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Resistivity01:22

Resistivity

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When a voltage is applied to a conductor, an electrical field is generated, and charges in the conductor feel the force due to the electrical field. The current density that results depends on the electrical field and the properties of the material. In some materials, including metals at a given temperature, the current density is approximately proportional to the electrical field. In these cases, the current density can be modeled as:
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Resistance01:19

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When a current moves through any conductor, the conductor causes some level of difficulty for the current to flow. The measure of that difficulty is known as the resistance of the material and is represented by R. Every material has its own resistance. In the case of conductors, heat is emitted whenever a current passes through them. Resistance depends on the resistivity of the material. Resistivity is a characteristic of the material used to fabricate electrical components, whereas the...
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Equivalent Resistance01:16

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In circuit analysis, situations often arise where resistors are neither in series nor parallel configurations. To tackle such scenarios, three-terminal equivalent networks like the wye (Y) (Figure 1 (a)) or tee (T) and delta (Δ) (Figure 1 (b)) or pi (π) networks come into play. These networks offer versatile solutions and are frequently encountered in various applications, including three-phase electrical systems, electrical filters, and matching networks.
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Resistance and Conductance01:25

Resistance and Conductance

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A conductor's DC resistance at a given temperature is influenced by its resistivity, length, and cross-sectional area. Resistivity is an inherent property of the conductor material, with annealed copper serving as the international standard for measurement. For instance, the resistivity of hard-drawn aluminum at 20 degrees Celsius is 61% of the standard conductivity of annealed copper.
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Rolling Resistance01:21

Rolling Resistance

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When a solid cylinder rolls steadily on a rigid surface, the normal force applied by the surface on the cylinder is perpendicular to the tangent at the contact point. However, since no materials are entirely rigid, the surface's reaction to the cylinder involves a range of normal pressures.
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Vascular resistance is a critical concept in understanding blood flow dynamics in the circulatory system. It refers to the resistance that blood encounters as it flows through the blood vessels. This resistance is a key factor in determining blood pressure and cardiac workload.
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Field evaluation of a novel tick control strategy aiming a reduced selection for drug resistance: A case for "host-level chemical mosaic".

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Susceptibility of Rhipicephalus microplus tick populations from western Pará to different acaricides.

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Resistance to ivermectin, doramectin and moxidectin in Rhipicephalus microplus isolates from Rio Grande do Sul,

Marcelo Becker1, Tiago Gallina2, Jose Reck1

  • 1Instituto de Pesquisas Veterinárias Desidério Finamor (IPVDF), 6000. Eldorado do Sul, Estrada do Conde 92990-000, Brazil.

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PubMed
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Cattle tick resistance to macrocyclic lactones is a growing problem in Brazil. This study found significant side-resistance between ivermectin, doramectin, and moxidectin in Rhipicephalus microplus ticks, impacting control strategies.

Keywords:
AcaricideBioassaysCattleMacrocyclic lactonesSide-resistance

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Area of Science:

  • Veterinary Parasitology
  • Entomology
  • Drug Resistance Studies

Background:

  • Rhipicephalus microplus is a major cattle parasite causing economic losses in Brazil.
  • Resistance to macrocyclic lactones in R. microplus has been documented in Rio Grande do Sul since 2001.
  • Side-resistance among macrocyclic lactones complicates parasite control and resistance management.

Purpose of the Study:

  • To evaluate the occurrence of side-resistance among ivermectin, doramectin, and moxidectin.
  • To assess R. microplus populations from various locations in Rio Grande do Sul, Brazil.
  • To determine the correlation of resistance among these macrocyclic lactone drugs.

Main Methods:

  • Collection of engorged female ticks from 30 premises in Rio Grande do Sul.
  • Larval progeny tested using the larval immersion test (LIT) for acaricide efficacy.
  • Probit analysis to determine LC50 values and resistance ratios (RR); Fisher's test for resistance correlation.

Main Results:

  • 57% of isolates were resistant to ivermectin (RR: 2.06-17.81).
  • 47% of isolates were resistant to doramectin (RR: 2.07-19.03).
  • 30% of isolates were resistant to moxidectin (RR: 2.00-17.28).
  • Significant associations were found between resistance to ivermectin and doramectin (p=0.02), ivermectin and moxidectin (p=0.015), and doramectin and moxidectin (p=0.019).

Conclusions:

  • This study is the first to demonstrate side-resistance among macrocyclic lactones in R. microplus from southern Brazil.
  • The findings highlight the importance of laboratory bioassays for detecting acaricide resistance.
  • Side-resistance among macrocyclic lactones significantly limits available treatment options for cattle tick control.