Clinical signs, lesions and viral antigen distribution in alpacas (Vicugna pacos) naturally infected with Rift Valley fever virus

Abstract

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease affecting both humans and ruminants, causing considerable morbidity and mortality, particularly during the rainy season in Africa. RVF virus (RVFV) infection was first confirmed in camelids in 1962 in Kenya. At the Western Cape Provincial Veterinary Laboratory, several alpaca mortalities were investigated during the 2010-2011 RVF outbreak in South Africa, using histopathology and immunohistochemistry (IHC). Seventeen alpacas were available for this study, with 13 alpacas positive for RVFV-infection by PCR and IHC testing. The 4 PCR- and/or IHC-negative cases were used as controls. In this retrospective study, clinical signs, lesions and viral antigen distribution in alpacas naturally infected by RVFV were documented. Additionally, the tissue and cell tropism in alpacas was studied using immunohistochemistry. Clinically, the majority of the alpacas presented with sudden death with no prior clinical signs. Some of the alpacas had a history of pyrexia, which rapidly progressed to recumbency, depression, respiratory distress, anorexia, and weakness. Unlike lesions typically described for sheep and cattle, macroscopically, the most significant lesions were pulmonary oedema and congestion accompanied by mild to severe ascites. In the majority of the alpacas cases the macroscopic lesions in the liver resembled those of young lambs infected with RVFV, where the necrotic foci were masked by congestion. In RVFV-infected alpacas, microscopic lesions were most consistently present in the liver and lungs and characterized by random, multifocal to extensive and coalescing, hepatic necrosis and pulmonary oedema and congestion. In affected areas in the liver, hepatocytes were swollen or had features typical for apoptosis (Councilman bodies) namely loss of adhesion between cells, condensed hypereosinophilic cytoplasm, chromatin condensation and pyknotic or karyolytic nuclei. Inflammation was minimal with only sparse neutrophils and Kupffer cells in the foci of hepatic necrosis. Comma-or rod-shaped eosinophilic intranuclear inclusion bodies and apoptosis (Councilman bodies) were readily observed in hepatocytes of all RVFV-infected alpacas. In sections of the lungs, there was moderate protein rich alveolar oedema with occasional intravascular macrophages. Only one case had interlobular septal oedema. Immunolabelling for RVFV-positive cases were most consistent throughout the liver samples, followed by the adrenal glands. Labelling was less frequently observed in the spleen, kidney, small intestine, lung, skin and lymph nodes. In these organs, viral antigen was in the cytoplasm of hepatocytes, adrenocortical cells, keratinocytes, large mononuclear cells in the spleen and lymph node most characteristic of macrophages, isolated epithelial cells in the proximal convoluted tubules and thin descending loop of Henle in the kidney, and in endothelial and smooth muscle cells in arterioles and small to medium sized blood vessels of the kidney and lungs. Viral antigen was also in monocytes and intravascular fragments or debris throughout various organs and extracellularly within capillaries in the lamina propria of the small intestines. There was no detectable labelling in the brain and thymus. NovaRed chromogen precipitant was present within the myelin sheaths in the white matter of the cerebrum and cerebellum in 2 of the RVFV-infected cases where brain tissue was available. This was interpreted as non-specific labelling since it was also present in the brain of all the RVFV-negative control cases. In the brain of one of the control cases, there was moderate mononuclear perivascular cuffing, neuronal necrosis, and scattered gliosis of small to medium sized blood vessels. There was also RVFV-positive immunolabelling in the cytoplasm of neurons. This finding was similar to brain lesions and labelling previously reported in alpacas that died following RVF Smithburn vaccination. Therefore, this specific case was likely not a natural infection with wild-type RVFV, but probably due to vaccination with Smithburn vaccine. Clinically alpacas respond like young lambs in that they show minimal clinical signs and die within 12 to 24 hours. Also, like young lambs, the macroscopic lesions in the liver of RVFV-infected alpacas may be obscured by congestion or haemorrhage. Therefore, a diagnosis of RVFV-infection will likely be missed during the initial clinical and post-mortem investigation. However, the microscopic lesions and immunolabelling for RVFV in the liver, adrenal glands, lungs, and spleen are similar to that of sheep. Additionally, apoptotic bodies and eosinophilic intranuclear inclusion typical for RVF were prominent in the liver of all 13 alpacas examined in this study. Therefore, an early diagnosis of RVFV-infection in alpacas is possible based on histology and IHC if tissue samples of the liver are available for examination. If adrenal glands, lungs, and spleen are also available for histological examination the diagnosis is virtually certain and confirmation using another diagnostic test a formality.