FASCIOLOSIS IS a parasitic disease caused by Fasciola (F.) hepatica and F. gigantic, commonly called as liver fluke. These are leaf shaped, pointed from the back and wider from the front, although the shape may vary. They have ability to infect livers of many animals including humans. The species of genus Faciola are prevalent throughout the world and affect more than 600 million animals and 2.4 million humans. It has been reported that 0.07 kg/week of the live body weight of animals is decreased depending on the level of infection. Fasciolosis causing a total estimated economic losses of more than 2000 million dollars per annum and causing major constraints in development of a profitable livestock industry.
Fasciolosis like other parasites requires snail as intermediate host (organism that supports the non-reproductive forms of a parasite). At present, Pakistan provides suitable climatic fasciolosis in approximately all areas of snail vectors, hence increasing the problem of fasciolosis in approximately all areas of the Punjab. However, this appears as a serious problem in low lying, swampy areas such as districts of Lahore, Gujranwala, Sheikhupura, Faisalabad, Multan, Muzaffargarh, Sargodha, Jhang, etc; where due to water-logged topography, snail causing more livestock population at risk of getting infection.
Developmental stages in the lifecycle of Fasciola are directly linked with the variation in the environmental temperature. Optimal temperature for development of larval stage is ranges from 24-26oC; however, Fasciola eggs are sensitive and cannot survive in dry conditions particularly at higher temperatures (43oC). It is believed that the survival rate of larval stage of fasciola is directly proportional to the moisture content of the environment and inversely proportional to the temperature. The larval development ceases or slows down in the body of intermediate host (snail) due to lower temperature (<16oC). As soon as temperature increases to 20oC, the development of larvae commences and at an optimum temperature range (25oC to 247oC), maximum shedding of cercariae (a larval stage) take place. These cercariae developed into metacercaria (infective larval stage) within 24 hours at 27oC. In July and August (rainy season) infective larval stage can be found in considerable number on vegetation. During grazing ruminants picked up the infection in post-rainy months, which resulted in higher infection and worm burden in September-October. These immature larval flukes migrate through the intestinal wall, the abdominal cavity and from liver tissue into the bile ducts where they gain maturity and produce eggs.
The clinical signs of the acute disease usually seen in autumn and early winter, are characterised by sudden deaths, weakness, anaemia, dyspnoea, progressive loss of condition, hypoalbuminaemia, emaciation, pallor of the mucous membranes, submandibular edema and ascites. Chronic fasciolosis is associated with mature flukes, and seen mainly in late winter/ early spring. In milder infections, clinical signs may or may not be readily observed; due to parasitic specific immune response however, a decreased appetite and interference with post-absorptive metabolism of protein, carbohydrates and minerals, may have a significant effect on the animal production. The level of liver condemnation at abattoir meat inspection in Pakistan varies according to the season. It has been observed based on the climatographs of fasciolosis that the rate of liver condemnation is higher in winter. The average rate of liver condemnation following dry summers is reported to be 5 per cent but can be as high as 10 per cent to 20 per cent following wet summers.
Study of climatic factors showed that after heavy rainfall in July and August (253mm and 108mm, respectively), humidity increases, which results in higher risk of infection in September, indicating positive relationship between infection and humidity. Lowest infection in May-June is related to progression of hot dry weather, which is well justified due to scarcity of snail vector during hot months. In January, prevalence increase again after winter rainfall due to increased humidity.
Conclusively, that water reservoirs and vegetation play an important role in the growth and propagation of snails (the intermediate host). Hence, the biological and geo-climatic conditions provide favourable condition required for the epidemiological distribution of fasciolosis. The very information can be applied in other topogeographic areas having similar ecological and environmental conditions.
The information on the frequency on the frequency distribution of fasciolosis in the livestock population in different climatic condition can be utilized to jot down an effective and sustainable control strategy with respect to the target area.
Overall, according to the average climatic condition of Pakistan, following measures could be adopted to control the nuisances of fasciolosis ;
Use strategic anthelmintic treatment (e.g. Triclabendazole), during winter to reduces pasture contamination before the snails become active. Otherwise, the contamination of pastures with fluke eggs will result in high fluke burdens in spring.
Reduce the number of intermediate host (snails) by using molluscicides and improved drainage, Draining marshy pastures and building dams may reduce snail habitats and increase grazing areas.
Rotational grazing (i.e. grazing animals in divided paddocks; grazing equines, then sheep etc.) and also avoiding missed grazing of animals of different age groups (Young animals are generally susceptible to helminthes infections).
The writers are associated with the Department of Parasitology, University of Agriculture, Faisalabad, Pakistan.