Christopher Dye

Infectiousness in a Cohort of Brazilian Dogs: Why Culling Fails to Control Visceral Leishmaniasis in Areas of High Transmission

The elimination of seropositive dogs in Brazil has been used to control zoonotic visceral leishmaniasis but with little success. To elucidate the reasons for this, the infectiousness of 50 sentinel dogs exposed to natural Leishmania chagasi infection was assessed through time by xenodiagnosis with the sandfly vector, Lutzomyia longipalpis. Eighteen (43%) of 42 infected dogs became infectious after a median of 333 days in the field (105 days after seroconversion). Seven highly infectious dogs (17%) accounted for >80% of sandfly infections. There were positive correlations between infectiousness and anti-Leishmania immunoglobulin G, parasite detection by polymerase chain reaction, and clinical disease (logistic regression, r2=0.08-0.18). The sensitivity of enzyme-linked immunosorbent assay to detect currently infectious dogs was high (96%) but lower in the latent period (<63%), and specificity was low (24%). Mathematical modeling suggests that culling programs fail because of high incidence of infection and infectiousness, the insensitivity of the diagnostic test to detect infectious dogs, and time delays between diagnosis and culling.

Immunization coverage required to prevent outbreaks of dog rabies

WHO recommends that 70% of dogs in a population should be immunized to eliminate or prevent outbreaks of rabies. This critical percentage (pc) has been established empirically from observations on the relationship between vaccination coverage and rabies incidence in dog populations around the world. Here, by contrast, we estimate pc by using epidemic theory, together with data available from four outbreaks in urban and rural areas of the USA, Mexico, Malaysia and Indonesia. From the rate of increase of cases at the beginning of these epidemics, we obtain estimates of the basic case reproduction number of infection, R0, in the range 1.62-2.33, implying that pc lies between 39% and 57%. The errors attached to these estimates of pc suggest that the recommended coverage of 70% would prevent a major outbreak of rabies on no fewer than 96.5% of occasions.

Detection of Leishmania infantum by PCR, serology and cellular immune response in a cohort study of Brazilian dogs.

The sensitivity and specificity of PCR, serology (ELISA) and lymphoproliferative response to Leishmania antigen for the detection of Leishmania infantum infection were evaluated in a cohort of 126 dogs exposed to natural infection in Brazil. For PCR, Leishmania DNA from bone-marrow was amplified with both minicircle and ribosomal primers. The infection status and time of infection of each dog were estimated from longitudinal data. The sensitivity of PCR in parasite-positive samples was 98%. However, the overall sensitivity of PCR in post-infection samples, from dogs with confirmed infection, was only 68%. The sensitivity of PCR varied during the course of infection, being highest (78-88%) 0-135 days post-infection and declining to around 50% after 300 days. The sensitivity of PCR also varied between dogs, and was highest in sick dogs. The sensitivity of serology was similar in parasite-positive (84%), PCR-positive (86%) and post-infection (88%) samples. The sensitivity of serology varied during the course of infection, being lowest at the time of infection and high (93-100%) thereafter. Problems in determining the specificity of serology are discussed. The sensitivity and specificity of cellular responsiveness were low. These data suggest that PCR is most useful in detecting active or symptomatic infection, and that serology can be a more sensitive technique for the detection of all infected dogs.

Maintenance of a microparasite infecting several host species: rabies in the Serengeti

Whether and how microparasites such as rabies persist in their host populations are among the fundamental questions of infectious disease epidemiology. Rabies is fatal disease of all mammalian species, but not all mammalian species can maintain the infection as reservoirs. The approach to control depends on which of the affected species do act as reservoirs. Bringing together old and new data, we examine here the role of wild and domestic animals in maintaining rabies in the Serengeti region of Tanzania, presenting our findings in two parts. In Part I, we argue that domestic dogs are the likely reservoirs because: (1) rabies has been continuously present in the dog population since its (re)introduction in 1977, whilst (2) wildlife cases have been very rare over this period, despite intensive study of Serengeti carnivores; (3) outbreaks of rabies in wild canids (jackals) elsewhere in Africa (Zimbabwe) have followed, rather than preceded, outbreaks in the dog population; (4) all viruses isolated from wild carnivores in the Serengeti ecosystem (including the Kenyan Masai Mara) are antigenically and genetically indistinguishable from the typical domestic dog strain; (5) dog rabies control in the Serengeti between 1958-77 apparently eliminated the disease from both dogs and wildlife. Having identified dogs as reservoirs, Part II explores some possible mechanisms of maintenance in dog populations. In theory, infection is more likely to be maintained at higher dog densities, and we provide evidence that rabies is maintained in one district with a dog density > 5/km2, but not in two other districts with densities < 1/km2. Because 5 dogs/km2 is much lower than the expected density required for persistence, we go on to investigate the role of atypical infections, showing: (1) from serology, that a substantial proportion of healthy dogs in the Serengeti have detectable serum levels of rabies-specific antibody; (2) from mathematical models that, whilst we cannot be sure what seropositivity means, persistence in low-density dog populations is more likely if seropositives are infectious carriers, rather than slow-incubators or immunes.

The epidemiology of canine leishmaniasis: transmission rates estimated from a cohort study in Amazonian Brazil.

We estimate the incidence rate, serological conversion rate and basic case reproduction number (R0) of Leishmania infantum from a cohort study of 126 domestic dogs exposed to natural infection rates over 2 years on Marajó Island, Pará State, Brazil. The analysis includes new methods for (1) determining the number of seropositives in cross-sectional serological data, (2) identifying seroconversions in longitudinal studies, based on both the number of antibody units and their rate of change through time, (3) estimating incidence and serological pre-patent periods and (4) calculating R0 for a potentially fatal, vector-borne disease under seasonal transmission. Longitudinal and cross-sectional serological (ELISA) analyses gave similar estimates of the proportion of dogs positive. However, longitudinal analysis allowed the calculation of pre-patent periods, and hence the more accurate estimation of incidence: an infection-conversion model fitted by maximum likelihood to serological data yielded seasonally varying per capita incidence rates with a mean of 8.66 x 10(-3)/day (mean time to infection 115 days, 95% C.L. 107-126 days), and a median pre-patent period of 94 (95% C.L. 82-111) days. These results were used in conjunction with theory and dog demographic data to estimate the basic reproduction number, R0, as 5.9 (95% C.L. 4.4-7.4). R0 is a determinant of the scale of the leishmaniasis control problem, and we comment on the options for control.

Host preferences of the phlebotomine sandfly Lutzomyia longipalpis in Amazonian Brazil

Abstract. Experiments were undertaken to determine the relative attractiveness of humans, dogs and chickens to Lutzomyia longipalpis, the principal vector of Leishmania chagasi causing American visceral leishmaniasis Field experiments in two villages on Marajo Island, Para State, Brazil, showed that one boy attracted significantly more flies than one dog or chicken, and slightly fewer flies than a group of six chickens. Experiments with laboratory‐bred female flies showed that a significantly greater number of flies engorged on a single human than on either a single dog or chicken, and man‐biting catches demonstrated the willingness of flies to bite in the field. It appears that Lu. longipalpis has catholic feeding habits, the attractiveness of different hosts being largely a function of their relative sizes. These results are discussed with reference to the epidemiology of visceral leishmaniasis in Brazil.

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.

Pheromones, kairomones and the aggregation dynamics of the sandflyLutzomyia longipalpis ☆

Abstract MaleLutzomyia longipalpis(Diptera: Psychodidae) form lek-like aggregations on a range of host animals, to which females migrate to mate and take a blood meal. In so doing, females act as the vectors of American visceral leishmaniasis in humans and canids. Host kairomones and a male pheromone are thought to be important for aggregation formation. Stimulated by interest in the development of a semiochemical-baited trap for fly control, a technique was developed to mark flies with minimum disruption of their natural behaviour, and employed in a set of field experiments to investigate the role of host and fly factors in aggregation dynamics. Males arrived at aggregations earlier than females, at a rate dependent on the abundance of resident flies and hosts. The immigration rate of females was dependent on fly abundance alone. The emigration rate of males decreased as fly and host abundance increased. The emigration rate of females was greater than males, and increased with host abundance, but decreased with female abundance. It is argued that male behaviour maximizes mating success, whereas female behaviour depends on the rate of bloodfeeding and the reduction of travel costs. Between nights, most males returned to the site of their previous nights activity, suggesting that flies may memorize a ‘familiar area map’. These results raise the possibility that, without the addition of pheromone baits, insecticide spraying programmes that do not achieve blanket coverage of aggregation sites would not significantly reduce the fly population, and might increase parasite transmission between suceptible hosts.

Maximum likelihood for parasitologists

In quantitative biology, observed data are fitted to a model that captures the essence of the system under investigation in order to obtain estimates of the parameters of the model, as well as their standard errors and interactions. The fitting is best done by the method of maximum likelihood, though least-squares fits are often used as an approximation because the calculations are perceived to be simpler. Here Brian Williams and Chris Dye argue that the method of maximum likelihood is generally preferable to least squares giving the best estimates of the parameters for data with any given error distribution, and the calculations are no more difficult than for least-squares fitting. They offer a relatively simple explanation of the methods and describe its implementation using examples from leishmaniasis epidemiology.

Replication, differentiation, growth and the virulence of Trypanosoma brucei infections

This study had 2 objectives: first, to investigate how the processes of slender form replication, of differentiation from dividing slender to non-dividing stumpy forms, and of stumpy mortality, combine to determine the initial (acute-phase) growth rate of Trypanosoma brucei populations; second, to determine how acute-phase growth rates influence parasite densities during the subsequent, chronic phase of infection. During the acute phase, slender and stumpy populations both grew approximately exponentially, the latter more slowly than the former. Mathematical models showed how this difference in slender and stumpy growth rates can be explained in terms of heterogeneous replication and differentiation rates. Stumpy life-expectancy was determined for one stock and found to be age-dependent with a half-life of 48-72 h, much larger than observed population doubling times of 5-10 h. A comparison of cloned stocks showed that the highest parasite densities during the chronic phase were associated with the highest acute-phase growth rates of both the whole parasite population and of the subpopulation of slender forms. By contrast, high chronic-phase parasitaemias artificially produced following rapid syringe passage were associated with low acute-phase growth rates of slender forms. Syringe-passaging is a laboratory procedure which selects for virulent parasites, but these parasites behave differently from naturally virulent stocks.