ABSTRACT
Parasitic helminth infections are one of the most important causes of production loss in livestock worldwide. Grazing cattle on dairy farms are exposed to an array of different worm species, all of which can affect health, welfare and productivity to varying degrees. Infection can result in diarrhoea, weight loss, oedema, anaemia and anorexia, and, importantly, can produce subclinical effects that have negative impacts on growth and production, particularly in youngstock (Ploeger et al., 1990a,b; Eysker and Ploeger, 2000). In milking cows, helminth infections can result in reduced milk yield, which can lead to economic loss (Verschave et al., 2014). Because helminths can detrimentally affect health and performance, control measures must be implemented to reduce infection levels of helminths. Usually, cattle will harbour several different helminth species simultaneously and the species present will determine the clinical signs of infection, as well as the type of treatment and control strategies that need to be instigated. For several decades, control has been largely been dependent on the frequent use of broad-spectrum anthelmintics; however, the application of frequent treatments to control infection needs to be balanced
with a requirement to avoid selection pressure for anthelmintic resistance. There is obvious conflict between the use of regular anthelmintic treatments to minimise infection levels and the need to preserve the efficacy of these medicines and, because of large differences in the prevalence of helminths and their epidemiology, even at farm level (see below), the balance needs to addressed on a case-by-case basis. A reduction in anthelmintic usage requires the use of knowledge on local epidemiology, farm history and parasite diagnostics (faecal egg count [FEC] analysis, primarily, at the current time) and needs close interaction of the farmer with their veterinarian or animal health advisor. The advisors should use their expert knowledge and a limited number of well-validated diagnostic tools (see below) to build a picture of the likely parasite threats present, the stock that are most susceptible to these threats and the anthelmintic sensitivity of the helminth population to guide treatment decisions. Well-validated diagnostic tools are needed on which to make these decisions. Moreover, there are no studies on the cost/benefit ratio of using such tools in control strategies, as opposed to simply applying all-group (‘blanket’) anthelmintic treatments. It should be acknowledged that if resistance is not addressed, as there are no new chemical classes near to market for use in cattle, once efficacy is reduced, significant losses will be incurred as levels of infection will increase such that the impact will not only impinge on economic viability, but also on the incidence of clinical disease. This chapter describes the likely helminth threats to grazing dairy cattle, followed by an update on what diagnostic tools are available to help evidence-based decisions for treatment. Finally, progress on the development of anti-helminth vaccines is described. Such vaccines are a long way off commercial availability, but recent research progress suggests that these could form part of a sustainable solution to helminth control on dairy farms in future.
