Chad has an estimated herd of 91,522,170 small ruminants. These livestock provide livelihoods and food security as well as employment opportunities for 300 pastoralist families [1] .

However, the Chadian herd is subject to multiple constraints that hinder its development. Among these, the pathological constraints are not the least. In this case, the constraints related to peste des petits ruminants, which is enzootically rampant on the national territory. Peste des Petits Ruminants (PPR) is a highly contagious transboundary viral disease of small ruminants, considered to be one of the most damaging to livestock. PPR now affects nearly 76 countries in Africa, the Middle East and Asia [2] , China, most of the Indian subcontinent and some European countries, such as Türkiye [3] . The risk period for PPR is between January and April, with a peak observed in February. The key risk factors for PPR are the environment in which the farm is located, the introduction of new animals into the farm, the gathering of animals to pasture or water together, the wandering of animals and transhumance with respective opinion percentages of 54%, 33%, 27%, 22% and 7%. PPR is generally characterized by the onset of depression, mouth sores, respiratory problems [4] , fever, oculonasal discharge, stomatitis, diarrhoea and pneumonia with foul-smelling breath leading to death [5] . It can lead to high morbidity and mortality rates, reaching up to 100% and more than 90% respectively in naïve herds. Mortality occurs between 5 and 10 days after infection, and convalescent animals develop strong lifelong immunity. In Chad, the disease has been endemic since 2013, and the isolation of its virus was carried out in 2014 [6] [7] . PPR control mainly relies on vaccination, isolation of infected animals and restriction of animal movements. As a prelude to the global and pan-African strategies for the control and eradication of PPR, Chad has set up its national strategy for the control and eradication of this disease on the model of these two strategies. Given that vaccination is the key element in the control and eradication of PPR [8] [9] , Chad, through the Ministry of Livestock and Animal Production (MEPA), organized two initial mass vaccination campaigns of small ruminants throughout the national territory, in 2017 and 2018 respectively. This was followed by two more mass campaigns in 2019 and 2023. Chad has made vaccination against PPR compulsory (Order No. 009 of March 24, 2023) [10] . During the last mass vaccination campaign against PPR, 9,485,373 small ruminants were vaccinated throughout the national territory [1] .

This study is undertaken with a view to estimating the seroconversion of PPR Virus (PPRV), through the analysis of small ruminant sera collected in the field from sheep and goats in the study area consisting of the provinces of N’Djamena, Chari-Baguirmi, Bahr El Gazal, Kanem and Lac. The small ruminant herd in this area is estimated at 4,881,959 head in 2022. The 2022-2023 mass vaccination campaign reached 2,155,649 heads of small ruminants [1] .

The results of PPR seroconversion across the study area are presented in Table 1 . Out of a total of 1687 sera analyzed, 1246 are positive. The post-vaccination seroconversion for the study area was 73.56%.

This result is in line with the rate set by the national PPR control and eradication strategy as well as with the sub-regional and international PPR control and eradication strategies. This rate is higher than that obtained in Senegal (67.3%) [11] , that obtained in Niger (73%) [12] , Tanzania (71%) [13] and Rajasthan (54.95%) [14] . However, this rate is lower than that obtained in Burkina Faso (76.3%) [15] .

The results of PPR seroconversion at the provincial level are presented in Table 2 . The post-vaccination seroconversion of PPR by province is not homogeneous and has some differences between them. The results indicate a distribution that varies between 68.67% and 84.21%. Thus, 4 out of 5 provinces have a rate of more than 70% and one province whose seroconversion is less than 70%. To this end, the results obtained for the Provinces of Chari-Baguirmi, N’Djamena, Kanem and Lac meet the first indicator of the 3 PPR control and eradication strategies (national, sub-regional and international strategy), all of which aim to achieve a seroconversion greater than or equal to 70%. The rates obtained respectively of 68.78% and 69.84% in Lac and Bahr Elgazal are close to that set by the national strategy for the control and eradication of PPR. This difference could be linked to constraints during the execution of vaccination campaigns.

The results of PPR seroconversion according to sex presented in Table 3 show that the seroconversion of males exceeds the rate set by the MEPA and is 72.44%, which is lower than that of females (73.36%). These results show that sex does not influence PPR seroconversion (p-value greater than 0.05).

The results of seroconversion according to the performance of the vaccination act are presented in Table 4 . These results show a very significant difference between the seroconversion rate of vaccinated animals, which is 74.92% and that of the unvaccinated, which is 7.89%. The presence of immunized subjects among the unvaccinated could be a confusion on the part of the owners of the animals during the investigation, i.e. they were already in contact with the virus.

The results of PPR seroconversion according to the municipalities presented in Table 5 show that out of 17 communes, 7 have a rate above 80%, 4 have a rate above 71%, 4 have a rate above 61% and which is close to 70% (indicator of the national and global strategy) and 2 communes have the lowest rate: Amsileb with a rate of 49.02% and the first district of the city of N’Djamena with a rate of 54.29%. Seroconversion rates that are less than 70% could be due to the absence of some of the animals during vaccination campaigns, either due to the vaccination act which may not be well accomplished and could be due to technical constraints to ensure the cold chain.

The results of seroconversion of marked and unmarked animals are shown in Table 6 . Marked animals have a seroconversion rate of 71.67% while unmarked animals have a rate of 84.13%. These results are in line with the national, sub-regional and international strategy for the control and eradication of PPR. This high rate of seroconversion of unlabelled animals could be the result of previous vaccinations, during which the animals were not tagged, or it is a post-infection immunity.

The results of PPR seroconversion according to the age of the animals are presented in Table 7 . These results show a PPR seroconversion of between 72% and 75%. These results are significantly higher than the rate set (70%) by the national strategy for the eradication and control of PPR in Chad.

A total of 1687 sera were collected from 17 communes and 52 villages across the study area. These samples were analysed at the Institute of Research in Livestock for Development (IRLD) serology laboratory. The results obtained show a good performance of the effectiveness of PPR vaccination campaigns. The objectives set by the Ministry of Livestock and Animal Production, in its strategy for the control and eradication of PPR by 2030, have been partially achieved. It would be suggested that the General Directorate of Veterinary Services (GDVS), through its Directorate of Animal Health, intensify its efforts in municipalities with low seroconversion and maintain the achievements in those with seroconversion above the target of 70%. In this context, it is recommended that the GDVS to ensure that vaccines are administered appropriately and that the cold chain functions properly.

Our sincere thanks go to the Regional Support Project for Pastoralism in the Sahel Phase II (PRAPS-2-TCD) and the Institute for Research, Livestock and Development (IRED), without which the accomplishment of this work will be impossible.