Effectiveness of Fipronil as a Systemic Control Agent Against Xenopsylla cheopis (Siphonaptera: Pulicidae) in Madagascar

Fipronil was evaluated as a systemic control agent for the rat flea Xenopsylla cheopis (Rothschild), the main vector of Yersinia pestis (Yersin), the causative agent of plague, in Madagascar. The effectiveness of fipronil as a systemic control agent against X. cheopis was assessed by determining the toxicity values of the "Lethal Dose 50" (LD50). Two techniques were used to evaluate the systemic action of the insecticide on the vector: 1) an artificial feeding device filled with blood-fipronil mixture from which X. cheopis was fed and 2) rodent hosts, Rattus norvegicus (Berkenhout) and Rattus rattus (L.), which fed on fipronil-treated bait. As a standardized control method, the susceptibility of X. cheopis to fipronil was evaluated by exposure to impregnated paper within World Health Organization (WHO) insecticide test protocol to compare its effect to the systemic activity of the studied insecticide. Results showed that when administered in a systemic way, fipronil appears to be more effective: the toxicity level was evaluated to be ninefold higher compared with the WHO test. Compared with other methods, which require indiscriminate dusting of rodent burrows and human dwellings, fipronil applied in a systemic way enables the direct targeting of the plague vector. Thus, this method appears to be a superior alternative to fipronil-dusting for the control of the main plague vector in Madagascar. However, subsequent tests in the field are necessary to confirm the suitability of fipronil administration in a systemic way on large scales.

Susceptibility to Yersinia pestis experimental infection in wild Rattus rattus, reservoir of plague in Madagascar

In Madagascar, the black rat, Rattus rattus, is the main reservoir of plague (Yersinia pestis infection), a disease still responsible for hundreds of cases each year in this country. This study used experimental plague challenge to assess susceptibility in wild-caught rats to better understand how R. rattus can act as a plague reservoir. An important difference in plague resistance between rat populations from the plague focus (central highlands) and those from the plague-free zone (low altitude area) was confirmed to be a widespread phenomenon. In rats from the plague focus, we observed that sex influenced plague susceptibility, with males slightly more resistant than females. Other individual factors investigated (weight and habitat of sampling) did not affect plague resistance. When infected at high bacterial dose (more than 10⁵ bacteria injected), rats from the plague focus died mainly within 3-5 days and produced specific antibodies, whereas after low-dose infection (< 5,000 bacteria), delayed mortality was observed and surviving seronegative rats were not uncommon. These results concerning plague resistance level and the course of infection in the black rat would contribute to a better understanding of plague circulation in Madagascar.

CCR5 polymorphism and plague resistance in natural populations of the black rat in Madagascar

Madagascar remains one of the world's largest plague foci. The black rat, Rattus rattus, is the main reservoir of plague in rural areas. This species is highly susceptible to plague in plague-free areas (low-altitude regions), whereas rats from the plague focus areas (central highlands) have evolved a disease-resistance polymorphism. We used the candidate gene CCR5 to investigate the genetic basis of plague resistance in R. rattus. We found a unique non-synonymous substitution (H184R) in a functionally important region of the gene. We then compared (i) CCR5 genotypes of dying and surviving plague-challenged rats and (ii) CCR5 allelic frequencies in plague focus and plague-free populations. Our results suggested a higher prevalence of the substitution in resistant animals compared to susceptible individuals, and a tendency for higher frequencies in plague focus areas compared to plague-free areas. Therefore, the CCR5 polymorphism may be involved in Malagasy black rat plague resistance. CCR5 and other undetermined plague resistance markers may provide useful biological information about host evolution and disease dynamics.

Genetic structure of black rat populations in a rural plague focus in Madagascar

The genetic structure of reservoir populations is a key characteristic in understanding the persistence of infectious diseases in natural systems. In the Highlands of Madagascar, where plague has persisted since 1920, the black rat, Rattus rattus (L., 1758), is the sole species acting as a reservoir of the disease. Ecological surveys have shown a clear correlation between the locations of the plague-persistence area in Madagascar (above 800 m elevation) and the distribution area of one endemic plague vector, the flea Synopsyllus fonquerniei , which is found exclusively on rats living outdoors. This clear habitat segregation has led to the suggestion that R. rattus populations in the central highlands are divided into indoor- and outdoor-dwelling populations. Using eight microsatellite markers, we analysed the genetic structure of R. rattus populations living within a human plague focus in relation to habitat and geographic distance. We found that habitat by itself was not a structuring factor, unlike geographic distance. Nevertheless, the significant genotypic differentiation of R. rattus populations that was found at a fine spatial scale might relate to differences in population dynamics between rats in indoor and outdoor habitats.

[Plague control in Madagascar: evaluation of the efficacy of Kartman baitboxes in urban areas]

A method associating an anticoagulant rodenticide and an insecticide called Kartman bait-box aimed both at fighting reservoir and vectors of plague. It was evaluated in two neighbourhoods of Antananarivo (Madagascar) from October 2002 to May 2003. It involved the local community in the control. The study was carried out in Ambodirano-Ampefiloha refered as treated neighbourhood in which the Kartman bait box were laid out with an anticoagulant rodenticide and an insecticide with a rapid action versus a "pilot neighbourhood", Ankorondrano-Andranomahery in which the boxes were provided with non poisoning bait and non insecticidal white powder. The rodenticide used was Baraki (difethialone 25 ppm) and the insecticide was a powder of Propoxur 3%. The evaluation of effectiveness of this method was based on the four following parameters: (1) the number of dead rats collected daily inside and in the vicinity of the houses, (2) the daily number of baits non consumed in the Kartman bait box, (3) the cheopis index of the rats trapped using the BTS trap, and (4) the flea carrier index of the rats captured monthly with BTS trap. The cheopis index and the flea carrier index of the rats were calculated monthly. The number of rats that died in the treated neighbourhood was of 968 versus 3 in the pilot neighbourhood. The other parameters reached a stable level after 3 months. Between days 120 and 180, the mean number of unconsumed baits was 2.79 in the treated neighbourhood versus 0.14 in the pilot neighbourhood, the flea carriage (percentage of parasitized hosts) was 0% versus 61% in the pilot neighbourhood, and the cheopis index was 0.0 versus 5.0 in the pilot neighbourhood. This study demonstrates that Kartman bait-boxes reached the rat borne and the vectors of plague found in urban area. We propose to use this method extensively both during epidemic and inter-epidemic contexts.