Introduction
Every year, 40,000-70,000 people die of rabies worldwide, and 10 million people undergo a post-exposure treatment, according to WHO. Most of rabies cases in humans occur in tropical developing countries and the dog is the main vector for human exposure. Human rabies can be prevented by an immediate full-course post-exposure treatment (PET) consisting of serum and vaccine. However, in developing countries PET is not assured due to lack of serum, shortages of vaccine and the high price of PET. In order to prevent human exposure, canine rabies can be eliminated through dog vaccination with threshold coverage of 70% in a dog population.

In Chad canine rabies has been endemic as shown by the long history of rabies diagnosis at the Chadian Veterinary Laboratory (Laboratoire de Recherches Vétérinaires et Zootechniques de Farcha, LRVZ). No official intervention strategy against rabies exists.

The study: aims, methods, results
The study on canine rabies in N’Djaména was a research partnership between the Swiss Tropical Institute in Basel (STI), and the Centre de Support en Santé Internationale au Tchad (CSSI/T) and the LRVZ in N’Djaména. A Swiss veterinarian (DVM candidate) collaborated with a Chadian geographer.

The study was divided in a study of incidence of canine rabies, a demography study of the dog population and a pilot vaccination campaign. The aim of the incidence study was to find the number of new cases in dogs, and to establish case histories with regard to ownership, confinement, prior rabies vaccination of the rabid dog and exposure of people. With the demography study we intended to estimate the owned dog population of N’Djaména as well as its vaccination coverage, and to establish the constraints to dog vaccination. The knowledge on rabies of the population and dog keeping practices were asked. The aim of the pilot vaccination campaign was to measure vaccination coverage after a free parenteral vaccination campaign, and to estimate the number of inaccessible ownerless dogs in the study zones.

Rabies cases were found passively but at the beginning of the incidence study we informed local authorities and the public on rabies symptoms and the new diagnostic facilities at the Chadian Veterinary Laboratory. Each animal brought for examination was registered with a questionnaire. To diagnose rabies we installed the Immunofluorescent Antibody Test facility at the LRVZ. For dog demography studies a representative household survey was conducted in 600 households. Households were chosen in clusters with a probability proportional to the size of the cluster. Every dog and all the persons belonging to the household were registered with a questionnaire. The dog population estimate was based on the dog per human ratio, calculated with a negative binomial model taking into account a random effect for clustering of dogs. To measure vaccination coverage after the campaign we used a capture-mark-recapture approach. All vaccinated dogs were marked with a collar, and in a household survey and on a transect line marked and unmarked dogs were recaptured. With a mathematical model the owned dog population, and the number of ownerless dogs in the respective study zone were then calculated and overall vaccination coverage was established.

We found a yearly incidence risk of canine rabies in N’Djaména of 1.4 (95% C.I.: 0.9-2.3) per 1000 unvaccinated dogs. Eighty-five percent of rabid dogs were owned, however 86% of them were free-roaming and 90% were never vaccinated against rabies. On average 2.4 persons were exposed per rabid dog. The dog population of N’Djaména was estimated at 23’560 (95% C.I.: 14,570-37,898) dogs with a vaccination coverage of 19%. Most of these dogs were free-roaming. Constraints to dog vaccination were the cost of vaccine and difficulty in transportation to the veterinarian. Seventy-six percent of the interviewed knew that rabies is a transmissible disease from dogs. The overall vaccination coverage after the pilot campaign was 87% (95% C.I.: 84-89%) in zone I, 71% (64-76%) in zone II, and 64% (58-71%) in zone III, and the ratio of ownerless dogs to owned dogs was 11 (95% C.I.: 0-31) per 1000, 76 (7-165) per 1000 and 106 (16-191) per 1000 in the respective study zone. Dog owner participation was high and the campaign was thoroughly feasible from a logistic point of view.

Conclusion
Dog rabies is endemic in N’Djaména, mostly transmitted by owned dogs. A full-course PET is not available and access to human vaccine is difficult due to financial constraints and shortages of vaccine. The dog population in N’Djaména is accessible for parenteral vaccination and a high vaccination coverage can be reached due to good dog owner participation in the free vaccination campaign and the low number of ownerless dogs. The control of human rabies in N’Djaména through regular dog vaccination campaigns can be recommended. However, the question of funding these campaigns still remains a barrier to clear.