Statistical analysis of the dynamics of antibody loss to a disease-causing agent: plague in natural populations of great gerbils as an example

Abstract

We propose a new stochastic framework for analysing the dynamics of the immunity response of wildlife hosts against a disease-causing agent. Our study is motivated by the need to analyse the monitoring time-series data covering the period from 1975 to 1995 on bacteriological and serological tests-samples from great gerbils being the main host of Yersinia pestis in Kazakhstan. Based on a four-state continuous-time Markov chain, we derive a generalized nonlinear mixed-effect model for analysing the serological test data. The immune response of a host involves the production of antibodies in response to an antigen. Our analysis shows that great gerbils recovered from a plague infection are more likely to keep their antibodies to plague and survive throughout the summer-to-winter season than throughout the winter-to-summer season. Provided the seasonal mortality rates are similar (which seems to be the case based on a mortality analysis with abundance data), our finding indicates that the immune function of the sampled great gerbils is seasonal.

Field evaluation of a 0.005% fipronil bait, orally administered to Rhombomys opimus, for control of fleas (Siphonaptera: Pulicidae) and phlebotomine sand flies (Diptera: Psychodidae) in the Central Asian Republic of Kazakhstan

“…fleas [ 32 ], particularly X . gerbili minax [ 33 , 34 ], and Phlebotomus spp. phlebotomine sand flies [ 35 , 36 ], respectively.…”

Dynamics of the plague–wildlife–human system in Central Asia are controlled by two epidemiological thresholds

“…Transmission to humans occurs occasionally through either contacting fleas that have fed on an infected animal (either a rodent or a secondary host) or eating or skinning infected animals (1). The records of human plague began in 1904, with most of the cases occurring before 1949, which was when plague monitoring and flea control began (8,10) in an effort to eliminate human plague outbreaks. The human plague cases are recorded annually and aggregated spatially, whereas the data on the gerbils and their fleas are recorded biannually.…”

“…The abundance of great gerbils and their fleas, and the prevalence of wildlife plague were estimated at up to 78 sites in the PreBalkhash each spring (May and June) and fall (September and October) from 1949 to 1995. Fleas (primarily Xenopsylla gerbilli minax) (7,8) infesting the gerbils are the main plague vector (3). Plague infection is known to persist in the population only when the gerbil density exceeds a threshold (3,9).…”

Modeling the epidemiological history of plague in Central Asia: Palaeoclimatic forcing on a disease system over the past millennium

“…Most of these cases occurred prior to 1949, when plague monitoring and control began as part of Soviet-wide efforts, and antibiotics and insecticides became increasingly available [18,30]. Thus, a consistent and important climate forcing on sylvatic plague should be detectable in a statistical connection between climate and the independent human case data, allowing us to look for associations between climate fluctuations and human plague on scales much larger than previously considered, as well as connecting these associations with the long-term, large-scale behavior of the plague system.…”

“…Their spatial distribution appears to be limited by soils suitable for burrowing, while their temporal density fluctuations are mainly influenced by an interplay between density dependence (which operates around the burrow systems as they disperse mostly at the local scale, i.e., <15 km per generation), and variability in vegetation cover (which is synchronous over larger distances due to spatially autocorrelated precipitation and temperature) [ 15 , 16 ]. The main sylvatic (i.e., wildlife) plague transmission vectors are fleas [ 17 , 18 ], and increasing vector abundance is probably one of the reasons why moist, relatively warm spring conditions tend to increase the prevalence of plague in great gerbils [ 12 , 16 , 19 ]. Great gerbils usually have few visible symptoms of plague infection, and only moderate increases in mortality [ 12 , 20 ], and there seems to be a critical threshold of host population density necessary for plague to persist in an area [ 10 , 12 , 21 ].…”