Surveillance and Control Strategies of Rabies in N’Djaména, Chad

Canine rabies is a neglected disease which causes worldwide 55’000 human death per year and 99% of all cases are transmitted by dog bites. In N’Djaména, the capital of Chad, rabies is endemic with an incidence of 1.71/1000 dogs (95% C.I. 1.45-1.98) in 2006.
The gold standard of rabies diagnosis is the direct immunofluorescent antibody (DFA) test, needing fluorescent microscope. The CDC developed a histochemical test using light microscopy, the direct rapid immunohistochemical test (dRIT). We evaluated the dRIT in the Chadian National Laboratory in N'Djaména by testing 35 sample with the DFA and dRIT. A retest by DFA and dRIT of 80 samples in Chad and 75 samples at CDC increases the quality of the evaluation. The evaluation of the dRIT showed ideal results. We found a 100% agreement of the dRIT and DFA in fresh samples. By retesting at CDC and in Chad, the results of dRIT and DFA depended on the samples condition (kappa value 0.87-1 for good condition, 0.13-0.52 for bad condition) and on whether the samples are stored in glycerol or not. If stored in glycerol, dRIT showed better results than DFA. The dRIT performed as reliable diagnostic method as good as the gold standard (DFA) if used in fresh samples. Its big advantage is that it needs only a light microscope which is 10 times less expensive than a fluorescent microscope. Thus there is a high potential of establishing rabies diagnosis in other cities and rural areas of Africa and as a consequence, to provide access to rabies diagnosis to huge populations yet not-covered.
In this project first dog-human deterministic transmission model of rabies with stochastic parameter specification of dog to dog and dog to human rabies transmission was developed. To fit model parameters, data from the Chadian National laboratory were collected. The weekly number of rabies positive dogs and the exposed people recorded in the laboratory over the last 305 weeks were used. On average we found 0.57 rabid dogs per week and 2.06 exposed people per rabid dog. Demographic data of the dog population was collected regularly every 1-2 months over 7 months on a cohort of 751 dogs in households with at least one dog. All birth, deaths, acquisitions and exits of the cohort were recorded. We found a birth rate of 0.68 newborns and a mortality rate of 0.30 deaths per dog and year. These data were included in the model as demographic parameters. With the fitted transmission parameters, different rabies control interventions can then be simulated and comparative cost-effectiveness can be assessed. Currently, based on model outputs we hypothesis, that by a single parenteral mass vaccination campaign with 70% vaccination coverage among all 23'560 dogs in N'Djaména the transmission of rabies could be interrupted over a period of about 6 years. A pilot parenteral dog vaccination campaign was organized in three quarters of N'Djaména. The only difference to a previous realized campaign which achieved 64-87% vaccination coverage was that we charged 3 € from the dog owners per dog vaccinated whereas in the last campaign the vaccination was free to owners. The aim was to assess the willingness to pay of the dog owners. If the participation of the animal owners and the resulting vaccination coverage would be sufficient, the financing of campaigns through the private sector would be possible. The reached vaccination coverage was only 24% which is not sufficient to interrupt transmission. Consequently, to achieve the needed vaccination coverage of at least 70% public investment is a precondition and the political will is central for its success. Based on our results, we recommend to reclassify rabies as public concern. Tied up to the results of this project, rabies control in N'Djaména as a showcase city in the Sahelian zone should be pushed forward.

Key words: canine rabies, diagnostics, transmission model, Chad

Adresses

Project manager:
Jakob Zinsstag, Swiss Tropical Institut, Socinstrasse 57, Postfach, CH-4002 Basel, Switzerland
Phone: +41 61 284 81 39, Mail: jakob.zinsstag@unibas.ch

Swiss DVM:
Salome Dürr, Eptingerstrasse 15, 4052 Basel, Schweiz
Phone: +41 61 312 24 77, Mail: salome.duerr@unibas.ch

Chadian Partners:
Rolande Mindekem, Centre de Support en Santé International (CSSI/T), BP 972, N’Djaména, Chad, mrola2002@yahoo.fr