevalence, such as S. mansoni, soil-transmitted helminths, S. stercoralis and intestinal protozoa [28, 35–41].
Although we could not identify cases with paragonimiasis in the current study, we cannot conclude that paragonimiasis is entirely absent from Côte d'Ivoire. Paragonimiasis, similar to other trematode infections, has a highly focal distribution. Typically transmission foci are detected with the diagnosis of a severely sick index case seeking care at local health services , which acts as a trigger for more in-depth investigation in the location of origin and a characterisation of the foci . Large-scale surveillance is therefore necessary to detect paragonimiasis foci actively. A simple questionnaire approach has been proven useful for settings in Southeast Asia , with which also the case detection rate of tuberculosis patients could be improved . It would be interesting to validate this questionnaire approach in an African setting. Recent studies showed that Paragonimus eggs are not destroyed by the widely used Ziehl-Neelsen staining techniques . Therefore, paragonimiasis cases can be detected retrospectively by re-examination of Ziehl-Neelsen stained sputum samples kept at tuberculosis centres. Finally, the surveillance of shellfish, checking for the presence of metacercariae, should be encouraged, which can be integrated in critical point monitoring of food safety activities.
We could not confirm previous reports of Paragonimus infection in the south of Côte d'Ivoire. The low endemic level of the infection in animals and humans in this area, the moderate sensitivity of the diagnostic tools utilised and the lack of active surveillance are important limitations. Due to the sustained socio-political crisis, food consumption habits might have changed, altering transmission pathways to humans. Hence, heightened vigilance is required.