Biomedical subjects

G Hasibeder

Publications and source records attributed to G Hasibeder.

6 recordsLinked to original sources

PubMed1996

Estimation from current-status data in continuous time.

The nonparametric maximum likelihood estimator for current-status data has been known for at least 40 years, but only recently have the mathematical-statistical properties been clarified. This note provides a case study in the important and often studied context of estimating age-specific immunization intensities from a seroprevalence survey. Fully parametric and spline-based alternatives (also based on continuous-time models) are given. The basic reproduction number R0 exemplifies estimation of a functional. The limitations implied by the necessarily rather restrictive epidemiological assumptions are briefly discussed.

Adolescent↗

PubMed1996

On estimating the basic reproduction number for Schistosoma haematobium.

Existing estimates of the basic reproduction number, Ro, for human schistosomes are mostly in the range 1-4, implying that schistosomes should be relatively easy to eliminate from endemic areas, which is contrary to practical experience. An estimate of Ro for a site in Zimbabwe is obtained here using a mathematical model explicitly incorporating two features believed to be epidemiologically significant; age-dependent exposure and acquired immunity. Parameter estimates are, as far as possible, obtained independently, but the coefficients representing man-snail and snail-man transmission, as well as parameters representing effects of acquired immunity, must be estimated indirectly by fitting the model to field data. Heterogeneity in human exposure and contamination is crudely incorporated by considering "wormy' and non-wormy' fractions of the population. The results suggest Ro to be in the range 4-5 or more, higher than previous estimates and despite only moderate levels of infection at this site. It is shown that this estimate is sensitive to the form of the underlying model. The application of less realistic models may lead to less reliable estimates of Ro.

Animals↗

PubMed1992

Epidemiology of canine leishmaniasis: prevalence, incidence and basic reproduction number calculated from a cross-sectional serological survey on the island of Gozo, Malta.

Assessment of the resilience of canine leishmaniasis to control or, more ambitiously, the effort needed to eradicate infection, requires an estimate of the basic case reproduction number (R0). This paper applies the theoretical results of Hasibeder, Dye & Carpenter (1992) to data from a cross-sectional survey on the Maltese island of Gozo in which dogs of known age, sex and occupation (pet, guard etc) were subjected to three different serological tests for the presence of specific antibody (IFAT, DAT and ELISA). Difficulties in interpreting these test results, and hence of determining the proportion of dogs infected, present the main obstacle to estimating R0: estimates are critically dependent on the choice of threshold separating seropositives from seronegatives. The data do, however, allow a robust comparative analysis of risk which shows that the force of infection experienced by working dogs is about three times higher than that of pet dogs, a degree of non-homogeneous contact which actually has little effect on estimates of R0. We suggest a cautious point estimate of R0 congruent to 11, and comment briefly on its significance for leishmaniasis control.

Age Factors↗

PubMed1992

Mathematical modelling and theory for estimating the basic reproduction number of canine leishmaniasis.

The paper describes a mathematical model for canine leishmaniasis and presents formulae which can be used to estimate the basic reproduction number, R0. The primary concern has been to devise methods of estimation which make best use of those data most easily obtained by fieldwork, e.g. surveys of prevalence in dog (by age) and sandfly populations. A range of formulae are offered which are more or less demanding of data, and which consequently give more or less precise estimates of R0. They include methods for assessing the influence on R0 of heterogeneous biting rates of sandflies on dogs, in which the essence of heterogeneous transmission can be captured merely by measuring relative rather than absolute contact rates.

Animals↗

PubMed1988

Population dynamics of mosquito-borne disease: persistence in a completely heterogeneous environment.

We investigate the persistence of a mosquito-borne disease (malaria) in a system where mosquitoes and hosts are grouped in patches containing any number of individuals. A mosquito from any one of vector patches can bite, and take blood meals, in any one of m host patches. We confirm our earlier result (C. Dye and G. Hasibeder, 1986, Trans. R. Soc. Trop. Med. Hyg. 80, 69-77) that nonhomogeneous host selection by mosquitoes leads to basic reproductive rates (which measure the persistence of infection in the system) greater than or equal to those obtained under uniform host selection. We find, in addition, that strong associations between particular groups of mosquitoes and hosts lead to still higher basic reproductive rates. Exacting fieldwork would be required to find out how much higher.

Animals↗

PubMed1986

Population dynamics of mosquito-borne disease: effects of flies which bite some people more frequently than others.

The consequences of non-homogeneous mixing (non-random contact) between vectors and hosts for the persistence, prevalence, and hence control, of any mosquito-borne disease are explored. When mosquitoes concentrate on certain hosts, the basic reproductive rate, R, of the disease (a measure of persistence) and the vectorial capacity are both greater than or equal to their values under homogeneous mixing. Field data suggest that R could be more than two and a half times as large as it would be under homogeneous mixing. Our calculations underestimate the importance of heterogeneity by ignoring mosquito patchiness and stochastic effects. Host selection limits the dependability of predictions, made from population models which assume homogeneous mixing, about the success of disease control (vaccines, chemotherapy, vector control). In particular, eradication or the maintenance of low prevalence will be more difficult than expected, unless localized control makes the distribution of infective bites on a community more nearly uniform. Age may explain most of the variability in biting in small communities (e.g., rural villages), in which case models incorporating age-specific biting will be appropriate tools in control programmes.

Anopheles↗