Private veterinary practitioners (PVPs) play an important advisory role in parasite control in livestock but best practice on this topic has changed through the years, particularly with the rise in the level of anthelmintic resistance. The Parasite Control Technical Working Group of Animal Health Ireland distributed a survey to PVPs in late 2018 and early 2019 in order to gain an understanding of what is currently being advised to farmers with regards to parasite control for cattle. PVPs were approached through conferences and discussion groups and 131 people participated in the survey. The majority of PVPs were in mixed practice (79%), worked in the provinces of Leinster (38%) and Munster (34%) and had been in practice for a period of 10-20 years (32%).
Many PVPs routinely gave parasitological advice on the sale of anthelmintics (58%) and reported that at least 30% of their cattle-farming clients have sought this information from them in the previous year. This advice was usually given in response to a current problem on farm (63%). Eighty per cent of PVPs reported that fewer than 30% of their clients had established a formal parasite-control plan with them and that this was rarely reviewed annually, many having been drawn up as part of a Knowledge Transfer Programme. Short-acting macrocyclic lactones (eg. ivermectins) were reported as the anthelmintic group most commonly sold by PVPs (66%) for gut worms, followed by benzimidazoles (19%) and levamisoles (8%). The majority of PVPs (92%) advised rotation of drug classes.
Farmers may be tempted to dose animals frequently because of the perceived production benefits and low cost of anthelmintics. However, this approach is unsustainable as exposure to anthelmintics risks an increase in the development of resistance. In Ireland, levels of resistance to drug classes seem to reflect the usage as seen in this survey, with ivermectin resistance commonly detected in studies. The veterinary profession needs to encourage the prudent usage of anthelmintics to minimise production losses while also slowing the reduction in the long-term efficacy of anthelmintics associated with the emergence of resistance. While the majority of farmers that sought advice reported a current problem on the farm, there is an opportunity for PVPs to discuss strategic parasite control as part of a wider herd health plan as a service to farmers. This supports a shift in focus to preventative veterinary medicine. If parasite-control plans are being followed, they need to be reviewed on a regular basis as worm burdens are highly dependent on environmental conditions and, therefore, may change from year to year.
Questioned on grazing management, 20% of PVPs supported immediately moving first grazing season calves to new pastures after treatment with an anthelmintic for stomach/gut worms. This is no longer considered best practice as it encourages anthelmintic resistance because any resistant worms are likely to become dominant on the clean pastures. Rather, the concept of parasites in refugia is now being promoted, which involves leaving a proportion of the worm population unexposed to an anthelmintic. This can be achieved by not dosing 10-20% of animals before moving to new pastures, or by leaving animals on the original pastures after treatment for a number of days to dilute any potentially resistant worm populations in the animals. Adult animals, which often do not require treatment for stomach/gut worms, can also be a source of refugia. Additionally, rotational grazing practices, such as calves following adult animals or sheep, or not placing high risk dairy calves on the same pastures each year, can reduce the reliance on anthelmintics by matching the animal risk with the pasture risk.
Faecal egg counts
Survey responses concerning faecal egg counts (FEC) were extremely varied. For the FEC treatment threshold for gut worms in first-season dairy calves, the responses ranged from 50-2,000 eggs per gram (EPG) of faeces, with the majority of PVPs advising treating above an FEC threshold of 200 EPG. Many PVPs advised FEC monitoring in spring-born suckler (66%) and dairy calves (76%).
The varied responses for an FEC treatment threshold was understandable as a definitive cut-off oversimplifies the additional considerations when interpreting FECs, such as whether samples have been pooled, the age of the animals and what type of roundworm eggs are seen in the sample. The concentration of eggs (EPG) is further influenced by the volume of faeces produced and the distribution of eggs in the faeces. Additionally, overdispersion in groups is common ie. a few animals may account for the majority of the parasite burden. FEC values often do not correlate well to the individual worm burden in an animal and eggs are only seen if mature female worms are present. In the cases of Nematodirus or lungworm for example, pathology is usually associated with immature parasites and FECs are then not reliable during times of clinical disease. FECs can be a useful tool to determine the optimal timing of treatments or whether anthelmintic treatment is necessary but should be interpreted with an understanding of the limitations of the test alongside the animal or group history, clinical signs and the epidemiology of the parasites. For cattle, general guidelines for ranges of FEC are summarised in Table 1.
PVPs reported advising dosing of first grazing season calves: at intervals (26%); based on clinical signs of scour or weight loss (11%); on FEC (15%); and on FEC and liveweight gain (46%). Production losses due to parasitic gastroenteritis may be significant before clinical disease becomes apparent. Therefore, dosing based on FEC and liveweight gain or other performance measures are considered best practice, although dosing at strategic intervals is a practical alternative for many farms. The timing of these intervals can be reviewed alongside FEC results. The majority of PVPs (63%) advised that the first sampling for FECs should be performed six to eight weeks after turnout for spring-born dairy calves. This is in line with current advice, as FEC during the early summer for first grazing season calves can be a good indicator of parasitic gastroenteritis risk later in the season. Spring-born suckler calves, in contrast to dairy calves, do not routinely require intensive monitoring or dosing in their first grazing season until weaning approaches.
When lungworm is suspected, the majority of PVPs advised immediate treatment with or without sampling (88%). Death in acute cases of lungworm challenge can occur before larvae appear in faecal samples and bronchoalveolar lavage is the preferred diagnostic test for prepatent infection. Detection of antibodies in serum or milk by ELISA is not useful in acute cases, as seroconversion may take four to six weeks after infection and antibody titres persist for up to seven months. Treatment based on clinical signs and grazing history is advised.
It is encouraging that most PVPs are following best practice guidelines in the sphere of parasite control although they can be challenging to implement. A recent report by the Health Products Regulatory Authority1 found that antiparasitic medicines in food-producing animals should not be exempt from prescription. For this reason, the role of the PVP in advising on anthelmintic treatments may become even more important in the future. PVPs should, therefore, be familiar with dosing guidelines and anthelmintics available beyond the reactionary treatment of immediate parasite problems on-farm.