The pathology and role of bacterial and endoparasitic diseases in mass mortality of lesser flamingos in Kenya

Abstract

The lesser flamingo (Phoeniconaias minor) is the most abundant waterbird species in Kenya and a major attraction for ecotourism in many parts of Africa. Mass deaths in the species have become more frequent in recent years and are believed to contribute to declining numbers of the species. Previous studies into the mass deaths have suggested various toxicological and infectious causative agents but most of these studies have been conducted during outbreaks. This study examined pathology and bacterial and endoparasitic diseases in lesser flamingos during non-outbreak periods and compared these with those of outbreak periods, through retrospective study. The study also assessed environmental factors that could contribute to disease in the species. The study was done in Lakes Bogoria and Nakuru in Kenya. During 4 field visits to the lakes, a total of 57 lesser flamingos comprising 17 that were found dead, 18 sick and 22 healthy ones were examined clinically and by necropsy and samples taken from them were analyzed for haematology, histopathology, bacteriology and parasitology. In addition, tissue samples from 134 lesser flamingos collected over a period of 6 years (1997-2004) and preserved in 10% buffered formalin were processed and examined for histopathological lesions. Environmental data on rainfall, water quality and waterfowl populations for the lakes was collated over periods of 3 to 10 years and analyzed to determine trends that could be associated with disease occurrence. xxi Weakness, coma and death in good body condition were common signs observed in lesser flamingos during the field survey. They were accompanied by heterophilia and lymphopaenia. Congestion, haemorrhage and extensive necrosis in the visceral organs associated with infiltration with heterophils and mononuclear inflammatory cells were major and consistent histopathological lesions in lesser flamingos examined during the field survey and those examined retrospectively. These lesions were most severe in samples obtained during the mass mortality period. Abscesses and marked lymphocytic infiltration into various organs were observed in healthy as well as sick lesser flamingos. Mycobacterial granulomas were demonstrated in 2/57 lesser flamingos examined during the field survey and 1/134 of those examined retrospectively. Pasteurella multocida and Salmonella gallinarum were isolated from livers and spleens of 4/46 (9%) and 1/46 (2%), respectively. One of the birds from which P. multocida was isolated was healthy. Two cestodes, Cladogynia phoeniconiadis and Gynandrotaenia stammeri were found in 50/53 and 1/53, respectively of the birds. One nematode, Striatofilaria phoenicopteri, was found in 2/53 of the birds. Lesions of helminthiasis comprising extensive burrowing by the worms into the intestinal mucosa accompanied by thickening of the latter were observed in one bird at histology. Major fluctuations in water quality parameters, waterfowl populations and weather patterns were observed in the study sites. Conductivity of water in Lake Nakuru, recorded monthly over a period of three years, showed a strong and statistically significant negative correlation with lake depth (r = -0.834, p = 0.01). Concentration of nitrate compounds in the lake water increased after the rains and high levels of Arthrospira sp, the primary food for lesser flamingos, were recorded xxii following the rise in concentration of nitrogen compounds. The lesser flamingo populationhad a positive and statistically significant correlation with the concentration of Arthrospira sp. (r = 0.968, p = 0.032). The population of lesser flamingos in lakes Nakuru and Bogoria were negatively correlated (r = -0.503, p=0.497) and mass mortality events of the lesser flamingo were shown to coincide with concentrations of 500,000-800,000 birds in Lake Nakuru. This study underscores that the causes of mass mortality in lesser flamingo remain as sub-clinical infections in the host and flare up to acute severe disease with high mortalities when suitable conditions prevail. The causes of such mortalities have a multifactorial perspective. Pathogenic bacteria, namely P.multocida, Mycobacterium spp are present in the lesser flamingo population as sub-clinical infections that can flare up to cause mass mortalities when the population is subjected to environmental stressors. This is supported by the range of pathological lesions observed during the study. Isolation of S.gallinarum, a pathogen of domestic poultry, points to possible disease transmission at the lesser flamingo-poultry interfase.