In recent research on a farm in Scotland, clinical signs and significant numbers of lungworm larvae were found in bovine faeces after treatment with ivermectin and moxidectin. Lead researcher Paul Campbell from the University of Glasgow reports.
Parasitic bronchitis is caused by the bovine lungworm Dictyocaulus viviparus. Infections can cause mild to severe respiratory distress, inappetence and in severe cases, death and can therefore have severe welfare and financial impact.
When exposed to a high level of challenge, youngstock with limited immunity are at significant risk of clinical disease. In the UK, lungworm outbreaks have been increasing since 2009 and are most pronounced in Scotland and northern England, with a shift in seasonality from late summer to autumn, and now occurring at any time of year, although still peaking in September.
Despite the availability of a vaccine (Huskvac), there has been a decrease in its use since the advent of anthelmintic products belonging to the macrocyclic lactone (ML) class. Reliance on a single anthelmintic class would be expected to select for resistance, but to our knowledge, only one case of confirmed resistance has been reported worldwide.
Lungworm investigation
Lungworm monitoring was carried out on a commercial dairy unit located in central Scotland, as part of a larger study investigating anthelmintic inefficacy in roundworms using the faecal egg count reduction test (FECRT).
Thirty-three spring-born Holstein-Friesian heifer and dairy x beef first grazing season (FGS) calves were turned out onto 4.4ha of permanent pasture on 2 May 2023, at a stocking density of 7.5 individuals/ha. The pasture had not been grazed that year and was only used for turnout of first season grazing (FSG) calves in previous years.
The calves are usually set stocked from turnout in late April/early May until mid-September and then moved to new grazing until housing in mid-October.
For seven years, all FSG calves were treated with moxidectin at turnout and ivermectin at housing. In previous years, calves were reported to have maintained good body condition and growth rates, with no reported diagnoses of lungworm.
In the study, the 33 calves were not wormed at turnout, and faecal egg counts (FECs) and body condition were monitored for roundworm infection fortnightly from early July 2023 until the FEC reached a group average of around 100 eggs per gram (epg) on 21 July. Opportunistic faecal samples were taken for lungworm detection.
On the 4 August (day 0 of the FECRT), 16 calves were treated with moxidectin by subcutaneous ear injection, and the other 17 were treated with subcutaneous ivermectin. Individual per-rectum samples were collected from all animals for FEC on days 0 and 15. After suspicion of lungworm post-treatment, nine samples per group were randomly selected for lungworm detection.
Results
Fifteen Baermann filtrations were performed on a random subset of faecal samples collected 21 and 14 days before the FECRT (faeces collected from the pasture), and no lungworm larvae were detected. On these visits the calves were fit and well.
On day 15 post-treatment, several calves were coughing intermittently. Of the 18 per-rectum samples chosen for lungworm detection, 12 were positive for lungworm. The larval counts of the lungworm-positive individuals varied greatly, ranging from 2 to 340 larvae per 30 grams of faeces.
These cases were reported to the Veterinary Medicines Directorate (VMD) as an adverse event with a suspected lack of efficacy. All calves were given levamisole and a supportive NSAID treatment and moved to low-risk grazing on silage aftermath. Fourteen days after levamisole treatment, samples were collected from nine randomly sampled individuals, and no larvae were detected.
Conclusions
Both the moxidectin and ivermectin treatments were ineffective in this situation, with the findings highly suggestive of an ML-resistant population of D. viviparus.
We hypothesised that this resistant population arose from repeated and prolonged exposure over multiple years to a moxidectin long-acting injectable. As this field was only ever grazed by FGS calves, always treated with the same moxidectin product at turnout, the presence of a susceptible lungworm population (refugia) to survive would be severely limited.
The prolonged period of protection offered by MLs, coupled with their ease of application, makes them a popular choice for controlling lungworm and roundworms.
However, it is imperative that producers and their advisers – SQPs and vets, consider the possibility of anthelmintic resistance in lungworms and implement sustainable parasite control strategies. In particular, vaccination against lungworm infection in calves can be used successfully, alongside other management strategies which can reduce anthelmintic usage while still protecting youngstock.
Mr Campbell’s paper can be found at: Campbell P, Forbes A, McIntyre J, Bartoschek T, Devine K, O’Neill K, et al. Inefficacy of ivermectin and moxidectin treatments against Dictyocaulus viviparus in dairy calves. Vet Rec. 2024;e4265 https://doi.org/10.1002/vetr.4265