# Drug Resistance in Filarial Parasites Does Not Affect Mosquito Vectorial Capacity

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## Abstract

**:**

## 1. Introduction

## 2. Results

#### 2.1. Establishment Load

^{2}= 19.772; p < 0.001; vector isolate: χ

^{2}= 8.694; p = 0.003; parasite isolate × vector isolate: χ

^{2}= 3.716; p = 0.294). The most striking effect we observed was that the Yazoo isolate had the lowest establishment load in both mosquito species. However, this seems to be an isolate-specific effect, and contrary to expectation (see Figure 1), there was no overall reduction in establishment load associated with infection by drug-resistant vs. drug-susceptible parasite isolates (Figure 2A).

#### 2.2. Vector Efficiency

^{2}= 7.434; p = 0.059; vector isolate: χ

^{2}= 77.422; p < 0.001; parasite isolate × vector isolate: χ

^{2}= 10.609; p = 0.014). However, again contrary to expectation (see Figure 1), there was no overall difference in vector efficiency between mosquitoes infected with drug-susceptible (i.e., GA-2 and Missouri) vs. drug-resistant (i.e., Metairie and Yazoo) parasite isolates. Vector species plays an important role in vector efficiency, with the Metairie isolate of Ae. albopictus having very low vector efficiency compared to the Liverpool Blackeye isolate of Ae. aegypti (Figure 2B).

#### 2.3. Survival to EIP

^{2}= 80.145; p < 0.001; vector isolate: χ

^{2}= 1.120; p = 0.290; parasite isolate × vector isolate: χ

^{2}= 31.512; p < 0.001). It is important to recognize that the term “parasite isolate” in this analysis also included the uninfected controls as one category. Thus, the global effects reported here incorporate both the differences in survival between infected and uninfected mosquitoes, as well as differences in survival between the different parasite isolates. One of the most interesting findings of this analysis was the exceptionally large reduction in survival between infected and uninfected Ae. aegypti mosquitoes (Figure 2C). However, in the case of Ae. albopictus, the magnitude of difference in survival between infected and uninfected mosquitoes was lower, with no significant difference in the case of GA-2 (Figure 2C). Overall, we found that both mosquito species had higher survival when infected by GA-2 compared to other parasite isolates (Figure 2C). However, contrary to expectation (see Figure 1) there was no overall difference in survival to EIP between mosquitoes infected with drug-susceptible (i.e., GA-2 and Missouri) vs. drug-resistant (i.e., Metairie and Yazoo) parasite isolates.

#### 2.4. Vectorial Capacity

_{TOT}= 0.275; 95% confidence interval (CI): 0.037, 0.467], vector efficiency (β

_{TOT}= 0.609; CI: 0.526, 0.700) and survival to EIP (β

_{TOT}= 0.304; CI: 0.219, 0.383).

## 3. Discussion

## 4. Materials and Methods

#### 4.1. Simulation Model

#### 4.2. Experimental Procedures

#### 4.2.1. Establishment Load

#### 4.2.2. Vector Efficiency

#### 4.2.3. Survival to EIP

#### 4.3. Study System

#### 4.4. Data Analyses

#### 4.4.1. Establishment Load

#### 4.4.2. Vector Efficiency

#### 4.4.3. Survival to EIP

#### 4.4.4. Vectorial Capacity

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Predicted differences in mosquito vectorial capacity when infected with drug-susceptible (blue symbols) vs. drug-resistant (red symbols) isolates of D. immitis. The heat map on the left shows the interacting effects of establishment load (i.e., initial load of establishing parasites) and vector efficiency (proportion of establishing parasites that develop to L3) on vectorial capacity. Vectorial capacity was calculated as the combined product of establishment load, vectorial efficiency and probability of survival to the extrinsic incubation period (EIP). The latter was calculated using a logistic function dependent on predicted L3 parasite load at EIP. We assume that drug resistance will lead to a reduction in establishment load (due to reduced parasite fitness) and increased vector efficiency (due to reduced pathogenicity). These parameters interact to produce a complex set of expectations, with either significant (filled symbols) or non-significant (non-filled symbols) differences between vectorial capacity between drug-susceptible (blue symbols) or drug-resistant (red symbols) parasite isolates (

**A**–

**H**; see text for details).

**Figure 2.**Effects of parasite isolate on parasite transmission risk in Ae. aegypti Liverpool Blackeye (

**A**–

**D**, left side of graphics) and Ae. albopictus (

**A**–

**D**, right side of graphics). Parasite isolates include two drug-susceptible (GA-2 and Missouri; blue symbols) and drug-resistant (Metairie and Yazoo; red symbols) isolates. Separate graphs are given for: (

**A**) establishment load measured as the number of viable microfilaria (mf) counted immediately after infection; (

**B**) vector efficiency measured as the number of infective L3 parasites developing per mf in mosquitoes surviving to the extrinsic incubation period (EIP); (

**C**) survival to the EIP measured as the proportion of surviving mosquitoes; (

**D**) vectorial capacity measured as the number of infective L3 parasites developing per mf, accounting for the probability of survival to EIP. Least square means standard error are error bars and least square mean values from the GLMER models show significance when they do not share a letter.

**Figure 3.**Causal pathways driving D. immitis vectorial capacity in Ae. mosquitoes. (

**A**) The final structural equation model for vectorial capacity (Vect. Cap.) showing the direct and indirect effects of establishment load (Est. load), vector efficiency (Vect. Eff.) and survival to EIP (Surv. EIP). In the path diagram, circles indicate variables (with model r2 values, if applicable). Arrows indicate significant positive (black lines) or negative (red lines) relationships, with standardized coefficients indicated in bold. (

**B**–

**F**) Partial residual plots of the predictor (x axis) and response (y axis) variables associated with all significant SEM paths. Individual points represent values obtained for each experimental replicate for Ae. aegypti (circles) and Ae. albopictus (squares) mosquitoes infected by either parasite isolates that were drug susceptible (GA-2 and Missouri; blue symbols) or drug resistant (Metairie and Yazoo; red symbols) with respective 95% confidence intervals (CIs; gray bands). (

**G**) Indirect (white bars), direct (gray bars) and total (circles) effects of establishment load, vector efficiency and survival to EIP on vectorial capacity. Error bars are 95% CIs for the estimated total effects.

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**MDPI and ACS Style**

Neff, E.; Evans, C.C.; Jimenez Castro, P.D.; Kaplan, R.M.; Dharmarajan, G.
Drug Resistance in Filarial Parasites Does Not Affect Mosquito Vectorial Capacity. *Pathogens* **2021**, *10*, 2.
https://doi.org/10.3390/pathogens10010002

**AMA Style**

Neff E, Evans CC, Jimenez Castro PD, Kaplan RM, Dharmarajan G.
Drug Resistance in Filarial Parasites Does Not Affect Mosquito Vectorial Capacity. *Pathogens*. 2021; 10(1):2.
https://doi.org/10.3390/pathogens10010002

**Chicago/Turabian Style**

Neff, Erik, Christopher C. Evans, Pablo D. Jimenez Castro, Ray M. Kaplan, and Guha Dharmarajan.
2021. "Drug Resistance in Filarial Parasites Does Not Affect Mosquito Vectorial Capacity" *Pathogens* 10, no. 1: 2.
https://doi.org/10.3390/pathogens10010002