10 Sept 2024
Sarah Hewitt and Rob Howe cover the what, why and how for implementing this system.
Figure 1. Acrossus rufipes (the night-flying dung beetle). Image: Rob Howe and the Dung Beetle Trust.
Gastrointestinal nematodes can cause production losses in cattle, but an over-reliance on chemical control measures has seen the development of resistant parasites and a reduction in efficacy of some anthelmintics.
In addition to the development of resistance, concerns are increasing about the unintended negative effects of anthelmintics on the environment and soil health.
Integrated parasite management involves using non-chemical control measures alongside anthelmintic best practice in a holistic way, taking into account the farm type and system. This can offer a valuable opportunity for vets to discuss sustainable and regenerative farming practices with farmers.
Integrated parasite management strategies can be grouped into chemical and non-chemical methods.
Chemical control of parasitic gastroenteritis includes the use of one or more of three classes of broad-spectrum wormer available for cattle: group 1 benzimidazoles (white drenches); group 2 levamisole (yellow drenches); group 3 macrocyclic lactones (clear drenches).
An “as little as possible but as much as necessary” approach should be taken by using targeted selective treatments and aiming to maximise the population of parasites in refugia (not exposed to the anthelmintic at a treatment event).
Non-chemical control methods aim to maximise the resistance and resilience of the animal (for example, optimising general health and immunity) and reduce the risk of infection (for example, use of grazing strategies).
Good health and optimal nutrition both improve ruminant resilience (the ability to cope with parasitism) and their resistance (the ability to mount an immune response; Burke and Miller, 2020; Coop and Kyriazakis, 1999).
Monitoring body condition scores, collecting weight data, metabolic profiling and forage analysis can all help to monitor and optimise herd nutrition. Protein availability and balanced mineral supply have been shown to be important in improving resilience to nematode infections in sheep (Sykes and Coop, 2001), and protein supplementation has been shown to reduce the periparturient reduction in immunity seen in ewes around lambing (Donaldson et al, 2001).
Studies in cattle are less extensive, but calves given a low protein and energy diet have been shown to have elevated worm egg counts, reduced growth rates and an impaired immune response compared to those with a high protein and energy diet (Mansour et al, 1992).
Grazing strategies may incorporate non-susceptible species to effectively reduce stocking density of susceptible species (co-grazing strategies), or to ingest infective stages and reduce contamination of pasture. Rotational grazing, forage species and sward height may also play a role in parasite management.
Co-grazing
The most important gastrointestinal nematodes in cattle (Ostertagia species and Cooperia species) are unlikely to cause disease in other species, and so alternate grazing between cattle and sheep or horses (or any non-susceptible species) can be a useful way to reduce larval burdens on pasture.
Co-grazing can also be used as a way of reducing the stocking density of the susceptible species. It should be noted that Trichostrongylus axei can infect horses and ruminants, so care should be taken if this parasite is present. Although rare, Ostertagia ostertagi can occasionally infect sheep. Haemonchus contortus can also occasionally cause disease in young cattle, while H placei, which is largely found in tropical regions, can cause disease in cattle and sheep (Barger, 1999).
Leader-follower systems follow a similar approach and involve low-risk animals (healthy adults or a non-susceptible species) grazing high-risk or medium-risk pastures before high-risk animals (young or naive animals of a susceptible species) to remove some of the infective larvae without heavily contaminating the pasture.
Rotational grazing
Rotational grazing involves frequent moves of cattle between small paddocks.
It can be helpful for parasite management, as cattle graze pasture for short periods of time and then move on. If this period of time is less than the pre-patent period of the parasite of concern (generally around three weeks for Ostertagia species), any parasites ingested will not have time to complete their life cycle and add to pasture contamination. The limiting factor is how long the pastures can be left empty before grazing again.
Temperate summers mean larval survival can extend to cover the whole grazing season. Rotational grazing is often employed to optimise use of grass, and so the system also needs to consider grass growth.
A danger also exists of cattle returning to pasture at peak infectivity (when all the eggs deposited at the previous grazing have developed into infective larvae), and grazing cattle at higher stocking densities may encourage grazing closer to pats, increasing larval ingestion. Although considerations must be made when employing rotational grazing strategies, it can be a useful tool; work is ongoing to explore its potential as a means of parasite management and of optimising soil health and biodiversity (Sands et al, 2024).
Sward height
Larvae tend to only travel around one inch from the ground (Maqbool et al, 2017), so grazing longer grass and not allowing cattle to graze too close to the ground can help reduce the risk of cattle ingesting infective larvae.
Larvae tend to only travel around one inch from the ground (Maqbool et al, 2017), so grazing longer grass and not allowing cattle to graze too close to the ground can help reduce the risk of cattle ingesting infective larvae.
A sward height of more than 8cm has been suggested by Control of Worms Sustainably (COWS) as a low-risk sward height in terms of parasite management (COWS, 2014).
Bioactive forages
Bioactive forages are plants containing compounds with anti-parasitic properties, such as lactones, alkaloids and tannins (Athanasiadou et al, 2007). Tannins are the most commonly reported; they can affect protein availability to the animal, which may influence the response to parasitism, and have been shown to directly affect parasites, too.
Reports are largely in small ruminants and involve Haemonchus contortus. Less has been reported in cattle, but reports exist of in vitro activity of condensed tannins against Cooperia species and Ostertagia species (Burke and Miller, 2020).
Application of in vitro results to in vivo situations should be done with caution, however, and potential toxicity risks should be considered (Athanasiadou et al, 2007).
Biological control measures include nematode-trapping fungus, earthworms and dung beetles.
Duddingtonia flagrans is a fungus which traps nematode larvae while they are in the faeces. Fungal spores are fed to livestock, passed in faeces and trap larvae in the dung, preventing them migrating up the grass and being ingested.
D flagrans does not seem to have adverse effects on free-living nematodes and was not detectable in the soil after two months. It requires a management system that can accommodate daily feeding, however, and the spores need to be fed for 60 to 120 days (Burke and Miller, 2020).
Earthworms and dung beetles are important members of soil ecosystems, and integral to soil health. They both break down and remove faeces from pasture, removing infective larval stages at the same time. Maintaining and monitoring populations of these invertebrates by promoting soil health and limiting anthelmintic use can reduce the risk of parasitic gastroenteritis (Sands and Wall, 2017).
Use of estimated breeding values to breed individuals with genetic resistance and resilience to parasitism is more developed in sheep than cattle, but as both resistance and resilience are moderately heritable, this may be something that is developed further in future; for example, although not currently commercially available, Meat and Livestock Australia has developed some preliminary estimated breeding values for internal parasite resistance in Angus cattle (Honey, 2013).
Reliance on chemical control measures has led to development of anthelmintic resistance in cattle, with resistance to all classes of broad-spectrum wormer reported (Bartley, 2011; Kelleher et al, 2020).
In addition, concern is increasing about the effects of anthelmintics on non-target species within the environment, and on soil and plant health (de Souza and Guimarães, 2022).
Although less common than it occurs in sheep, anthelmintic resistance in cattle is a growing problem (Bartley, 2011).
A study assessing gastrointestinal nematode resistance in cattle in Ireland found widespread resistance (Kelleher et al, 2020). Fenbendazole resistance was identified on 60% of the farms in the study, as well as levamisole resistance on 18%, ivermectin resistance on 100% and moxidectin resistance on 73%.
It should be noted that the study was of a relatively small number of farms (23), and that these were self-selected, so may be farmers concerned about resistance in their herds. It is likely, however, that a move away from reliance on chemical control measures for parasite management will occur in the future (Forbes, 2023).
In addition to concerns about resistance, the impact of anthelmintics on the environment is becoming more evident, with effects on non-target invertebrate species, plant and soil health (de Souza and Guimarães, 2022).
Macrocyclic lactones have tended to cause the most concern, with large amounts being excreted unchanged in the dung (80% to 98%) and remaining active on pastures for a prolonged period of time (up to two months in temperate climates). As well as lethal and sub-lethal (reproductive) effects on dung beetles and other soil invertebrates, these anthelmintics also have negative effects on plants, such as reduced root germination and growth, and chromosomal instability (de Souza and Guimarães, 2022; Laber et al, 2023).
Dung beetles and earthworms are important in dung degradation and nutrient cycling, and reducing anthelmintic treatments through targeted selective treatment has been shown to reduce negative impacts on dung fauna populations (Cooke et al, 2017).
Parasite management through chemical control is well established, and products are relatively cheap, convenient to use and widely available.
However, vets are well placed to have discussions with farmers about system changes, combining knowledge of the farming system with understanding of the epidemiology of the likely parasites. A study exploring farmer behaviour around parasite control identified factors influencing low uptake of new strategies to include low awareness of the risk of anthelmintic resistance, a positive attitude about the current strategy, and a perceived lack of link between treatment failure and anthelmintic use (Vande Velde et al, 2018). Veterinary communication is an active area of research – particularly in relation to how vets can facilitate behaviour change.
Previously, the assumption tended to be that decisions were based only on rational, economic and practical factors.
However – particularly with occupations such as farming which are often more of a lifestyle than a job – many other personal and social factors are involved (Vande Velde et al, 2018). Techniques such as motivational interviewing are being used by vets to facilitate change (Bard et al, 2022) and can readily be applied to integrated parasite management.
Combining epidemiological and farm system knowledge, awareness of likely barriers and effective communication methods, affords vets the skills to explore integrated parasite management options with farmers.
This has been demonstrated through a clinical trial set up in 2018 by one of the authors (Rob Howe).
The aim of the trial was to determine if learnings from innovative farmers, research scientists and others could be applied to an advisor-led service in a commercial vet practice. Five years on, the initial pilot has resulted in most of the practice’s dairy farms being enrolled on a vet-led integrated parasite management plan.
This case series is being written up to provide evidence for the approach, as well as the benefits for farmers and practices.
