Dicrocoelium dendriticum and dicrocoeliosis: a review of our research

Dicrocoeliosis, caused by D. dendriticum, is an important parasitic disease from an economic and health viewpoint. This trematode, whose adults live in the liver and bile ducts of numerous mammal species, mainly ruminant, which act as defmitive hosts in several countries in Europe, Asia, America and North Africa, needs land molluscs and ants as first and second intermediate hosts, respectively, to complete it life cycle. D. dendriticum occasionally affects humans. The application of efficacious prophylactic and control measures against dicrocoeliosis -which have not been satisfactory so far-, requires precise early diagnosis, a previous epidemiological study and experimental research to help interpret field observations and extend knowledge of the parasite/host relationships. We therefore decided to carry out studies on: epidemiology; strategic treatments; experimental infection in molluscs, hamsters and lambs; parasite isoenzymatic characterisation and genetic variability; and antigenic molecules as targets for specific diagnosis and protection. Our research on D. dendriticum started 30 years ago, mainly focusing on the specific determination, distribution and natural infection by trematodes of 23 species of Helicidae (Mollusca) collected over 12 years in more than 350 villages throughout the province of León (NW Spain). This parasite was detected in 11 mollusc species collected in 95 of these villages. Likewise, the monthly kinetic of D. dendriticum egg elimination in faeces of sheep and cattle chosen at random in 5 localities in the upper and middle Porma river basin was followed for one year. We found D. dendriticum eggs in 63.6% and 37.6% of the 995 and 1251 sheep and cattle samples examined, respectively. The highest egg-elimination period in both was the end of autumn-winter. Moreover, simultaneous and integrated studies conceming the transmission of D. dendriticum were carried out on 81 labelled sheep and lambs, molluscs and ants, over two consecutive years
According to our results, shedding of D. dendriticum eggs with the faeces of the ruminants occurs uninterruptedly throughout the year, but the highest values are recorded at the end of autumn-winter. At these times survival of D. dendriticum eggs is very high (low temperatures do not affect egg survival), so pastare contamination by viable eggs is very great in spring, when the molluscs are very abundant and active. Those infected at the beginning of this period could shed slimeballs with cercariae at the end of summer and during autumn, whilst those infected later can shed slimeballs the following year, beginning in spring, if they survive the harsh winter. Approximately 45 days later the cercariae ingested by ants will have become infected metacercariae for the definitive hosts. This will allow the parasite cycle to be completed when the ants are ingested by ruminants on grazing, during the active period of the ants (March-November). Nevertheless, some infected ants survíve in their nest during the winter, and they are responsible for definitive host infection at the beginning of spring. As the ant activity increases, so does metacercariae ingestion by the definitive hosts and their D. dendriticum worms increases. Thus, egg elimination also increases during this period, reaching the highest values in January-February (Manga-González et al., 2001). Two treatments applied in November and January were the most effective to reduce egg shedding by natural infected sheep. The ovine experimental dicrocoeliosis studies were carried out on 32 lambs: 12 infected with 1000 D. dendriticum metacercariae, 12 with 3000 and 8 controls. Half the lambs in each group were slaughtered on day 60 and 180 p.i., respectively. The percentage of metacercariae established as adult worms in the total of infected lambs was 17%. The worms recovered on necropsy of each animal was 110-2063 worms (dose 3000) and 30-437 worms (dose 1000). Egg elimination was first detected between days 49 and 79 p