CAN WE USE MOLECULAR DETECTION METHODS TO INDENTIFY LIVER FLUKE AND ITS INTERMEDIATE HOSTS IN THE ENVIRONMENT?

Fasciolosis, or liver fluke infection, results in reduced production and economic losses in dairy cattle and other livestock species. The lifecycle of the fluke, Fasciola hepatica, involves lymnaied snails as the intermediate host and depends on the development and survival of larval stages both in the snail and in the environment. This implies that the infection risk will depend on pasture and other areas harbouring the snails, as well as climatic and other environmental influences. However, although the general distribution of Fasciola and their intermediate hosts in SA is known, there is a lack of information regarding its micro-distribution in relation to the potential of snail species to harbour F. hepatica, and therefore variation in specific fluke infection risk from farm to farm and also within farm. Thus, it is important to identify these risk areas so as to strengthen infection risk models.

Traditional monitoring techniques such as physical identification and counting of individuals and species have been used to study the distribution of Fasciola and snails. However, these methods have shortcomings that include difficulties in correct identification of especially juvenile stages, dependence on weather conditions, shortage of expertise in taxonomy, the use of non-standardised sampling methods and the invasiveness of the field techniques. In contrast, environmental DNA (eDNA) methods are accurate, require less effort, are efficient and they offer good alternatives for monitoring the distribution of the species and larval stages. To test the possibilities, the objectives of the study cited below were to: (1) confirm the snail species involved in the transmission of  F. hepatica on farms; (2) detect F. hepatica infection in snails, and (3) detect snail- and fluke-derived DNA in the environment of both field and laboratory-based snails in relation to their presence, prevalence and seasonal cycling.

Fresh water snails, mud and water samples were collected from different farm spots and laboratory cultures. DNA from both snail and immature stages of F. hepatica in both snails themselves and their environment (eDNA) was detected using PCR, followed by cloning and sequencing of amplifications.

PCR and sequence analysis confirmed the identity of the snail species Galba truncatula and Pseudosuccinea columella. F. hepatica DNA was detected in 9 of the 10 pre-selected G. truncatula snails (90%) after immature stages of the parasite had been detected in them, and in all 3 P. columella snails (100%). Snail DNA was detected in laboratory samples in which snail species had been cultured: 66.6% and 50% of water and mud samples respectively for G. truncatula and from 2 out of 9 water samples (22%) for P. columella. Interestingly, the phylogenetic analysis showed the P. columella sequences grouped with those from South America.

The results suggest that eDNA techniques may be useful in identification and quantification of Fasciola and its snail host, and may therefore support management decisions to disrupt the lifecycle of the parasite.

Reference:

Byaruhanga, C., Oosthuizen, M.C., van Rensburg, L.J., Wolmarans, C.T. & van Wyk, J.A., 2019. Molecular detection of Fasciola hepatica and snail intermediate hosts and their environmental DNA, for sustainable management of fasciolosis. Report of a study on fasciolosis funded by Milk SA, Pretoria.