First report of the cattle tick Rhipicephalus microplus resistant to ivermectin in Mexico

Area of Science:

• Veterinary Entomology
• Parasitology
• Antiparasitic Resistance

Background:

• Ivermectin is crucial for controlling Rhipicephalus microplus (cattle ticks).
• Tick resistance to ivermectin poses a significant threat to cattle farming in Mexico.
• Previous observations suggested potential ivermectin resistance in Yucatan tick populations.

Purpose of the Study:

• To investigate and confirm ivermectin resistance in Rhipicephalus microplus field populations from Yucatan, Mexico.
• To quantify the levels of resistance in different tick populations.
• To establish the presence of ivermectin resistance in Mexican tick populations for the first time.

Main Methods:

• Larval immersion test used on progeny of field-collected Rhipicephalus microplus.
• Dose-mortality regressions and probit analysis to estimate lethal concentrations (LC50, LC99).
• Resistance ratios (RR) calculated by comparing field populations to a susceptible reference strain.

Main Results:

• All three studied tick populations (SFDO, SPN, LUADY) exhibited significantly higher LC50 and LC99 values than the susceptible reference strain.
• Quantified resistance ratios (RR) indicated varying levels of ivermectin resistance, with SFDO showing the highest (RR50: 6.84-8.59, RR99: 54.17-87.86).
• No significant differences in resistance levels were observed between two collection times for any population, indicating stable resistance.

Conclusions:

• This study provides the first evidence of field populations of Rhipicephalus microplus resistant to ivermectin in Mexico.
• The identified resistance, with varying levels across populations, necessitates strategic management to mitigate economic losses in cattle farming.
• Continued monitoring and development of resistance management strategies are crucial for effective tick control.