Impact of biodiversity and seasonality on Lyme-pathogen transmission

Abstract

Lyme disease imposes increasing global public health challenges. To better understand the joint effects of seasonal temperature variation and host community composition on the pathogen transmission, a stage-structured periodic model is proposed by integrating seasonal tick development and activity, multiple host species and complex pathogen transmission routes between ticks and reservoirs. Two thresholds, one for tick population dynamics and the other for Lyme-pathogen transmission dynamics, are identified and shown to fully classify the long-term outcomes of the tick invasion and disease persistence. Seeding with the realistic parameters, the tick reproduction threshold and Lyme disease spread threshold are estimated to illustrate the joint effects of the climate change and host community diversity on the pattern of Lyme disease risk. It is shown that climate warming can amplify the disease risk and slightly change the seasonality of disease risk. Both the "dilution effect" and "amplification effect" are observed by feeding the model with different possible alternative hosts. Therefore, the relationship between the host community biodiversity and disease risk varies, calling for more accurate measurements on the local environment, both biotic and abiotic such as the temperature and the host community composition.

Figure 1

Schematic diagram for the Lyme disease transmission. To describe the tick development and biting activities, the tick population is divided into 7 stages, stratified further as the uninfected or infected epidemiological classes for postegg stages. Immature ticks can feed on two host species, the mice (H ₁) and an alternative host (H ₂), while adult ticks are assumed to feed only on deer in this study.

Figure 2

30 year normal mean monthly temperature under two settings near Long Point. The blue solid and red dashed curves represent the monthly temperature for the periods 1961−1990 period and 1981−2010, respectively. We set monthly temperature to be 0°C if it is lower than 0°C. Both are collected from Environment Canada website [46].

Figure 3

Development rates and feeding rates of I. scapularis ticks within one year period. The blue solid and red dashed curves are related to the associated development rates and feeding rates under temperatures in the periods 1961−1990 and 1981−2010, respectively; The numbers at the left top corner in each subfigure indicate the areas under the associated curves, which are used to differentiate the differences of these rates under the two temperature settings.

Figure 4

The variations in the sizes of total questing nymphs and infected questing nymphs with the two temperature settings mentioned above. The red solid curves represent the outputs by seeding the model with 1961−1990 temperature data ( and in this case), while the blue dashed curves represent the model outputs by 1981−2010 temperature data ( and in this case). (a) Total questing nymphs; (b) infected questing nymphs in the 40 year simulation; (c) seasonality of questing nymphs at the steady state; (d) seasonality of infected questing nymphs at the steady state, where shaded portions in both (c) and (d) represent the active seasons of the questing nymphs.

Figure 5

Log plots of variations of ratios and against the number of alternative hosts. In the case of the eastern chipmunks, p ₁=0.4, p ₂=3.5, , , ; for the western fence lizard, p ₁=1, p ₂=1, , , ; for the Virginia opossum, p ₁=7.2, p ₂=36.9, , , . For all simulations, the temperature condition is fixed on the period 1981−2010.

Figure 6

Variations in the sizes of DON, DIN and NIP under different host sizes. The numbers on the left panel indicate that the sizes of associated alternative hosts are added into the host community. The scenarios where the eastern chipmunk is added are shown on the upper panel. The middle panel shows the scenarios where the Virginia opossum is considered as the alternative host; the bottom panel shows the situations where the western fence lizard is added. All the associated parameter values are the same as those in Figure 5 except the sizes of alternative hosts.

Figure 7

Relationship between the reproduction ratio , p ₁ and p ₂ for the three types of alternative hosts: the eastern chipmunk, the Virginia opossum and the western fence lizard. The number of alternative hosts is set as 30 and all other parameter values are same as those in Figure 5.