Persistence of Vertically Transmitted Parasite Strains which Protect against More Virulent Horizontally Transmitted Strains

Dhirasakdanon, Thanate; Thieme, Horst R.

doi:10.1142/9789814261265_0008

Cited by 8 publications

(12 citation statements)

References 15 publications

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“…This phenomenon has also been observed in the simulations of a discrete-time model [31]. The second (also observed in numerical simulations of somewhat different models [29,31]) is that the vertically transmitted strain that would go extinct by itself can persist by protecting the host against the more virulent horizontally transmitted strain [9].…”

Section: Discussionsupporting

confidence: 58%

“…This phenomenon has also been observed in the simulations of a discrete-time model [31]. The second is that the vertically transmitted strain that would go extinct by itself can persist by protecting the host against the more virulent horizontally transmitted strain [9]. (This had already been observed in simulations [29,31].…”

Section: Introductionsupporting

confidence: 55%

“…One class of mechanisms is the presence of multiple hosts and/or multiple parasites or multiple strains of one parasite [3,4,20,24,26,27,28,44,45]. This paper continues the investigation of an endemic model with one host and two strains of one parasite (or two different parasites) where one strain is exclusively vertically transmitted (the VT strain) and the other strain is horizontally (and possibly also vertically) transmitted (the HT strain) [9,13]. A similar scenario in a somewhat different model has been investigated in [1] (see Section 8 for a more detailed comparison).…”

Section: Introductionmentioning

confidence: 97%

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Stability of the Endemic Coexistence Equilibrium for One Host and Two Parasites

Dhirasakdanon

Thieme

2010

Math. Model. Nat. Phenom.

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Abstract. For an SI type endemic model with one host and two parasite strains, we study the stability of the endemic coexistence equilibrium, where the host and both parasite strains are present. Our model, which is a system of three ordinary differential equations, assumes complete crossprotection between the parasite strains and reduced fertility and increased mortality of infected hosts. It also assumes that one parasite strain is exclusively vertically transmitted and cannot persists just by itself. We give several sufficient conditions for the equilibrium to be locally asymptotically stable. One of them is that the horizontal transmission is of density-dependent (mass-action) type. If the horizontal transmission is of frequency-dependent (standard) type, we show that, under certain conditions, the equilibrium can be unstable and undamped oscillations can occur. We support and extend our analytical results by numerical simulations and by two-dimensional plots of stability regions for various pairs of parameters.

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Section: Discussionsupporting

confidence: 58%

Section: Introductionsupporting

confidence: 55%

Section: Introductionmentioning

confidence: 97%

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Stability of the Endemic Coexistence Equilibrium for One Host and Two Parasites

Dhirasakdanon

Thieme

2010

Math. Model. Nat. Phenom.

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“…Increased levels of virulence have been shown, both theoretically and experimentally, to favor horizontal transmission but to reduce the probability of vertical transmission, as virulence is generally negatively correlated to host fitness [1,2,4]. Thieme and collaborators [40,41,42] studied a model for a directly-transmitted pathogen in which two competing strains were able to coexist in a single host population, but in addition to the direct transmission, the coexistence required that one strain be transmitted purely vertically and that both strains induce either a reduced fertility or additional mortality in hosts. In our model, we did not consider infected hosts to be submitted to an additive mortality rate due to infection compared to susceptible hosts, as is usually done in models of pathogen evolution.…”

Section: Discussionmentioning

confidence: 99%

The role of the ratio of vector and host densities in the evolution of transmission modes in vector-borne diseases. The example of sylvatic Trypanosoma cruzi

Pelosse

Kribs-Zaleta

2012

Journal of Theoretical Biology

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Pathogens may use different routes of transmission to maximize their spread among host populations. Theoretical and empirical work conducted on directly-transmitted diseases suggest that horizontal (i.e., through host contacts) and vertical (i.e., from mother to offspring) transmission modes trade off, on the ground that highly virulent pathogens, which produce larger parasite loads, are more efficiently transmitted horizontally, and that less virulent pathogens, which impair host fitness less significantly, are better transmitted vertically. Other factors than virulence such as host density could also select for different transmission modes, but they have barely been studied. In vector-borne diseases, pathogen transmission rate is strongly affected by host-vector relative densities and by processes of saturation in contacts between hosts and vectors. The parasite Trypanosoma cruzi which is transmitted by triatomine bugs to several vertebrate hosts is responsible for Chagas' disease in Latin America. It is also widespread in sylvatic cycles in the southeastern U.S. in which it typically induces no mortality costs to its customary hosts. Besides classical transmission via vector bites, alternative ways to generate infections in hosts such as vertical and oral transmission (via the consumption of vectors by hosts) have been reported in these cycles. The two major T. cruzi strains occurring in the U.S. seem to exhibit differential efficiencies at vertical and classical horizontal transmissions. We investigated whether the vector-host ratio affects the outcome of the competition between the two parasite strains using an epidemiological two-strain model considering all possible transmission routes for sylvatic T. cruzi. We were able to show that the vector-host ratio influences the evolution of transmission modes providing that oral transmission is included in the model as a possible transmission mode, that oral and classical transmissions saturate at different vector-host ratios and that the vector-host ratio is between the two saturation thresholds. Even if data on parasite strategies and demography of hosts and vectors in the field are crucially lacking to test to what extent the conditions needed for the vector-host ratio to influence evolution of transmission modes are plausible, our results open new perspectives for understanding the specialization of the two major T. cruzi strains occurring in the U.S. Our work also provides an original theoretical framework to investigate the evolution of alternative transmission modes in vector-borne diseases.

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“…Most theoretical studies of T. cruzi transmission have heretofore been limited to domestic transmission involving humans, including by transfusion [36,48], but modelling has been used to study many other vector-borne diseases, including vertical transmission in dengue vectors [14]. Variable adaptation to vertical transmission has been studied in infections transmitted purely horizontally [1,9,15], including Dhirasakdanon and Thieme's [9] finding of persistence in vertically transmitted parasite strains that provide cross-immunity against infection by more virulent strains transmitted purely horizontally, a situation not unlike the competition hypothesized between T. cruzi I and IV here. The contact process(es) that drive the transmission of pathogens between two distinct populations, however, saturate in one or the other population, as a function of the ratio of the two densities.…”

Section: Introductionmentioning

confidence: 99%

The role of adaptations in two-strain competition for sylvaticTrypanosoma cruzitransmission

Kribs-Zaleta¹,

Mubayi²

2012

Journal of Biological Dynamics

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This study presents a continuous-time model for the sylvatic transmission dynamics of two strains of Trypanosoma cruzi enzootic in North America, in order to study the role that adaptations of each strain to distinct modes of transmission (classical stercorarian transmission on the one hand, and vertical and oral transmission on the other) may play in the competition between the two strains. A deterministic model incorporating contact process saturation predicts competitive exclusion, and reproductive numbers for the infection provide a framework for evaluating the competition in terms of adaptive trade-off between distinct transmission modes. Results highlight the importance of oral transmission in mediating the competition between horizontal (stercorarian) and vertical transmission; its presence as a competing contact process advantages vertical transmission even without adaptation to oral transmission, but such adaptation appears necessary to explain the persistence of (vertically-adapted) T. cruzi IV in raccoons and woodrats in the southeastern United States.

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Persistence of Vertically Transmitted Parasite Strains which Protect against More Virulent Horizontally Transmitted Strains

Cited by 8 publications

References 15 publications

Stability of the Endemic Coexistence Equilibrium for One Host and Two Parasites

Stability of the Endemic Coexistence Equilibrium for One Host and Two Parasites

The role of the ratio of vector and host densities in the evolution of transmission modes in vector-borne diseases. The example of sylvatic Trypanosoma cruzi

The role of adaptations in two-strain competition for sylvaticTrypanosoma cruzitransmission

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