Field study of a homologous vaccine against peste des petits ruminants in northern Cameroon

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  • Technical Note

    Field study of a homologous vaccine against peste

    des petits ruminants in northern Cameroon

    A. Martrenchara,*, N. Zoyema, A. Njoyab, A.-C. Ngo Tamab, D. Bouchelb, A. Dialloc

    a Laboratoire National Veterinaire de Bokle, BP 503, Garoua, Cameroonb Institut de Recherche Agricole pour le developpement, Station polyvalente, BP 1073, Garoua, Cameroon

    c CIRAD-EMVT, Campus International de Baillarguet, BP 5035, Montpellier Cedex 01, France

    Accepted 20 April 1998

    Abstract

    A homologous vaccine against peste des petits ruminants (PPR) was investigated in northern Cameroon. The vaccine strain

    was the attenuated PPRV 75/1 strain. The cost-effectiveness of the vaccine was studied by vaccinating flocks of small

    ruminants kept in their traditional environment. After 1 year, the mortality rates of the flocks were compared with those of a

    control group. The results demonstrated that the mortality rates were significantly decreased in the vaccinated flocks. The

    annual benefit for the herder was estimated to be about 13 400 FCFA (about US$ 22) for a flock of 45 animals. # 1999Elsevier Science B.V. All rights reserved.

    Keywords: Peste des petits ruminants; Homologous vaccine; Field study; Sheep; Goats

    1. Introduction

    Peste des petits ruminants (PPR) is a highly con-

    tagious disease which is endemic in northern Came-

    roon. The prevalence rate of PPR was assessed to be

    about 64% in the Far North Province and 14% in the

    North Province (Martrenchar et al., 1995). Earlier

    studies demonstrated that PPR, associated with capri-

    pox infections, could explain partly the loss of pro-

    ductivity in the flocks (Martrenchar et al., 1997a). A

    homologous vaccine against PPR has been developed

    but a field evaluation still needed to be carried out

    (Diallo et al., 1989). The present study was conducted

    in order to assess the influence of a field vaccination

    with the PPR homologous vaccine on the mortality

    rates of small ruminants flocks.

    2. Material and methods

    2.1. Animals

    Local flocks, consisting of both sheep and goats,

    were used. Overall, 1562 animals were involved in the

    study (438 sheep and 1124 goats). The mean age of the

    animals at the beginning of the study was 20 months

    (min. 1 week, max. 9.5 years). The sheep were ofthe local Fulbe and Oudah breeds. The goats were of

    the local Kirdi breed.

    Small Ruminant Research 31 (1999) 277280

    *Corresponding author. Fax: +33-2-96-01-62-23; e-mail:

    a.martrenchar@ploufragan.cneva.fr

    0921-4488/99/$ see front matter # 1999 Elsevier Science B.V. All rights reserved.P I I : S 0 9 2 1 - 4 4 8 8 ( 9 8 ) 0 0 1 3 8 - 2

  • 2.2. Virus

    2.2.1. Vaccine strains

    Homologous PPR vaccine strain (Diallo et al.,

    1989) was used. The strain was cultured on Vero cells

    at the Bokle Veterinary National Laboratory and then

    stored in freeze-dried form at 48C until use. Freeze-dried vaccines were titrated on Vero cells using a

    microtitre system as described elsewhere (OIE,

    1992). The titre was calculated with the Reed and

    Muench method after 10 days of culture. The vaccine

    was used at the 103 tissue culture infective dose 50

    (TCID 50). The RM 65 strain used for the control of

    sheep pox (Ramyar and Hessami, 1968) was also

    included in the vaccine at the 103 TCID 50 dose.

    2.3. Field experiment

    Two groups of ten flocks, each of which were

    concurrently part of a zootechnic trial in the traditional

    environment of northern Cameroon, distributed in

    eight localities were constituted in July 1993 (see

    Fig. 1). One group (792 small ruminants) was vacci-

    nated with the homologous PPR vaccine. Having

    regard to herders willingness, animals 3 monthsof age were not vaccinated. The second group (770

    small ruminants) was used as control. There was at

    least one test and one control flock in each of the eight

    localities to minimize the geographical effect. Within

    each locality, the flocks were randomly assigned to

    one treatment. The causes of mortality during the trial

    were established through herders and field agents

    observations. All the animals were identified by ear

    tags and a field agent visited each flock at least every

    two weeks to record the dead animals. After 1 year the

    mortality rates were calculated using the PIKBEU

    software programmes (Planchenault and Sahut,

    1989). The influence of vaccination on mortality

    was tested in creating different dichotomous variables.

    The dependent variable was the mortality which took

    the value 1 if the animal had died during the observa-

    tion period and 0 if it had not. The independent

    variables were the vaccination status of the animals

    (value 1 if the animal had been vaccinated and 0 if it

    had not) and the localities (817 variables); thelatter variables were created to account for clustering

    effect (each of the 7 variables took the value 1 if the

    animal was in this locality and 0 it he was not) and

    were forced into the model (McDermott et al., 1994).

    The variables were entered in a multiple logistic

    regression (Norusis, 1992). Two analyses were con-

    ducted according to the age class (less or more than 1

    year old).

    3. Results

    Mortality was significantly lower in the vaccinated

    flocks than in the control flocks: 13% against 19% (P

  • characteristic of sheep and goat pox were not seen in

    either the vaccinated or control groups.

    4. Discussion

    As no signs of capripox were observed during the

    study and as another experiment carried out after the

    beginning of our field vaccination trial has shown

    (Martrenchar et al., 1997b) that the capripox valence

    was not effective against goat pox, it can be supposed

    that the main effect of the vaccine was due to the PPR

    valence.

    Prior infection status of the flocks was not deter-

    mined and this may appear as a cause of concern.

    Nevertheless, due to the study design, this drawback

    should be minimized. As a matter of fact, PPR is a

    highly contagious disease and it has been previously

    shown (Martrenchar et al., 1997a) that during a sero-

    conversion outbreak, all the animals of the flocks

    seroconverted at the same time. It can be assumed

    that within the small area of a locality, where flocks

    Fig. 1. Location of the survey sites (* denotes localities).

    A. Martrenchar et al. / Small Ruminant Research 31 (1999) 277280 279

  • mixed frequently during daily grazing, all the flocks

    have the same PPR serologic prevalence; hence, the

    prior PPR status should have been the same between

    control and treatment groups. Furthermore, Diallo

    et al. (1989) have shown that the PPR vaccination

    strain did not diffuse from vaccinated animals to

    unvaccinated animals.

    Conventional good practise does not allow to treat

    individuals as the statistical unit when the allocation

    has been on the basis of flocks (Elbers and Schukken,

    1995). In the case of such a serious disease as PPR, it

    was interesting to assess the effect of vaccination on

    the mortality rates of the flocks and not on the

    occurrence of the disease within a flock; this could

    provide a direct economic consequence of vaccina-

    tion. In this way, a comparison of means of mortality

    rates between flocks (comparison of two samples of

    ten and nine units, respectively) would have not

    accounted for the number of animals used to calculate

    these mortality rates. Hence, the statistical unit was

    chosen to be the animal.

    Economic analyses based on a theoretical 5-year

    dynamic herd model in Niger (Stem, 1993) have

    outlined the high cost effectiveness of a PPR vaccina-

    tion campaign using rinderpest vaccine. Our field

    experiment allows an other estimate of the economic

    benefits of PPR vaccination using homologous vac-

    cine. If we consider a flock of 15 young animals and 30

    adults, the annual productivity of the flock will be

    increased by approximately one young animal

    (0.0615) and three adults (0.130); this correspondsto a benefit of (20001)(5000 3) 17 000 FCFAper year (FCFA comes from the French: Franc Com-

    munaute Financie`re Africaine [Franc African Finan-

    cial Community]; 600 FCFA being roughly equal to

    US$ 1). The cost of the vaccine is 20 FCFA per dose.

    At the present time in northern Cameroon, the PPR

    vaccine is sold 80 FCFA per dose, including 60 FCFA

    to cover the cost of the fields agents needed to carry

    out the vaccination. The annual cost of the vaccination

    will be: 80453600 FCFA while the annual benefitto the herder will be 13 400 FCFA (about US$ 22).

    Furthermore, in case of severe outbreaks of PPR this

    benefit would certainly strongly increase. However, as

    other zootechnical parameters have not been taken

    into account, this result may not be an absolutely exact

    estimate.

    Acknowledgements

    This work was supported by grants from the

    research regional project on small ruminants (Coop-

    eration Aid Fund, Ministry of French Cooperation).

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