incidence of canine rabies in n’djaména, chad

Preventive Veterinary Medicine 61 (2003) 227–233

Incidence of canine rabies in N’Djaména, Chad

U. Kayalia,b, R. Mindekemb, N. Yémadjib,d, A. Oussiguéréc,S. Naıssengarc, A.G. Ndoutamiac, J. Zinsstaga,∗

a Swiss Tropical Institute, Socinstrasse 57, P.O. Box, CH-4002 Basle, Switzerlandb Centre de Support en Santé Internationale, Boˆıte Postale 972, N’Djaména, Chad

c Laboratoire de Recherches Vétérinaires et Zootechniques de Farcha, Boˆıte Postale 433, N’Djaména, Chadd Département de Géographie, Université de N’Djaména, N’Djaména, Chad

Received 4 September 2002; accepted 3 July 2003

Abstract

This work describes for the first time the incidence risk of passively reported canine rabies, andquantifies reported human exposure in N’Djaména (the capital of Chad). To diagnose rabies, weused a direct immunofluorescent-antibody test (IFAT). From January 2001 to March 2002, we werebrought 34 rabies cases in dogs and three cases in cats. Canine cases were geographically clustered.The annual incidence risk of canine rabies was 1.4 (95% CI: 1.2, 1.7) per 1000 unvaccinated dogs.Most of the rabid dogs were owned—although free-roaming and not vaccinated against rabies. Mostshowed increased aggressiveness and attacked people without being provoked. Eighty-one personswere exposed to rabid dogs and four persons to rabid cats (mostly children< 15 years old). Mostof the exposed persons were neighbours or family members of the animal owner. Most exposureswere transdermal bites, but nearly half of all exposed persons did not apply any first wound careor only applied a traditional treatment. In N’Djaména, humans are often exposed to canine rabiesbut do not use the full-course post-exposure treatment and wound care is insufficient. Most rabiddogs would be accessible to parenteral vaccination. Pilot vaccination campaigns are needed to de-termine the success of dog mass vaccination in N’Djaména as a way to prevent animal and humanrabies.© 2003 Elsevier B.V. All rights reserved.

Keywords:Canine rabies; Incidence; Human exposure; N’Djaména; Chad

1. Introduction

Rabies is a viral disease that affects all mammals including humans. It is spread byreservoir and vector species. Despite effective canine rabies vaccines, the domestic dog

∗ Corresponding author. Tel.:+41-61-284-81-39; fax:+41-61-271-79-51.E-mail address:[email protected] (J. Zinsstag).

0167-5877/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.prevetmed.2003.07.002

228 U. Kayali et al. / Preventive Veterinary Medicine 61 (2003) 227–233

is the most-important source of human rabies infection—especially in urban areas ofless-developed and developing countries. Of the 413,450 people in Africa who receivedpost-exposure treatment in 1998, 94.4% had been exposed by a domestic dog (WHO, 1999).Africa reported 204 human rabies-related deaths (161 from domestic dogs).Cleaveland et al.(2002)estimated that the true incidence of human rabies in Tanzania is 10–100 times higherwhen bite incidence is used as an indicator for human cases.

In Africa and elsewhere, only a few studies on canine rabies incidence exist.Kitalaet al. (2000)report 8.6 rabid dogs per 1000 dogs with an active surveillance systemin Machakos District of Kenya during 1992–1993.Beran et al. (1972), Beran and Frith(1986)found an annual incidence risk of 0.4–1.0 per 1000 dogs in six Philippine townsin 1968 and in 1969 and of 1.5 per 1000 dogs in Guayaquil, Ecuador during 1980–1986.In Chad, rabies is endemic as shown by the long history of rabies diagnosis at the Na-tional Veterinary Laboratory (Laboratoire de Recherches Vétérinaires et Zootechniquesde Farcha, LRVZ). Apart from one report on six cases of human rabies (Sirol et al.,1971), no other studies on rabies have been conducted in the country. Within a researchpartnership between the Swiss Tropical Institute (STI), the Centre de Support en SantéInternationale au Tchad (CSSI/T) and the LRVZ in N’Djaména, our aims were: (1) toestimate the incidence risk of canine rabies in N’Djaména, (2) to quantify human andanimal exposure, and (3) to analyse the animal case histories in view of rabies risk andprevention.

2. Materials and methods

2.1. Study site

The city of N’Djaména had a reported human population of 530,965 inhabitants in the1993 census (Ministère du plan et de la coopération, 1995); according to the same source(Ministère du plan et de la coopération, 2001), the extrapolated estimate for 2001 is 775,020persons. The owned-dog population in 2001 is estimated to be∼23,560 (95% CI: 14,570,37,898) dogs with a vaccination coverage of 19% (Mindekem et al., in press). The city isdivided into eight administrative districts. Some residential areas of N’Djaména are mostlypopulated by people from Southern Chad who are Christians or animists; these areas aremore likely to have dogs than the areas populated mostly by Muslims (to whom dogs are“unclean”). Our rabies laboratory (on the premises of the LRVZ) is located in the westernoutskirts of N’Djaména, about 15 km away from the city centre.

2.2. Information campaign

Rabies cases were found passively during the study period (January 2001–March 2002).However, at the beginning of our work, meetings were held with local authorities in all ad-ministrative districts to inform them about rabies. Posters advertising the diagnostic workat the LRVZ were distributed to each local chief (who represents traditional authority andthe city administration). Collaboration with health institutions and the only veterinary clinicwas established to refer suspected cases in animals to the LRVZ. Over a period of 2 weeks

U. Kayali et al. / Preventive Veterinary Medicine 61 (2003) 227–233 229

at the beginning of the study, radio broadcasts were used to inform the population on clin-ical signs of dog rabies, the steps to be taken, and the availability of improved diagnosis.However, diagnostic services were not free (to assure the independent functioning of thelaboratory after the study).

2.3. Questionnaire

For each animal brought to the laboratory for examination, a questionnaire was filled in:species, age, sex, origin and status of ownership were recorded. If a dog was owned, weasked whether it was confined to the family compound, and its rabies vaccination statuswas checked in the vaccination certificate. Furthermore, we recorded symptoms of disease,and the number of exposed persons and animals. For exposed persons, age, sex, originof the patient and his or her relation to the animal owner were recorded. Furthermore,nature of exposure, location on the body and first wound care were noted. Interviews werein French—but sometimes, the questions needed to be translated orally to Arab, Sara orNgambai.

For ethical reasons, and to ensure post-exposure treatment for all people exposed to adiagnosed rabid animal during the study, a stock of human antirabies vaccine (Lyssavac byBerna) was kept at the LRVZ. This vaccine was used in times of shortages but otherwisewas not promoted (to guarantee continuation of supply in pharmacies). Vaccine at no chargewas given to people who could not afford to buy it. All other persons were referred to amedical service for post-exposure treatment at their charge.

2.4. Diagnosis of rabies

To diagnose rabies, we introduced the direct immunofluorescent-antibody test (IFAT)at the LRVZ. This test is the laboratory method most-widely used to diagnose rabies inhumans and in animals (WHO, 1999). It is the standard diagnostic tool for rabies, and it isdescribed inDean et al. (1996). The sensitivity of IFAT is 99%, and specificity is 99.5%(Swiss Rabies Centre, unpublished data). Incidence estimates were adjusted according toRogan and Gladen (1978). For staining, we used the SAD-RNP/FITC conjugate by the SwissRabies Centre. A fluorescent negative result from an animal that had bitten someone, andhad shown suspect behaviour, was confirmed by the mouse-inoculation test (for which fiveyoung mice were inoculated). The sensitivity of the mouse-inoculation test is nearly 100%(Swiss Rabies Centre). Some suspected dogs were killed with a barbiturate (PentobarbitalNatricum®) after sedation (Prequillan®/Sedalin®). However, most dogs were killed bytheir owners through a blow with a stick or a stone or died alone, before they were broughtto us.

2.5. Data entry and analysis

Data were entered and analysed in Epi Info 6.04 (software from Centers for DiseaseControl and Prevention, US Department of Health and Human Services). All rabies casesin dogs were localised in Mapinfo 6.5©, on a digital town map of N’Djaména.


3. Results

3.1. Dog rabies incidence

The test result was positive for 34 of 47 dogs (72%), for three of three cats, and forzero of one monkey. Twice, there was no analysis possible due to the advanced decay ofthe material. The cases were received continually through the whole study period. Theannual incidence risk of canine rabies in N’Djaména was 1.4 (95% CI: 1.2, 1.7) per 1000unvaccinated dogs. Of the 34 rabid dogs, 19 came from the seventh administrative districtof N’Djaména and 9 from the sixth administrative district. None were reported in the third,fourth and fifth administrative districts. Eighty-five percent of rabid dogs were owned, but90% of these dogs had never been vaccinated against rabies (Table 1). The most-commonsigns in rabid dogs were aggressiveness (21 of 34 cases, against 2 of 13 test negative dogs;P < 0.05; OR= 8.8; 95% CI (odds ratio): 1.4, 69.4) anorexia (11 of 34 cases) and changingbehaviour (6 of 34 cases). We observed more male and free-roaming dogs among the rabidthan among the population of a household survey (Mindekem et al., in press). For threerabid dogs, the owner mentioned that the animal previously had been bitten by another dog.

3.2. Exposed persons

A total of 85 persons were reported exposed to the 37 rabid dogs and cats (2.3 personsper rabies case). Dogs and cats accounted for 81 and 4 human exposures, respectively. For58 exposed persons, the questionnaire was available. Children<5 years old made up for19% and 45% were >15 years old. In 33% of the cases, a family member of the animal

Table 1Characteristics of 34 confirmed rabid dogs, N’Djamena, Chad (January 2001–March 2002)

Characteristic Number Percentage

Owned 29 85Ownerless 3 9Ownership unknown 2 6

Always confineda 2 7Partially confineda 2 7Free-roaminga 25 86

Vaccinated several timesa 0 0Vaccinated oncea 3 10Never vaccinateda 26 90

Younger than 3 months 1 33 months–1 year 13 38Older than 1 year 20 59

Male 28 82Male castrated 3 9Female 3 9Female castrated 0 0

a Owned dogs only.


Table 2Details of human exposure, N’Djamena, Chad (January 2001–March 2002)

Detail Number Percentage

Nature of exposureTransdermal bite 53 91Transdermal bite and scratch 3 5Scratch 2 3

Body locationHead 4 7Arm 29 50Leg/trunk 25 43

Wound careNo first wound care at all 12 24Traditional treatment 11 22Water 3 6Soap 2 4Desinfection 21 42Other 1 2

owner was the victim—and in 44% of exposures, a neighbour. In 81% of exposures, theanimal attacked without being provoked. In 24% of exposures, there was no first woundcare at all applied—and in 22%, only a traditional treatment was given (usually by rubbinghair of the suspected dog or salt into the wound) (Table 2). To our knowledge, none of thevictims developed clinical rabies.

3.3. Exposed animals

Most people did not know if and how many other animals had been exposed to the rabidanimal. Nevertheless, there were eight other dogs, three cats, one sheep, one goat, three ducksand three chickens reportedly exposed to rabid dogs and one dog was exposed to rabid cats.

4. Discussion

Passive surveillance does not find all cases—especially, we suspect that rabid animalsthat had not bitten people might not have been brought to us. Exposed people, on thecontrary, usually inform the owner of the animal and claim treatment costs. It is thereforelikely that these cases were brought to us to confirm rabies in the animal (even though ourdiagnostic services were not free), because the cost of post-exposure treatment would stillbe much more costly and treatment would be taken only in case of a positive test result.Underreporting also might have occurred due to transportation difficulties (because thelocation of the laboratory is in the outskirts of N’Djaména, about 20 km away from thedistricts where most dogs are kept). The true incidence of canine rabies in N’Djaména istherefore likely to be higher than the calculated one.

A high percentage of the animals taken to the laboratory for rabies analysis were actuallypositive. Our findings are similar to the rates of positive dogs found in a region of Tanzania


within an active surveillance system (Cleaveland et al., 2002). Most rabies cases occurred inthe sixth and the seventh administrative district of N’Djaména. These are residential areasinhabited mostly by people from Southern Chad who are Christians or animists. They arethe administrative districts with probably the highest dog populations (Mindekem et al.,in press). Most rabid dogs were owned but free-roaming, and old enough for vaccination.Ownership and age distribution of rabid dogs reflected the dog population of N’Djaména(Kayali et al., in press; Mindekem et al., in press).

As expected, most people were exposed by a dog (Wandeler et al., 1993). Mostly, itwas the neighbour who was bitten and reported. Social conflicts arising from the incidentare plausible. Nearly half of all humans exposed did not apply correct first wound care,even though chemical or physical elimination of rabies virus at the site of infection isrecommended as the most-effective mechanism of protection (WHO, 1997). Most humanexposures were transdermal bites. These category-III exposures need to be treated witha complete post-exposure treatment consisting of serum and vaccine (WHO, 1997). Nofull-course treatment could be offered, because there is no serum available in Chad. Childrenare more likely to be exposed than adults (Fagbami et al., 1981). All head-region exposuresoccurred in children. Exposures at this body location have a higher risk of developing rabiesif untreated than exposures on the hand, trunk or leg (Fishbein, 1991).

We believe that the immediate situation of exposed persons in N’Djaména can be im-proved further by the permanent supply of human antirabies vaccine in pharmacies.

Pilot vaccination campaigns are needed to determine the success of dog mass vaccinationin N’Djaména as a way to prevent animal and human rabies.

5. Conclusion

In N’Djaména humans are often exposed to canine rabies but do not use the full-coursepost-exposure treatment, and wound care typically is insufficient.

Most rabid dogs we found would have been accessible to parenteral vaccination againstrabies.

Acknowledgements

We thank the Swiss Development Agency, the Federal Veterinary Office, Emdo-Foundation, and Zürcher Hochschulverein for financing this study. Berna supplied us withhuman antirabies vaccine for which we are grateful. Our appreciation goes to the SwissRabies Centre, which provided us with conjugate and technical support. We also thank thecollaborators, and the collaborating institutions, for the fruitful intercultural collaboration.

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incidence of canine rabies in n’djaména, chad

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