Contagious bovine pleuropneumonia (CBPP) in the Maasai ecosystem of south-western Kenya: Evaluation of seroprevalence, risk factors and vaccine safety and efficacy.

Abstract

Contagious bovine pleuropneumonia (CBPP) is a bovine bacterial disease of major economic importance in sub-Saharan Africa. Vaccination has been recommended to control the disease in endemic areas such as the Maasai ecosystems of Kenya and Tanzania; however, the currently used live attenuated vaccine has been reported to have poor vaccine safety and efficacy. To compare standard (current) and an improved (buffered) version of the live CBPP-vaccine, several epidemiological studies were carried out in Maasai cattle in Kenya between 2006 and 2008. Specifically, the aims were to estimate CBPP seroprevalence at herd and animal level; to identify risk factors for seroprevalence at both levels; to investigate the spatial distribution of seroprevalence; to compare post vaccination adverse events in cattle vaccinated with a standard and a buffered vaccine, and finally to compare efficacy of the two vaccines to induce seroconversion and to prevent development of clinical signs suggestive of CBPP. A cross-sectional study was carried out in 6872 cattle in 175 randomly selected herds from Loita and Mara divisions. A competitive ELISA revealed that 85% of the herds in the area had at least one seropositive animal and that seropositive herds were harbouring 11% seropositive cattle. A complement fixation test revealed that 46% of the herds had at least one seropositive animal and that seropositive herds were harbouring 4% seropositive cattle. A multivariable logistic regression analysis of the seroprevalence indicated that previous vaccination against CBPP, a history of CBPP outbreaks in the herd, animal age and the location of the herd in the division of Mara were positively correlated to seroprevalence. To investigate the observed difference in herd seroprevalence between the two divisions further, a spatial analysis was conducted. A SatScan test revealed clusters in Mara in areas identified by veterinary personnel as CBPP ‘hot spots’. A logistic regression using spatial information identified that location in the midland agro-ecological zone or close to a river and vaccination were positively associated with seroprevalence. To compare safety and efficacy of a standard and a buffered vaccine, two cohorts of approximately 40,000 cattle were used. The study showed that within 100 days post vaccination, 6.2 cattle per 1000 vaccinates developed adverse events, 4.1 of which were specifically attributable to vaccination and ranging from swelling of the tail to the tail sloughing off. This study revealed a slightly higher incidence of adverse events in cattle vaccinated with the buffered vaccine compared to the standard vaccine. A comparison of the efficacy of the two vaccines revealed that cattle vaccinated with the buffered vaccine had higher odds of seroconversion and lower odds of developing symptoms of CBPP, three and twelve months post vaccination respectively. The epidemiological studies conducted clearly show wide spread seroprevalence in the Maasai cattle. Given the (spatial) heterogeneity observed, control measures should probably be targeted in areas of increased risk (clusters). However, positive association of vaccination and seropositivity call for better diagnostics tests that can differentiate vaccinated from infected animals. Vaccination with buffered vaccine resulted in increased seroconversion, decreased clinical signs indicative of CBPP post vaccination and low seroprevalence post ‘outbreak’. Nevertheless, the increase in adverse events related to the buffered vaccine calls for further research into safer CBPP vaccines.