ECOLOGY AND EPIDEMIOLOGY OF ANTHRAX IN THE ETOSHA-NATIONAL-PARK, NAMIBIA

Analysis of mortality records has revealed distinct patterns in the in cidence of anthrax in elephant and plains ungulates. The seasonal peak among the former is in November at the end of the dry season, while a mong the latter it occurs in March towards the end of the rainy season . Among elephants, there has been a notable spread of the disease to t he west of the Park. Age and sex analyses indicate that, except for ze bra, proportionally greater numbers of adult males die of anthrax amon g the species predominantly affected; however, zebra carcases are diff icult to sex. In a study to identify possible environmental sources of infection, B. anthracis was detected in 3,3% of 92 water and 3,0% of 230 soil samples collected at different times of the year from 23 site s not associated with known cases of anthrax. Slight seasonal differen ces were noted with 5,7% positives occurring in the cold-dry period (M ay to August), 3,5% in the hot-dry season (September to December) and 1,4% in the hot-wet season (January to April). Higher rates (26,0% of 73 samples) were found in water from waterholes in the western part of the Park at the time of an outbreak in elephants. The possible import ance of scavenger faeces was confirmed with > 50% of vulture, jackal a nd hyaena faeces collected from the vicinity of confirmed anthrax carc ases yielding B. anthracis, sometimes in substantial numbers, while no spores were found in faeces not associated with known anthrax carcase s. Despite terminal B. anthracis levels of usually > 10(7) cfu/ml in t he blood of animals dying of anthrax, spore levels in soil contaminate d by such blood at sites of anthrax carcases ranged from undetectable to a few tens of thousands. The rapid loss of viability in soil and wa ter of anthrax bacilli was monitored experimentally and the importance of soil type demonstrated. Survival and extent of sporulation of the bacilli in water were shown to be dependent on the rate at which the b lood was diluted out. Other relevant parameters examined were backgrou nd flora, pH and sunlight.