In this paper, an insect-parasite-host model with logistic growth of triatomine bugs is formulated to study the transmission between hosts and vectors of the Chagas disease by using dynamical system approach. We derive the basic reproduction numbers for triatomine bugs and Trypanosoma rangeli as two thresholds. The local and global stability of the vector-free equilibrium, parasite-free equilibrium and parasite-positive equilibrium is investigated through the derived two thresholds. Forward bifurcation, saddle-node bifurcation and Hopf bifurcation are proved analytically and illustrated numerically. We show that the model can lose the stability of the vector-free equilibrium and exhibit a supercritical Hopf bifurcation, indicating the occurrence of a stable limit cycle. We also find it unlikely to have backward bifurcation and Bogdanov-Takens bifurcation of the parasite-positive equilibrium. However, the sustained oscillations of infected vector population suggest that Trypanosoma rangeli will persist in all the populations, posing a significant challenge for the prevention and control of Chagas disease.
Citation: Lin Chen, Xiaotian Wu, Yancong Xu, Libin Rong. Modelling the dynamics of Trypanosoma rangeli and triatomine bug with logistic growth of vector and systemic transmission[J]. Mathematical Biosciences and Engineering, 2022, 19(8): 8452-8478. doi: 10.3934/mbe.2022393
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In this paper, an insect-parasite-host model with logistic growth of triatomine bugs is formulated to study the transmission between hosts and vectors of the Chagas disease by using dynamical system approach. We derive the basic reproduction numbers for triatomine bugs and Trypanosoma rangeli as two thresholds. The local and global stability of the vector-free equilibrium, parasite-free equilibrium and parasite-positive equilibrium is investigated through the derived two thresholds. Forward bifurcation, saddle-node bifurcation and Hopf bifurcation are proved analytically and illustrated numerically. We show that the model can lose the stability of the vector-free equilibrium and exhibit a supercritical Hopf bifurcation, indicating the occurrence of a stable limit cycle. We also find it unlikely to have backward bifurcation and Bogdanov-Takens bifurcation of the parasite-positive equilibrium. However, the sustained oscillations of infected vector population suggest that Trypanosoma rangeli will persist in all the populations, posing a significant challenge for the prevention and control of Chagas disease.
Lyme disease accounts for over 90
Ticks are capable of moving only very short distances independently, so their fast and large scale spatial spread cannot be attributed solely to their own mobility. Rather, large-scale changes in tick distribution arise as a consequence of the movement of ticks by the vertebrate hosts to which they attach while feeding (see, e.g., [4,5,10,15]). Among such hosts are, in the order of the distances they can move, white-footed mice Peromyscus leucopus, white-tailed deer Odocoileus virginianus, and some migratory birds. Mice can be infected by this bacterium and therefore can transmit the pathogen, and can also transport the tick nymphs. In [4], a reaction diffusion system is proposed to model the advance of the natural infection cycle mediated by the white-footed mouse. Although white-tailed deer diffusion is also mentioned in the model, since the deer cannot be infected and accordingly do not transmit the bacterium the focus of [4] is on the transmission dynamics, the role of deer diffusion in the spatial spread of the pathogen is not discussed in detail in [4]. In relation to birds, in addition to the works [3,18], there have been some works that quantitatively model the role of bird migration in the tick's range expansion, see, e.g., [23].
This paper focuses on the role of white-tailed deer in spreading the ticks. Over the past 50 years, white-tailed deer populations have undergone explosive population growth due to reversion of agricultural lands to forest and restrictions on hunting. This expanding deer population is believed to have facilitated blacklegged tick expansion throughout the Northeast and Midwest [2]. To understand this, we first point out an important difference between birds and deer in transporting I. scapularis and B. burgdorferi. On the one hand, birds carrying the infected immature stages of the tick are capable of traveling longer distances than deer. On the other hand, if immature ticks dropping from birds are to establish a new population they must survive one or two moults and then find a mate, which will be unlikely if they are dropped far from existing populations. In contrast, during fall deer will be carrying numerous already-mated female ticks, each of which becomes engorged with blood while on the deer and then falls to the ground ready to lay approximately 2000 eggs that can form the basis of a new tick population at that location. This observation seems to suggest that deer play a more important role in the tick's range expansion in regions inhabited by white-tailed deer.
In this study we use a spatial model to quantitatively investigate the role of white-tailed deer dispersal in the spatial spread of I. scapularis (and hence B. burgdorferi). Our model combines age structure with the dispersal of deer leading to a system with two time delays and spatial nonlocality resulting from the dispersal of the deer when the adult ticks are attached to them enjoying blood meals. We will begin, in the next section, with a detailed derivation of the model.
To assess the rate at which deer can transport blacklegged ticks into new areas, we develop a differential equation model with spatial effects that describes the stage-structured tick population and its transport by deer. Blacklegged ticks typically undergo a 2-year life cycle in which the larvae quest for a host (typically a small mammal or bird), and if successful feed for several days, drop back to the ground, and later moult into a nymph. The nymph then quests, feeds and moults -again typically on a small mammal. The final adult life stage (which is male or female) then quests and feeds (typically on a deer), falls to the ground when fully engorged and then produces approximately
The mouse population (which feeds the immature ticks) and the deer population (which feeds the adult ticks) are assumed to be homogeneous and constant over time in both the tick-infested and tick-free regions. Mouse home ranges are much smaller than those of deer, so the only significant movement of ticks is by deer transporting adult females while they feed. Because of this, and for simplicity, we assume that larvae and nymphs do not disperse. Since the average time a tick spends attached to a deer is around one week, the relevant deer movements are assumed to be those undertaken in the course of each deer's normal home range activity, rather than long-distance directional movements associated with natal dispersal or seasonal migration.
Consider a spatial domain
{∂L(x,t)∂t=br4e−d4τ1Af(x,t−τ1)−d1L(x,t)−r1L(x,t),∂N(x,t)∂t=r1g(L(x,t))−d2N(x,t)−r2N(x,t),∂Aq(x,t)∂t=r2N(x,t)−d3Aq(x,t)−r3Aq(x,t),∂Af(x,t)∂t=r32∫Ωk(x,y)e−d3τ2Aq(y,t−τ2)dy−r4Af(x,t)−d4Af(x,t), | (1) |
where the parameters are defined in Table 1. The table also gives the values of the parameters for the stage-structured components of the model which were all, except for
Parameters | Meaning | Value |
Birth rate of tick | ||
average time that a questing larvae needs to feed and moult | ||
average time that a questing nymph needs to feed and moult | ||
average time that a questing adult needs to successfully attach to a deer | ||
Proportion of fed adults that can lay eggs | 0.03 | |
per-capita death rate of larvae | 0.3 | |
per-capita death rate of nymphs | 0.3 | |
per-capita death rate of questing adults | 0.1 | |
per-capita death rate of fed adults | 0.1 | |
average time between last blood feeding and hatch of laid eggs | 20 days | |
average time tick is attached to a deer |
The structure of system (1) can be visualized with the help of the diagram in Fig. 1.
A very important aspect of model (1) is the term with the integral, which models the transport of adult ticks by deer. Note that
(∂∂t+∂∂a)udeer(x,t,a)=−d3udeer(x,t,a)+D∇2udeer(x,t,a),forx∈Ω,a∈(0,τ2),t>0, | (2) |
where
Adeer(x,t)=∫τ20udeer(x,t,a)da. | (3) |
Differentiating (3) and using (2) gives
∂Adeer(x,t)∂t=udeer(x,t,0)−udeer(x,t,τ2)−d3Adeer(x,t)+D∇2Adeer(x,t). | (4) |
The rate at which ticks drop off the deer after feeding is
uξdeer(x,a)=udeer(x,a+ξ,a)ed3a. | (5) |
Differentiating with respect to
∂uξdeer(x,a)∂a=D∇2uξdeer(x,a),x∈Ω. |
This is the heat equation, and its solution can be expressed in the form
uξdeer(x,a)=∫ΩK(x,y,a)uξdeer(y,0)dy | (6) |
where the Green's function
∂K(x,y,a)∂a=D∇2xK(x,y,a),K(x,y,0)=δ(x−y) | (7) |
and the boundary conditions to which the deer are subjected at
udeer(x,t,τ2)=e−d3τ2∫ΩK(x,y,τ2)udeer(y,t−τ2,0)dy. |
But
udeer(x,t,0)=r3Aq(x,t). |
Therefore
udeer(x,t,τ2)=e−d3τ2∫ΩK(x,y,τ2)r3Aq(y,t−τ2)dy. |
After the insertion of a factor
k(x,y)=K(x,y,τ2). |
The kernel
k(x,y)=K(x,y,τ2)=1l[1+∞∑n=1{cosnπl(x−y)+cosnπl(x+y)}e−D(nπ/l)2τ2]. |
If
k(x,y)=1l∞∑n=1{cosnπl(x−y)−cosnπl(x+y)}e−D(nπ/l)2τ2. |
We are interested mainly in the case when
k(x,y)=Γ(x−y),whereΓ(z):=1√4Dτ2πe−z24Dτ2 | (8) |
We point out that, just recently, starting from a version of the McKendrick-von Foerster equation without spatial effects but with temporal periodicity, Liu et al [14] also derived a periodic model with age structure for a tick population. A threshold dynamics result is obtained for the model in [14].
To prevent the tick population from increasing to unrealistic levels, density dependence is incorporated into model (1) through a simple nonlinear relationship between questing larvae and questing nymphs. The biological basis for this relationship is that there should be an upper limit to the number of larvae that the mouse population is able to feed -an equivalent relationship was assumed in [21]. This leads us to propose the following expression for the function
g(L)={NcapLcapLforL∈[0,Lcap],NcapforL∈[Lcap,∞). | (9) |
The function
g(L)=Ncapk2Lk1+k2L=NcapLk1/k2+L=NcapLh+L, | (10) |
which is smooth and yet captures the main features of the function given by (9). In the remainder of this paper, we always use (10) for
For the majority of this paper we have in mind model (1) for
Associated to (1) are the following biologically and mathematically meaningful initial conditions:
{L(x,0),N(x,0)are continuous forx∈ΩwithL(x,0)≥0,N(x,0)≥0;Aq(x,s)is continuous for(x,s)∈Ω×[−τ2,0]withAq(x,s)≥0;Af(x,s)is continuous for(x,s)∈Ω×[−τ1,0]withAf(x,s)≥0. | (11) |
Using the method of steps, one can easily see that the initial value problem (1)-(11) has a unique solution for
{L(x,t)=L(x,0)e−(d1+r1)t+br4e−d4τ1∫t0Af(x,s−τ1)e(d1+r1)(s−t)ds,N(x,t)=N(x,0)e−(d2+r2)t+r1Ncap∫t0L(x,s)h+L(x,s)e(d2+r2)(s−t)ds,Aq(x,t)=Aq(x,0)e−(d3+r3)t+r2∫t0N(x,s)e(d3+r3)(s−t)ds,Af(x,t)=Af(x,0)e−(d4+r4)t+r32e−d3τ2∫t0∫Ωk(x,y)Aq(y,s−τ2)e(d4+r4)(s−t)dyds. | (12) |
Let
Next, we prove an important property of the kernel
Proposition 1. If
∫Ωk(x,y)dx=1,forally∈Ω. |
If
∫Ωk(x,y)dx<1,forally∈Ω. |
Proof. When
∂∂a∫ΩK(x,y,a)dx=D∫Ω∇2xK(x,y,a)dx=D∫∂Ω∇xK(x,y,a)⋅ndS=0 |
where
∫Ωk(x,y)dx=∫ΩK(x,y,τ2)dx=∫ΩK(x,y,0)dx=∫Ωδ(x−y)dx=1. |
In the case of homogeneous Dirichlet boundary conditions we have
Next, we show that the solution is bounded for
∂N(x,t)∂t≤r1Ncap−(d2+r2)N(x,t). |
This implies that
lim supt→∞N(x,t)≤r1Ncapd2+r2,for allx∈Ω, |
proving boundedness of
Irrespective of the domain
{∂u1(x,t)∂t=br4e−d4τ1u4(x,t−τ1)−(d1+r1)u1(x,t),∂u2(x,t)∂t=r1Ncaphu1(x,t)−(d2+r2)u2(x,t),∂u3(x,t)∂t=r2u2(x,t)−(d3+r3)u3(x,t),∂u4(x,t)∂t=r32∫Ωk(x,y)e−d3τ2u3(y,t−τ2)dy−(d4+r4)u4(x,t). | (13) |
Consider the case
R0=br4e−d4τ1d1+r1⋅(r1/h)Ncapd2+r2⋅r2d3+r3⋅(r3/2)e−d3τ2d4+r4=Ncapb2he−(d3τ2+d4τ1)4∏i=1ridi+ri. | (14) |
From the biological interpretation of
Substituting the ansatz
{λψ1(x)=br4e−d4τ1e−λτ1ψ4(x)−(d1+r1)ψ1(x),λψ2(x)=r1Ncaphψ1(x)−(d2+r2)ψ2(x),λψ3(x)=r2ψ2(x)−(d3+r3)ψ3(x),λψ4(x)=r32e−d3τ2e−λτ2∫Ωk(x,y)ψ3(y)dy−(d4+r4)ψ4(x). | (15) |
We shall show that the dominant eigenvalue
Proposition 2. If
Proof. Let
ψ4(x)=λ+d1+r1br4e−d4τ1e−λτ1ψ1(x). | (16) |
Similarly, from the second and third equations of (15),
ψ1(x)=(λ+d2+r2)hr1Ncapψ2(x), | (17) |
and
ψ2(x)=λ+d3+r3r2ψ3(x). | (18) |
The fourth equation of (15) then yields
e−λτ1e−λτ2∫Ωk(x,y)ψ3(y)dy=2h(λ+d4+r4)(λ+d1+r1)(λ+d2+r2)(λ+d3+r3)bNcape−d4τ1e−d3τ2r1r2r3r4ψ3(x),=2h∏4i=1(λ+di+ri)bNcape−(d3τ2+d4τ1)∏4i=1riψ3(x) |
so that
e−λτ1e−λτ2∫Ωk(x,y)ψ3(y)dy=∏4i=1(λ+di+ri)R0∏4i=1(di+ri)ψ3(x). | (19) |
Integrating with respect to
f1(λ)=f2(λ) | (20) |
where
f1(x)=R0e−(τ1+τ2)x4∏i=1(di+ri), | (21) |
f2(x)=4∏i=1(x+di+ri). | (22) |
Noting that
R0e−(τ1+τ2)(Reλ)4∏i=1(di+ri)=4∏i=1|λ+di+ri|≥4∏i=1|Reλ+di+ri|, |
that is,
Finally, by the aforementioned properties of
Remark. The above argument fails in the case of homogeneous Dirichlet boundary conditions applied to a finite domain
∏4i=1|λ+di+ri|R0∏4i=1(di+ri)|ψ3(x)|=e−(Reλ)(τ1+τ2)|∫Ωk(x,y)ψ3(y)dy|≤e−(Reλ)(τ1+τ2)∫Ωk(x,y)|ψ3(y)|dy. |
Integrating over
∏4i=1|Reλ+di+ri|R0∏4i=1(di+ri)≤∏4i=1|λ+di+ri|R0∏4i=1(di+ri)≤e−(Reλ)(τ1+τ2) |
which implies that
(Fϕ)(x)=∫Ωk(x,y)ϕ(y)dy,x∈Ω | (23) |
where
In the case
L+=h(R0−1),N+=d3+r3r2A+q,A+q=d4+r4r32e−d3τ2A+f,A+f=d1+r1br4e−d4τ1L+. |
When this persistence (positive) steady state exists, it is locally asymptotically stable. The arguments are similar to those just described for studying the linear stability of the extinction steady state. Linearizing system (1) at
{∂v1(x,t)∂t=br4e−d4τ1v4(x,t−τ1)−(d1+r1)u1(x,t),∂v2(x,t)∂t=r1NcaphR20v1(x,t)−(d2+r2)v2(x,t),∂v3(x,t)∂t=r2v2(x,t)−(d3+r3)v3(x,t),∂v4(x,t)∂t=r32∫Ωk(x,y)e−d3τ2v3(y,t−τ2)dy−(d4+r4)v4(x,t). | (24) |
This linear system is the same as (13) except that
In this section, we consider
L(x,t)=φ1(x+ct),N(x,t)=φ2(x+ct),Aq(x,t)=φ3(x+ct),Af(x,t)=φ4(x+ct). |
Here,
{cφ′1(s)=br4e−d4τ1φ4(s−cτ1)−(d1+r1)φ1(s),cφ′2(s)=r1g(φ1(s))−(d2+r2)φ2(s),cφ′3(s)=r2φ2(s)−(d3+r3)φ3(s),cφ′4(s)=r32∫+∞−∞Γ(z)e−d3τ2φ3(s−z−cτ2)dz−(d4+r4)φ4(s), | (25) |
where
We are interested in traveling wave fronts that connect
lims→−∞φi=0,lims→+∞φi(s)=φ+i,i=1,2,3,4. | (26) |
Linearizing system (25) at the trivial equilibrium
{cφ′1(s)=br4e−d4τ1φ4(s−cτ1)−(d1+r1)φ1(s),cφ′2(s)=r1Ncaphφ1(s)−(d2+r2)φ2(s),cφ′3(s)=r2φ2(s)−(d3+r3)φ3(s),cφ′4(s)=r32∫+∞−∞Γ(z)e−d3τ2φ3(s−z−cτ2)dz−(d4+r4)φ4(s). | (27) |
The characteristic equation associated with (27) is
P(λ,c)=0 | (28) |
where
P(λ,c)=|cλ+d1+r100−br4e−d4τ1e−cτ1λ−r1Ncaphcλ+d2+r2000−r2cλ+d3+r3000−r32e−d3τ2e−cτ2λˉk(λ)cλ+d4+r4|, |
where
ˉk(λ)=∫+∞−∞Γ(y)e−λydy=1√4Dτ2π∫+∞−∞e−y24Dτ2e−λydy=eDτ2λ2. |
Evaluating the determinant on the left hand side of (28), we obtain
4∏i=1[cλ+(di+ri)]−r1r2r3r4bNcap2he−(d4τ1+d3τ2)eDτ2λ2−c(τ1+τ2)λ=0; |
that is
4∏i=1[cλ+(di+ri)]−R0[4∏i=1(di+ri)]eDτ2λ2−c(τ1+τ2)λ=0. | (29) |
Generically, the behaviour of solutions of (25)-(26) at
Set
H1(λ,c)=4∏i=1[cλ+(di+ri)],H2(λ,c)=R0[4∏i=1(di+ri)]eDτ2λ2−c(τ1+τ2)λ. |
Then (29) can be rewritten as
c∗=minc>0{c:H1(λ,c)=H2(λ,c)haspositiverealrootswithrespecttoλ} |
and is determined by the tangential conditions:
H1(λ,c)=H2(λ,c),and∂H1∂λ(λ,c)=∂H2∂λ(λ,c),λ>0. | (30) |
There is no explicit formula for
Based on (30), we may also numerically explore the dependence of
From the above we have seen that, for
Although we defer to another paper the proof that
(L+,N+,A+q,A+f)=(1.72×103,1.43×103,1.43×103,60.63). |
Fig. 4 shows that there is no traveling wave solution for
Next, we numerically simulate solutions of the original initial value problem (1) to observe the time evolution of solutions toward a traveling wave front. To estimate the spreading rate, we use the same approach as was mentioned in [16]. The idea is to assume some threshold population density
c=limt→∞dˆx(t)dt. |
With the model parameter values given above, Fig. 6(a)-(b) and Fig. 7(a)-(b) show the evolution of the
In this paper, based on the fact that blacklegged ticks are only capable of moving very short distances by themselves and the general belief that dispersal of ticks over appreciable distances is via transport on the white tailed deer on which the adult ticks feed, we developed a spatial differential equation model for a stage-structured tick population. In addition to well-posedness, we identified a basic reproduction ratio
We also discussed traveling wave front solutions to the model that connect
We also numerically simulated the solutions of the original initial value problem (1). The results not only demonstrate the evolution of solutions toward a traveling wave front, but also suggest that
Theoretically confirming that
In this paper, we have concentrated mainly on the case when the spatial domain is
To solve the wave equations (25) with asymptotic boundary condition (26) numerically, we truncate
{cφ′1(sj)=br4e−d4τ1φ4(sj−cτ1)−(d1+r1)φ1(sj),cφ′2(sj)=r1g(φ1(sj))−(d2+r2)φ2(sj),cφ′3(sj)=r2φ2(sj)−(d3+r3)φ3(sj),cφ′4(sj)=r32e−d3τ2∫+∞−∞k(sj−y−cτ2)φ3(y)dy−(d4+r4)φ4(sj). | (31) |
The asymptotic boundary conditions
φi(−M)=0,φi(M)=φ∗i;φi(s)=0,s<−M;φi(s)=φ∗i,s>M,i=1,…,4. |
It then follows that
f1(φ3,sj):=∫+∞−∞k(y)φ3(sj−y−cτ2)dy=∫+∞−∞k(sj−y−cτ2)φ3(y)dy=(∫−M−∞+∫M−M+∫+∞M)k(sj−y−cτ2)φ3(y)dy=∫M−Mk(sj−y−cτ2)φ3(y)dy+φ3(M)∫+∞Mk(sj−y−cτ2)dy. |
Applying the composite trapezium rule for integrals, we obtain
∫M−Mk(sj−y−cτ2)φ3(y)dy=△2[k(sj−s1−cτ2)φ3(s1)+22n∑l=2k(sj−sl−cτ2)φ3(sl)+k(sj−s2n+1−cτ2)φ3(s2n+1)], | (32) |
and
∫+∞Mk(sj−y−cτ2)dy=∫sj−M−cτ2−∞k(y)dy=12(1−∫−sj+M+cτ2sj−M−cτ2k(y)dy) |
=12{1−△2[k(sj−M−cτ2)+k(−sj+M+cτ2)+22[2n+m2−(j−1)]∑l=2k(sj−M−cτ2+(l−1)△)]}. |
Then
f1(φ3,sj)=△2n∑l=2k(sj−sl−cτ2)φ3(sl)+f2(sj) |
where
f2(sj)=△2[k(sj−s1−cτ2)φ3(s1)+k(sj−s2n+1−cτ2)φ3(s2n+1)]+φ3(M)∫+∞Mk(sj−y−cτ2)dy=△2k(sj−M−cτ2)φ∗3+φ∗3∫+∞Mk(sj−y−cτ2)dy=△2k(sj−M−cτ2)φ∗3+φ∗32{1−△2[k(sj−M−cτ2)+k(−sj+M+cτ2)]−△2[2n+m2−(j−1)]∑l=2k(sj−M−cτ2+(l−1)△)}=φ∗32{1−△2[2n+m2−(j−1)]∑l=2k(sj−M−cτ2+(l−1)△)}. |
Let
{cφ1(sj+1)−φ1(sj−1)2△=br4e−d4τ1φ4(sj−m1)−(d1+r1)φ1(sj),cφ2(sj+1)−φ2(sj−1)2△=r1g(φ1(sj))−(d2+r2)φ2(sj),cφ3(sj+1)−φ3(sj−1)2△=r2φ2(sj)−(d3+r3)φ3(sj),cφ4(sj+1)−φ4(sj−1)2△=r32e−d3τ2f1(φ3,sj)−(d4+r4)φ4(sj), | (33) |
for
{cφ1(sj+1)−cφ1(sj−1)−2△br4e−d4τ1φ4(sj−m1)+2△(d1+r1)φ1(sj)=0,cφ2(sj+1)−cφ2(sj−1)−2△r1g(φ1(sj))+2△(d2+r2)φ2(sj)=0,cφ3(sj+1)−cφ3(sj−1)−2△r2φ2(sj)+2△(d3+r3)φ3(sj)=0,cφ4(sj+1)−cφ4(sj−1)−△r3e−d3τ2f1(φ3,sj)+2△(d4+r4)φ4(sj)=0 | (34) |
for
cφ2(sj+1)−cφ2(sj−1)−2h△r1Ncapφ1(sj)+2△(d2+r2)φ2(sj)+chφ1(sj)φ2(sj+1)−chφ1(sj)φ2(sj−1)+2h△(d2+r2)φ1(sj)φ2(sj)=0. |
Thus, system (34) can be expressed as
[M1100M14M21M22000M32M33000M43M44][φ1(s2)⋮φ1(s2n)φ2(s2)⋮φ2(s2n)φ3(s2)⋮φ3(s2n)φ4(s2)⋮φ4(s2n)]+[C1C2C3C4]=0, | (35) |
where
Mii=[2△(di+ri)c0−c2△(di+ri)c⋱−c2△(di+ri)c0−c2△(di+ri)],i=1,…,4, |
M14=[0⋮0−2△br4e−d4τ1⋱−2△br4e−d4τ10…0], |
M21=[−2h△r1Ncap0⋱0−2h△r1Ncap], |
M32=[−2△r20⋱0−2△r2], |
ˆM43=[k(s2−s2−cτ2)k(s2−s3−cτ2)…k(s2−s2n−cτ2)k(s3−s2−cτ2)k(s3−s3−cτ2)…k(s3−s2n−cτ2)⋮k(s2n−s2−cτ2)k(s2n−s3−cτ2)…k(s2n−s2n−cτ2)], |
C1=[0⋮0cφ∗1],C3=[0⋮0cφ∗3],C4=[−△r3e−d3τ2f2(s2)−△r3e−d3τ2f2(s3)⋮cφ∗4−△r3e−d3τ2f2(s2n)], |
C2=[1hφ1(s2)[cφ2(s3)+2△(d2+r2)φ2(s2)]1hφ1(s3)[cφ2(s4)−cφ2(s2)+2△(d2+r2)φ2(s3)]⋮1hφ1(s2n−1)[cφ2(s2n)−cφ2(s2n−2)+2△(d2+r2)φ2(s2n−1)]cφ∗2+1hφ1(s2n)[cφ∗2−cφ2(s2n−1)+2△(d2+r2)φ2(s2n)]]. |
The algebraic system (35) can then be solved numerically using Matlab.
This work was initiated at the Current Topics Workshop: Spatial-Temporal Dynamics in Disease Ecology and Epidemiology, held at the Mathematical Biosciences Institute (MBI) at Ohio State University during October 10-14,2011. The workshop was organized by R. Liu, J. Tsao, J. Wu and X. Zou, and was funded by the NSF through the MBI. We thank Dr. G. Hickling for bringing this problem to a group discussion which stimulated this research project. We also thank those participants at the workshop who offered helpful suggestions and advice on the model, and particularly Drs. Hickling and Tsao for their valuable input during the discussions at the MBI and for providing some valuable references.
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Parameters | Meaning | Value |
Birth rate of tick | ||
average time that a questing larvae needs to feed and moult | ||
average time that a questing nymph needs to feed and moult | ||
average time that a questing adult needs to successfully attach to a deer | ||
Proportion of fed adults that can lay eggs | 0.03 | |
per-capita death rate of larvae | 0.3 | |
per-capita death rate of nymphs | 0.3 | |
per-capita death rate of questing adults | 0.1 | |
per-capita death rate of fed adults | 0.1 | |
average time between last blood feeding and hatch of laid eggs | 20 days | |
average time tick is attached to a deer |
Parameters | Meaning | Value |
Birth rate of tick | ||
average time that a questing larvae needs to feed and moult | ||
average time that a questing nymph needs to feed and moult | ||
average time that a questing adult needs to successfully attach to a deer | ||
Proportion of fed adults that can lay eggs | 0.03 | |
per-capita death rate of larvae | 0.3 | |
per-capita death rate of nymphs | 0.3 | |
per-capita death rate of questing adults | 0.1 | |
per-capita death rate of fed adults | 0.1 | |
average time between last blood feeding and hatch of laid eggs | 20 days | |
average time tick is attached to a deer |