Can we treat intestinal worm infections selectively in dairy cows?

Grazing ruminants around the globe continue to be exposed to gastrointestinal nematodes (GIN). In lactating cows, infection by GIN is frequently associated with a decrease in milk productivity [1]. For many years, anthelmintics have been the cornerstone to manage parasite infections and limit milk production losses. The emergence and fast development of anthelmintic resistance in many countries and for many classes of anthelmintics has forced us to rethink our approach regarding parasite control [2]. Among the different options, the simple but effective concept of “refugia” has gained in popularity [3]. Refugia strategies focus on maintaining a population of worms with high susceptibility by allowing some to survive either as adult worms in untreated animals or as infective larvae on the pasture. This helps to dilute the resistant worms, preventing them from becoming a dominant part of the overall worm population. For dewormers, the application of this concept means to stop using them systematically in all animals and to leave a significant proportion of a herd untreated. One question remains: can dairy farmers apply this approach easily and without losing too much productivity?     

Milk gains following deworming are not homogenous among dairy cows

To start addressing this question, we should realize that dairy cows are not equally susceptible to GIN infection and therefore will not have similar milk losses [4]. In fact, whether parasite infection will have negative or positive consequences on a given cow mainly depends on the ability of animals to resist or tolerate this exposure to parasites, and the number of parasites harboured by the host animals.

• Resilience to infection corresponds to a cow’s ability to withstand the pathogenic effects of parasites: the animal maintains its performance despite the presence of parasites. The underlying mechanisms are complex, and it is a genetic trait with low heritability [5]. 
• Infection resistance, i.e. immunity against gastrointestinal nematodes, is the ability of cows to control the number of parasites that settle in the digestive tract and their ability to reproduce. It can reduce the parasite load and limit faecal egg excretion. The resistance of an animal is determined by immunological mechanisms. Its development depends on the duration and extent of exposure to parasites [6]. The immunity of a herd of adult cows can therefore depend on the grazing management of heifers (one or two grazing seasons before first calving), and anthelmintic treatment programs for heifers (short- or long-term treatments). The immunity of an adult herd can therefore vary from one farm to another and from one cow to another.
• The number of gastrointestinal nematodes in the host cow, i.e. the parasite load or level of infection, varies from cow to cow, because parasites are not distributed evenly among a cattle population. Most animals have only a few parasites and a minority have many: depending on the study, 2% to 20% of dairy cows harbor more than 10 000 worms [7,8].  A recurrent problem is that there is no diagnostic tool for reliably quantify the parasite load in adult cattle. In fact, the number of eggs per gram of faeces (EPG) does not correlate well with the parasite load, and the pepsinogen assay, that is used in dairy heifers, is not easily interpreted in adult cattle.

Deciphering variability of milk gains after treatment is key to frame selective strategies

The existence of differences among dairy cows regarding the impact of GIN infection on their milk yield (MY) is an opportunity to develop selective treatment strategies. By understanding and characterizing this variability, it can be possible to accurately select cows likely to experience an increase in milk production after treatment and to discard the other cows.

Simple and effective criteria to select dairy cows are available

In a large field study performed in various regions in France that involved 120 grazing dairy cattle herds [9], researchers found that when treated at housing with eprinomectin injectable, it is the cows that calved during the grazing season that respond best, not the ones that calved during the previous winter and that are at the end of their lactation period. This corresponds to lactating dairy cows within 200 days in milk (DIM) at the time of treatment.

In addition,  the cows that calved during the grazing season and got off to a poor start with their lactation compared to other cows in the same parity in the herd that had the best milk production gains after treatment (~ 1 kg/cow/day). In the end, these are quite logical treatment criteria: it is the cow that calves during grazing that is "behind" the others when starting her lactation that needs to be treated.

When engaging in a selective treatment scheme, the choice of the formulation is important. Indeed, selective deworming is easily achieved with injectable formulations. Pour-on formulations are not considered suitable in dairy cattle as cows frequently lick themselves and their herd mates which makes it difficult to ensure proper dosing to individual animals.

Conclusion

The impact of gastrointestinal nematode infections on milk production varies between cows. Selective deworming is useful to retard the development of resistance and implies to treat only cows that have a good chance of improving their milk production. For treatment at housing with eprinomectin injectable, the cows to be selected are the ones that have calved during the grazing season (<200 DIM) and which are producing the lowest compared to the other cows in the herd in the same parity. For farmers, this approach can be attractive as it is readily available, cost effective and in line with current expectations regarding a better use of anthelmintics. 





Bibliography

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