High seroprevalence for Crimean-Congo haemorrhagic fever virus in ruminants in the absence of reported human cases in many regions of Bulgaria

Crimean-Congo haemorrhagic fever (CCHF) is a severe tick-borne zoonotic disease in humans caused by CCHF virus. It has been observed in Bulgaria since 1952 and over the years more than 1600 cases have been reported in the country. Close contact with viraemic livestock was shown as one of the main causes of the infection. Detection of CCHF virus specific antibodies in livestock can be used as an indicator for virus circulation and risk assessment. CCHF seroprevalence was investigated in 843 cattle, 88 goats and 130 sheep, originating from all 28 districts of Bulgaria. CCHF virus-specific IgG antibodies were observed in 165 cattle (19.6, 95% CI 17.0-22.4%), in 20 goats (22.7, 95% CI 15.2-32.5%) and in 10 sheep (7.7, 95% CI 4.2-13.6%). The highest seroprevalence was detected in the district of Kardzhali, South Bulgaria (86.7, 95% CI 73.8-93.7%), a well-known focus of CCHF in humans. The other two districts with human CCHF cases, Blagoevgrad and Burgas, located in southwest and southeast Bulgaria, showed overall seroprevalences in livestock of 41.9% (95% CI 28.4-56.7%) and 31.3% (95% CI 22.2-42.1%), respectively. Seroprevalences in districts with no history of human CCHF cases varied between 55% (95% CI 39.8-69.3%) and 22.5% (95% CI 12.3-37.5%). These results suggest frequent CCHF virus infections even in regions without human CCHF cases and an enhanced risk of infection for humans in close contact with the infected livestock.