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Abstract
New Zealand farmers are currently spending about $27.9 million/yr on anthelmintics to control helminth parasites in cattle. Productivity trials carried out over the last 30 years show that the benefit‐cost ratio of an effective drench programme is likely to be high, particularly in young animals in their first year, which tend to be very susceptible to infection. In New Zealand, cattle have been recorded as the definitive host of at least 27 nematode species, two trematode species, and one (possibly two) cestode species. Of these, three gastrointestinal nematode species/species complexes, Ostertagia ostertagi/O. lyrata, Trichostrongylus axei, and Cooperia oncophora/C. surnabada are generally considered to be of greatest and most widespread economic importance. In New Zealand, as overseas, bovine ostertagiosis occurs in two clinical forms, referred to as Type I and Type II. Type I ostertagiosis is the normal form of the disease seen in weaner cattle, while Type II ostertagiosis results from the rapid maturation of large burdens of arrested fourth stage larvae of Ostertagia spp., which can accumulate in the stomach wall of yearling and older cattle. While some of the nematode species that have been recorded from cattle were simply incidental infections resulting from cross transmission from different host species (particularly sheep), one species complex, Ostertagia leptospicularis/ kolchida, which is normally considered to be a cervid parasite, is known to cycle readily in young cattle and has been implicated in clinical disease. The liver fluke Fasciola hepatica can be economically significant in cattle under certain local conditions but, while its distribution has increased markedly over the last 20 years, it is still generally considered to be a relatively minor problem in the national sense.
Despite the value of cattle and dairy products to New Zealand's economy (1992 gross production ["farm gate” prices] estimated to be $3382 million), very little has been invested in research into the control of helminth parasitism in cattle in New Zealand. A limited amount of applied work was done in the period between the mid 1960s and the 1980s to assess the prevalence, epidemiology, and economics of control of nematode parasites in dairy and dairy‐beef type cattle in particular, but there is little published information on traditionally raised (single‐suckled) beef cattle. Over the last 6 or 7 years, various pharmaceutical firms have carried out efficacy and on‐farm production trials as they developed a growing market for bovine anthelmintics. However, currently there is no active research into helminth parasitism of cattle by Crown Research Institutes in New Zealand. This reflects the fact that finance available for research is restricted, and in contrast to the situation in sheep and goats, there has been little evidence to date that anthel‐mintic resistance has become widespread in bovine nematodes. So far only six cases of anthelmintic resistance in nematodes of cattle in New Zealand have been reported. However, the rapid growth in intensive dairy bull beef production in New Zealand, and a proportionately even greater expansion of the bovine anthelmintic market, is likely to result in the problem becoming more widespread in the future.