Fascioliasis in New Zealand: A review

Intermediate hosts of Fasciola hepatica in New Zealand are the native species Lymnaea tomentosa, and introduced species, L. columella and L. truncatula. The latter is of little significance in New Zealand. Fascioliasis has become more widespread since 1950 following colonisation of much of the North Island by L. columella. L. tomentosa can produce eggs at 5°C and L. columella at 2°C. L. columella produces about the three times as many eggs as L. tomentosa and the eggs of the former develop more rapidly and over a wider range of temperatures. Both species inhabit ponds and marshes, although L. columella is better suited to pond habitats. Neither species will survive in marsh habitats that dry out in summer. Both species tolerate wide ranges of Ca and Mg concentrations and pH. Marsh habitats of the two species are indistinguishable though there is evidence of subtle differences in habitat preferences. Snail populations are minimal in winter and fluctuate irregularly throughout the rest of the year. In general, L. tomentosa populations contain proportionally more mature snails than those of L. columella. The epidemiology of fascioliasis in New Zealand is discussed. The absence of outbreaks of acute disease, the multiplicity of snail hosts and variations in climate pose problems for investigators. Overwintering infections of snails have been observed in L. tomentosa, but not in L. columella. With the latter host, transmission of Fasciola occurred over 6–7 months from mid-summer to mid-winter.The rate of infection of tracer sheep over 5 years varied but was not related to temperature or increased summer rainfall. Infection rates tended to increase when summers were dry and animals forced to graze the snails habitats. Rates in infection were not related to snail densities but to the numbers of mature snails on habitats. The infection rate of L. columella was extremely low ($\text{1}\text{790}$ snails examined). Sheep appear to be more important than cattle in maintaining Fasciolainfections from year to year. The control of fascioliasis is discussed briefly. It is argued that prevention of infection is preferable to removing infections already acquired, and low this might be achieved by a strategic anthelmintic treatment when L. columella is involved is described. Further studies on the prevalence and epidemiology of fascioliasis in New Zealand are required.