# Elucidating the Mechanism of Trypanosoma cruzi Acquisition by Triatomine Insects: Evidence from a Large Field Survey of Triatoma infestans

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

_{bite})

^{S}

_{bite}is the probability of acquiring the parasite during any given stage (Note that when S ≤ 5 and p is sufficiently small, Equation (1) can be approximated by the linear function P(S) ≈ pS. For example, if p = 0.01 and S = 5, then P(S) = 0.049 and pS = 0.05. This is why we hypothesized that the stage-prevalence relationship might appear roughly linear if the Bites Hypothesis were correct). To test the Bites Hypothesis, we used nonlinear least squares regression to fit Equation (1) to the observed data.

_{blood})

^{B}

^{(S)}

_{blood}is the probability of acquiring the parasite after ingesting 1 mg of blood. The primary difference between Equations (1) and (2) is that the exponent of Equation (1) increases linearly with stage, whereas the exponent of Equation (2) increases nonlinearly. We estimated B(S) from T. infestans feeding data in a prior study [4]. Our estimates were B(1) = 7.1 mg, B(2) = 25.4 mg, B(3) = 87.8 mg, B(4) = 302.8 mg, and B(5) = 1072.8.0 mg (see Appendix A for derivation). We then used nonlinear least squares regression, with B(S) fixed at these values, to fit Equation (2) and estimate p

_{blood}given the observed data [4].

## 3. Results

_{bite}= 0.059, indicating a 5.9% probability of acquiring the parasite during any given stage. Visual inspection revealed that the Bites Hypothesis model fit the data well (Figure 4), and the model had an AIC of 11,672. In contrast, the Blood Hypothesis regression model was a very poor visual fit with a significantly higher (i.e., worse) AIC of 12,302. The observations of mean prevalence versus stage did not exhibit the excess prevalence in early nymphs that would occur if coprophagy were a primary driver of infection.

## 4. Discussion

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Appendix A

**Table A1.**Blood meal sizes observations for laboratory reared insects [4] and estimates of cumulative blood ingested by captured wild Triatoma infestans nymphs.

Stage | Median Quantity of Blood Ingested during this Stage (mg) | Estimated Cumulative Blood Ingested during Lifetime Prior to Capture (mg) |
---|---|---|

First instar | 7.1 | 7.1 |

Second instar | 18.3 | 25.4 |

Third instar | 62.4 | 87.8 |

Fourth instar | 215.0 | 302.8 |

Fifth instar | 770.0 | 1072.8 |

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**Figure 1.**Hypothetical shapes of the relationship between triatomine stage and prevalence of infection with Trypanosoma cruzi under three hypotheses. (

**A**) Acquisition of the parasite depends upon the quantity of blood ingested (Blood Hypothesis); (

**B**) Parasite acquisition depends upon the number of exposure opportunities (Bites Hypothesis); (

**C**) Early instar nymphs frequently acquire the parasite via coprophagy, while older instars acquire the parasite at a lower rate via ingestion of blood from mammalian hosts (Coprophagy Hypothesis). Note: the scale of the vertical axis is for illustrative purpose only and does not correspond to actual data; rather, panels (

**A**) through (

**C**) demonstrate three different theoretical shapes that could each produce 25% prevalence in fifth instars.

**Figure 2.**Distribution of developmental stage and Trypanosoma cruzi infection status in Triatoma infestans from nine households in Arequipa, Peru. Shown are the nine infected sites with the largest number of captured insects. White and black bars and left vertical axis labels: total number of insects. Black circles and right vertical axis labels: fraction of insects with T. cruzi.

**Figure 3.**Fraction of nymph Triatoma infestans infected with Trypanosoma cruzi at 188 sites with at least one infected insect. Black circles represent individual sites; sizes of circles are proportional to the logarithm of the number of insects captured. The bottom edges of the gray triangles show the mean fraction of insects with T. cruzi across all infected sites.

**Figure 4.**Fraction of triatomines infected with Trypanosoma cruzi as a function of stage. Filled circles: mean values observed in 15,252 insects captured at 188 infected sites. Dashed line: Best-fit regression model under the Blood Hypothesis that assumed an equal probability of infection with each milligram of blood ingested (Pr(infection) = 1 − (1 − 0.00036)^Nblood, where Nblood is the number of mg of blood ingested). Solid line: Best-fit regression model under the Bites Hypothesis that assumed an equal probability of infection during any given stage (Pr(infection) = 1 − (1 − 0.059)^N).

**Table 1.**Trypanosoma cruzi infection status by developmental stage, for 15,252 triatomines captured in 188 infected colonies in Arequipa, Peru.

Stage | Number Infected/Total Insects (%) |
---|---|

Second instar | 125/1037 (12.1) |

Third instar | 512/3352 (15.3) |

Fourth instar | 687/3310 (20.8) |

Fifth instar | 1060/3838 (27.6) |

Adult | 1326/3715 (35.7) |

Male | 853/2225 (38.3) |

Female | 473/1490 (31.7) |

Total | 3710/15,252 (24.3) |

**Table 2.**Results of regression modeling to test two hypotheses of T. cruzi transmission to T. infestans.

Model Name | Fitted Parameter | Best-Fit Parameter (95% Confidence Interval) | AIC |
---|---|---|---|

Bites Hypothesis | p_{bite}: probability of parasite acquisition during any given stage | p_{bite} = 0.059 (0.057 to 0.061) | 11,672 |

Blood Hypothesis | p_{blood}: probability of parasite acquisition with each milligram of blood ingested | p_{blood} = 0.00036 (0.00035 to 0.00038) | 112,302 |

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**MDPI and ACS Style**

Tustin, A.W.; Castillo-Neyra, R.; Tamayo, L.D.; Salazar, R.; Borini-Mayorí, K.; Levy, M.Z. Elucidating the Mechanism of *Trypanosoma cruzi* Acquisition by Triatomine Insects: Evidence from a Large Field Survey of *Triatoma infestans*. *Trop. Med. Infect. Dis.* **2020**, *5*, 87.
https://doi.org/10.3390/tropicalmed5020087

**AMA Style**

Tustin AW, Castillo-Neyra R, Tamayo LD, Salazar R, Borini-Mayorí K, Levy MZ. Elucidating the Mechanism of *Trypanosoma cruzi* Acquisition by Triatomine Insects: Evidence from a Large Field Survey of *Triatoma infestans*. *Tropical Medicine and Infectious Disease*. 2020; 5(2):87.
https://doi.org/10.3390/tropicalmed5020087

**Chicago/Turabian Style**

Tustin, Aaron W., Ricardo Castillo-Neyra, Laura D. Tamayo, Renzo Salazar, Katty Borini-Mayorí, and Michael Z. Levy. 2020. "Elucidating the Mechanism of *Trypanosoma cruzi* Acquisition by Triatomine Insects: Evidence from a Large Field Survey of *Triatoma infestans*" *Tropical Medicine and Infectious Disease* 5, no. 2: 87.
https://doi.org/10.3390/tropicalmed5020087