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INTRODUCTION
Dictyocaulus viviparus (Nematoda: Dictyocaulidae) is a rather common trichostrongylid nematode parasite in the lungs of grazing cattle in temperate climates (Eysker, 1994). In a recent Swedish survey 40% of the herds were found to be infected (Höglund et al . 2004b ). Cattle are infected by ingesting grass contaminated with larvae through faecal transmission, and this might induce a very pathogenic respiratory disease, dictyocaulosis (Schnieder et al . 1991). Although outbreaks of dictyocaulosis are sporadic, they have an economic impact due to production losses both in dairy and beef herds in Europe. The cost of control of a moderate outbreak in a dairy herd with 100 cows in the UK has been calculated at 30000-35000 EUR (Woolley, 1997). Unlike the situation in other trichstrongylids of veterinary importance, development of resistance to anthelmintics has not yet been reported.
The dispersal of the bovine lungworm D. viviparus has received attention in recent years in Sweden, in order to better understand and predict the risk of spread. With increased knowledge about the transmission biology it is possible to target weak points in the life-cycle and hence to improve current control methods. Clearly, a better understanding of the transmission patterns of D. viviparus could assist in the effective control of this important parasite. Ultimately this should lead to a reduction in the number of outbreaks of disease.
Although wildlife ruminants sometimes are infected by Dictyocaulus lungworms, both moose and roe deer are infected solely with different species closely related to D. viviparus but that are not infective to cattle (Divina et al . 2000; Divina and Höglund, 2002; Höglund et al . 2003). Thus, wildlife do not serve as reservoirs and are not involved in the transmission cycle of D. viviparus in Sweden. We are therefore now concentrating on how D. viviparus is transmitted both within and among cattle herds. Interestingly, dictyocaulosis in Sweden is sometimes encountered in relation to livestock movements between farms. There is then usually considerable confusion as to whether an outbreak is a result of the sudden introduction of the parasite into a susceptible (i.e. immunologically naive) herd, or introduction of susceptible stock into an immune herd with a low (subclinical) infection level. This is an issue of great practical importance to...