Fascioliasis is an important disease of livestock from production and economic point of view. Domestic and wild animals are particularly infected; whereas, humans are also occasionally infected with this disease. It is directly associated with the abundance of snail population which acts as intermediate host. The disease is caused by members of genus Fasciola; commonly known as liver fluke. The two important species, Fasciola (F.) hepatica and F. gigantica are etiological agents of fascioliasis. F. hepatica has a global distribution with more occurrences in temperate zones while F. gigantica is found primarily in tropical regions. In Pakistan, both aforementioned species have been reported; however F. gigantica is higher in distribution as compared to F. hepatica. Higher prevalence of fascioliasis has been recorded during autumn, followed by spring and winter while lowest in summer. The motile forms of Fasciola (cercariae) and snail population (vector) have been found to thrive best at 25-30 oC, this relation explains the reason of higher occurrence of fascioliasis in autumn as compared to other seasons. Temperature and humidity chiefly affect hatching of ova, viability of encysted cercarie and snail population. Economic loss due to fascioliasis in ruminants results from reduction in weight gain, milk and wool production which are determined by the intensity of infection.
The diagnosis of fascioliasis in domestic animals mainly relies on coprological examination for detection of Fasciola eggs in the faeces. Serological techniques are very useful and reliable tool for diagnosing fascioliasis due to their high sensitivity and specificity. Detection of antibodies in the infected animals with fascioliasis by secretary and excretory products of the parasites as antigens (ES antigens) and detection of antigens from faecal samples of infected animals (coproantigens). Advanced molecular methods like polymerase chain reaction (PCR) are also used for diagnosis of fascioliasis. The sensitivity of these techniques is extremely higher, as positive PCR signals can be obtained from the tissue of a snail penetrated by a single miracidium.
Chemotherapy is the chief approach for controlling fascioliasis in field conditions. In developed countries, strategic and tactical drenching programmes are in custom for the effective control of fascioliasis. Accurate drenching schedule is based on knowledge of the parasitic lifecycle and of its intermediate hosts. Resistance of flukes against conventional fasciolicides has been observed; however, some cocktails, like wormigold oral suspension (levamisole + oxyclozanide + cobalt sulphate) and oxyfen (oxyclozanide + oxfedazole + cobalt sulphate+ selenium) having less efficacious drugs appear to have a higher success rate. The use of shotgun drenching programme by treating 4-times a year with a cocktail fasciolicides is effective against young parasites, but it is only affordable in controlled farming systems.
Molluscicides are recommended for use in field to control the snails, as these serve as intermediate hosts of Fasciola. Extensive habitats such as rivers and irrigated larger fields make this type of treatment much expensive. A strategic approach is to apply molluscicides just before cercarial shedding, which begins in June, and repeats in September in Pakistan.
Successful biological control of fascioliasis by echinostomes flukes has also been demonstrated. The ability of larval echinostomes to aggressively displace other larval flukes from their snail hosts and parasitic castration of snails by larval echinostomes is well documented. This was achieved by adding faeces from 5 to 10 ducks naturally infected with Echinostoma revolutum to bovine faeces used as fertilizer in rice fields. Free-ranging ducks, geese and frogs, which eat snails, have also been proposed as a possible means for biological control of Fasciola in Pakistan especially under field conditions. Immunological control of fascioliasis appears to be a best choice under field conditions. Vaccine trials in cattle using several antigens of F. hepatica have been validated. The recombinant molecules have higher efficacy as compared to native antigens and vaccine cocktails (several antigens delivered in a single vaccine), which may have synergistic or additive protective effects. Nucleic acid vaccines can be used as an alternative to conventional immunological control. Nucleic acid cocktail of different antigenic portions of Fasciola (e.g. FABP, GST and cathepsin L) have effective protective coverage of animals leading to higher efficacy of herd protection. The application of nucleic acid vaccines against parasitic diseases has recently been reported. Fascioliasis is now recognized as an emerging food-borne zoonosis in many parts of the world. World Health Organization has also included fascioliasis on its list of priorities among neglected tropical diseases. Previous research done in Epidemiology laboratory on fascioliasis includes standardization of indigenous ELISA using local antigens (ES antigen) and application of commercial ELISA.
It is a well understood fact that epidemiology forms the foundation on which control strategies of parasitic diseases can be constructed. Further investigation of abundance and infection rate of the lymnaeid species is needed to assess the distribution together with studies of the role of other diseases in the epidemiology of fascioliasis. Apart from routine diagnosis and prophylactic treatment, associated risk factors influencing the prevalence of disease like age, area and feeding system/watering system should be taken into account and proper management techniques should be adapted. Provision of tap water rather than river or pond water will prevent the occurrence of disease. Similarly, proper grazing technique like rotational grazing should be adapted to minimize the chances of ingestion of infective metacercariae.
The distribution of disease must be determined using the most sensitive and reliable serological assays (in combination with conventional approaches) for prioritizing areas of highest risk can be targeted first. A combination of different control and diagnostic methods may work for an effective control strategy of fascioliasis in higher risk areas of Pakistan. Continuous education and extension programmes are need of the day for awareness regarding the transmission dynamics of fascioliasis. This kind of multidimensional approach can provide better sustainable control. At present, conventional and serological diagnostic facilities are available at epidemiology lab, department of Parasitology, university of agriculture, Faisalabad. Faecal sample preserved in 10% formalin and blood sample (serum) without anticoagulant can be dispatched for the diagnosis of fascioliasis.
The authors are from Department of Parasitology, Faculty of Veterinary Science, University of Agriculture, Faisalabad. Pakistan. The can be reached at <rubinadr@gmail.com>