RIFT VALLEY FEVER VIRUS IN NIGERIA: COMPARATIVE EXPERIMENTAL INFECTIONS OF THREE BREEDS OF INDIGENOUE SHEEP AND SERO-SURVEY OF HUMAN AND ANIMAL POPULATIONS

Abstract

Electron microscopic examination by negative staining and antigen controlled immunodiagnosis techniques on the Zinga virus used for this study showed that it is a strain of Rift Valley fever (RVF) virus. This is based on its morphology and specific reaction with reference to Rift valley fever virus antiserum. Retrospective and prospective immunity surveys in human and animal populations in different ecological zones of Nigeria showed endemic presence of Rift valley fever virus in the country. Out of 3,121 human sera tested for haemagglutination inhibition (HI) antibodies to RVF, 463 (14.8%) were positive of which 390 (85.4%) or 12.8% of total sera tested positive by plaque reduction neutralization test. The highest prevalence of neutralizing (NT) anti-bodies was found in the Sudan Savannah (17.9%) followed by derived savannah (17.7%). The prevalence rate in other zones included swamp forest (17.4%), Plateau (11.3%), rain forest (11.1%) and Guinea Savannah (4.5%). Antibody prevalence was significantly higher among livestock and forest related workers than hospital workers and the general population in different locations of all the ecological zones. Sex distribution analysis of all positive sera in the six ecological zones showed no significant difference between the male and female sex groups tested. In a longitudinal serological survey conducted at University College Hospital, Ibadan, a 5.5 RVF virus infection rate was demonstrated in the human population. There was a significantly higher infection rate during the wet than dry season of the same year. Human sera tested for specific RVF IgM in different ecological zones showed that 107 of 380 (27.4%) contained specific RVF IgM indicating recent exposure to the virus. Similar to results of HI and plaque reduction neutralization test (PRNT) , the highest incidence of RVF virus infection was found in the Sudan savannah. Results also showed significantly higher prevalence of specific RVF among livestock and forest-related workers than the general population and hospital workers. The highest prevalence of individuals with RVF virus specific IgM was found among cattle rearers at Maiduguri with over 50% positivity. Retrospective animal serological survey for RVF immunity showed that 261 of 2365 (11.0%) animal sera tested had HI and NT antibodies. The highest prevalence was found in sheep (17.8%) followed by goats (10.4%), cattle (10.2%) and horse (9.8%). Other animals included camel (3.3%), free range domestic fowls (3.4%) and pigs (0%). Closely related to findings in the human population, the highest prevalence of RVF antibody was found in the Guinea savannah (16.5%), followed by Sudan savannah (15.1%), swamp forest (11.4%), rain forest (4.8%) and derived savannah (4.0). A prevalence of 14.7% and derived savannah 14.7% was found among trade animals. Results showed that animals aged 3 years and above had higher prevalence of RVF antibodies than younger animals. Prospective animal serological survey using 10 sentinel flocks in 4 farms at Ibadan and Ile-Ife showed evidence of active circulation of the virus. This was demonstrated by seroconversions in 10 animals from 4 flocks in 2 farms, namely: University of Ibadan cattle, sheep and goats flocks and Iwo Roadairy cattle. One of the seroconversions took place during the early wet season (April), 6 occurred at the peak of rains, while the others sero converted towards the end of the wet season in the area. Experimental infection of three indigenous breeds of sheep in Nigeria, namely: West African dwarf (WAD), Yankasa and Ouda resulted in fatal disease with Zinga RVF virus. Infected sheep of the three breeds responded by pyrexia within 24 hours of infection. Fever peaked between the 2nd and 4th day p.i. and lasted 6 to 7 days. Viraemia coincided with pyrexia and peaked (10 9 PFU/ml) 3 days p.i. in two Yankasa and one West African dwarf sheep. Peak titre of 10 7.5 PFU/ml and above was detected in all other infected sheep. Infected sheep showed characteristic clinical changes which included hyperactivity, watery and mucoid nasal discharges, watery projectile and bloody diarrhoea, external haemorrhage and manifestations of nervous disorders. Viraemia was followed by mild antibody development in all the infected sheep. There was severe external haemorrhage in the Yankasa sheep but mild to moderate in the other breeds. Haematological changes included a sharp fall in the packed cell volume, haemoglobin concentration and total red blood cell count during the course of the disease. These changes were most severe in Yankasa with a drop of 10.3%, 40.0% and 13.3% in the PCV in WAD, Yankasa and Ouda breeds respectively on day 3 p.i. There were thrombocytopaemia, prolongation of the prothrombin and clotting times of all the infected sheep. Infected animals also showed progressive leucopaenia which was associated with severe lymphopaenia. Total protein dropped sharply during the early phase of the disease but rose gradually from day 5 p.i. The rise was associated with gradual increase in the globulin level from day 5. p.i. On the other hand, albumin level decreased progressively until the animals died. Evaluation of serum biochemical constituents of infected sheep showed sharp and progressive increase in the level of two liver enzymes, alanine aminotransferase and aspartate aminotransferase. The sodium level decreased gradually while potassium was initially stable but later increased in the level of blood urea nitrogen of all the infected animals from day 3 p.i. and this continued terminally. Gross and microscopic examinations of the carcasses of all the infected animals showed significant lesions in many organs which showed that Zinga RVF virus is pantropic for sheep and can also cause disseminated intravascular coagulation in this species.