Managing anthelmintic resistance in Parascaris spp.: A modelling exercise.

The research article presents a study on how managing the rate of anthelmintic resistance in the parasitic species Parascaris can be achieved through adjustments in treatment strategies.

Objective of the Research

• The main aim of the study was to modify a previously described model for the dynamics of the parasitic stages of Parascaris spp. This model now included details about eggs outside the host and the genetics of anthelmintic resistance. The model was then used to address three major questions concerning the development of resistance in the parasite.

Questions Addressed

• The first question probed the sustainability of the common practice of treating foals monthly during their first year of life. It asked if a year-round (12 treatments/year) treatment strategy is an effective or sustainable approach to managing the parasitic infection.
• The second question asked whether the timing of treatments could affect the development of resistance. In other words, it investigated if some treatments enhance resistance more strongly than others. This sought to understand if different time points in the parasite’s lifecycle might be more vulnerable to treatments.
• The last question sought to understand the effectiveness of the current recommended strategy of specifically targeting ascarid infections in young foals with two treatments in the first five months.

Findings and Conclusion

• The research findings revealed that 12 treatments at monthly intervals led to the development of resistance to all types of anthelmintics used, suggesting that a high frequency of treatment may not be sustainable.
• The study also determined that the timing of treatments can influence the development of resistance, with treatments at three and four months being more selective than treatments at other times.
• The study suggested that treatments active against migrating third stage larvae were more selective for resistance than those killing only intestinal worms.
• The results indicated that reducing the number of treatments to two, given at 2 and 5 months, slows down the development of resistance. This is because it allows a slight contribution from susceptible genotype worms to future generations.
• Resistance developed more rapidly if the interval between treatments was shortened. This shows the importance of allowing some susceptible worms to reach patency before administering the second treatment.
• Under a reduced treatment regimen with a designated ‘refugium’ of susceptible individuals, the use of effective actives in combination appears to delay the development of resistance.
• The study concluded that the model provides insights for developing more sustainable drug use strategies. It has also highlighted critical questions that need to be addressed in future research.