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 VeÂteÂrinaire de BokleÂ, BP 503, Garoua, Cameroonb Institut de Recherche Agricole pour le deÂveloppement, 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 ¯ocks of small
ruminants kept in their traditional environment. After 1 year, the mortality rates of the ¯ocks were compared with those of a
control group. The results demonstrated that the mortality rates were signi®cantly decreased in the vaccinated ¯ocks. The
annual bene®t for the herder was estimated to be about 13 400 FCFA (about US$ 22) for a ¯ock of 45 animals. # 1999
Elsevier 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 ¯ocks (Martrenchar et al., 1997a). A
homologous vaccine against PPR has been developed
but a ®eld evaluation still needed to be carried out
(Diallo et al., 1989). The present study was conducted
in order to assess the in¯uence of a ®eld vaccination
with the PPR homologous vaccine on the mortality
rates of small ruminants ¯ocks.
2. Material and methods
2.1. Animals
Local ¯ocks, 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 of
the local Fulbe and Oudah breeds. The goats were of
the local Kirdi breed.
Small Ruminant Research 31 (1999) 277±280
*Corresponding author. Fax: +33-2-96-01-62-23; e-mail:
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 ¯ocks, 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 months
of age were not vaccinated. The second group (770
small ruminants) was used as control. There was at
least one test and one control ¯ock in each of the eight
localities to minimize the geographical effect. Within
each locality, the ¯ocks were randomly assigned to
one treatment. The causes of mortality during the trial
were established through herder's and ®eld agent's
observations. All the animals were identi®ed by ear
tags and a ®eld agent visited each ¯ock 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 in¯uence 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 (8ÿ1�7 variables); the
latter 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 signi®cantly lower in the vaccinated
¯ocks than in the control ¯ocks: 13% against 19% (P
<0.01) for the age class <1 year old and 18% against
28% (P<0.003) in the age class >1 year old (see
Table 1). Mortality in the control group was mainly
caused by pulmonary diseases. Unfortunately, due to
communication problems, a laboratory diagnosis of
these diseases could not be established. Clinical signs
Table 1
Comparison of mortality rates (%) in control (C) and vaccinated
(V) flocks according to the age class
<1 year old �1 year oldLocation
C(n) V(n) C(n) V(n)
1 18.0 4.0 18.6 4.9
(50) (50) (59) (41)
2 2.9 4.9 24.0 0.0
(35) (41) (25) (37)
3 15.4 20.0 26.7 12.5
(26) (25) (15) (8)
14.3 31.6
(28) (19)
4 21.1 10.0 23.4 23.5
(109) (100) (94) (85)
5 25.5 23.6 35.3 25.0
(55) (72) (34) (32)
6 18.2 10.5 0.0 9.8
(22) (57) (6) (41)
7 17.8 21.9 47.5 40.0
(45) (32) (59) (40)
29.2 15.0 21.4 18.9
(24) (20) (14) (16)
8 27.3 12.9 33.3 16.0
(33) (70) (18) (25)
There were one control and one vaccinated flock per location
except in location 3 where there were two control flocks and one
vaccinated flock and in locality 7 where there were two control and
two vaccinated flocks. Overall, mortality rates were significantly
higher in C flocks compared with V flocks (P<0.01 for the age
class <1 year old, P<0.003 for the age class � 1 year old, logistic
regression)
278 A. Martrenchar et al. / Small Ruminant Research 31 (1999) 277±280
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 ®eld 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 ¯ocks 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 ¯ocks
seroconverted at the same time. It can be assumed
that within the small area of a locality, where ¯ocks
Fig. 1. Location of the survey sites (* denotes localities).
A. Martrenchar et al. / Small Ruminant Research 31 (1999) 277±280 279
mixed frequently during daily grazing, all the ¯ocks
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 ¯ocks (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 ¯ocks and not on the
occurrence of the disease within a ¯ock; this could
provide a direct economic consequence of vaccina-
tion. In this way, a comparison of means of mortality
rates between ¯ocks (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 ®eld
experiment allows an other estimate of the economic
bene®ts of PPR vaccination using homologous vac-
cine. If we consider a ¯ock of 15 young animals and 30
adults, the annual productivity of the ¯ock will be
increased by approximately one young animal
(0.06�15) and three adults (0.1�30); this corresponds
to a bene®t of (2000�1)�(5000 � 3) �17 000 FCFA
per year (FCFA comes from the French: Franc Com-
munaute FinancieÁre 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 ®elds agents needed to carry
out the vaccination. The annual cost of the vaccination
will be: 80�45�3600 FCFA while the annual bene®t
to the herder will be 13 400 FCFA (about US$ 22).
Furthermore, in case of severe outbreaks of PPR this
bene®t 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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