African Swine Fever in Two Large Commercial Pig Farms in LATVIA—Estimation of the High Risk Period and Virus Spread within the Farm

Abstract

African swine fever (ASF) was first detected in Latvia in wild boar at the Eastern border in June 2014. Since then ASF has continued to spread in wild boar populations covering almost whole territory of the country. Sporadic outbreaks occurred at the same time in domestic pig holdings located in wild boar infected areas. Here we present the results of the epidemiological investigation in two large commercial farms. Several parameters were analyzed to determine the high risk period (HRP) and to investigate the ASF virus spread within the farm. Clinical data, mortality rates and laboratory results proved to be good indicators for estimating the HRP. The measures for early disease detection, particularly the enhanced passive surveillance that is targeting dead and sick pigs, were analyzed and discussed. Enhanced passive surveillance proved to be a key element to detect ASF at an early stage. The study also showed that ASF virus might spread slowly within a large farm depending mainly on direct contacts between pigs and the level of internal biosecurity. Findings suggest improvements in outbreak prevention, control measures and may contribute to a better understanding of ASF spreading patterns within large pig herds. Culling of all pigs in large commercial farms could be reconsidered under certain conditions.

A Review of Risk Factors of African Swine Fever Incursion in Pig Farming within the European Union Scenario

“…Risk factors that can promote the introduction and spread of the virus at the farm level are multiple: poor farming practices and low biosecurity levels [ 5 , 7 , 21 , 22 ], swill feeding and slaughtering on the farm [ 23 ], introduction of purchased pigs [ 24 ] and products, human behaviours and activities, environmental factors, factors related to society, and the cultural background of the farmers [ 13 , 21 , 22 , 25 ]. In other cases, factors deemed to increase the risk of outbreaks at the farm level include the density or the size of the herd [ 26 ], a free-ranging husbandry system [ 27 ], the proximity to an infectious farm [ 18 , 28 ], usage of out-of-farm semen [ 29 ], ova or embryos on breeding farms, contact with wild boars and external pigs [ 2 , 16 ], and improper disposal of carcasses and manure [ 21 , 22 ]. The European Food Safety Authority (EFSA), assessing the risk of spread of ASF in south-eastern Europe in 2019, considered the following as the main risk factors for ASF spread in domestic pig populations: swill feeding, the presence of free-ranging pigs in some areas of a country, the presence of a substantially high number of smallholders in the country, and home-slaughtering [ 10 ].…”

“…Biosecurity measures on farms and especially at the farm entrance (thorough cleaning and disinfection of buildings, transport vehicles, and personal protective equipment) and health and safety regulations on farms have a role in the introduction of ASFV that remains difficult to quantify [ 12 , 30 ]. Lamberga et al [ 21 , 22 ] recently described an ASF outbreak in a large commercial pig farm in Latvia, where the weakest points identified were the entrances of the farm and the sanitary filters (locker rooms) for the staff that did not provide clear and clean separation between staff reception areas and the animal accommodation. Contaminated fomites and meat or meat products were a potential risk factor for the introduction of ASFV in the herd, considering that farm employees were allowed to bring and eat their own food inside the farm.…”

“…Contaminated fomites and meat or meat products were a potential risk factor for the introduction of ASFV in the herd, considering that farm employees were allowed to bring and eat their own food inside the farm. Additional risk factors identified in the study were contaminated vehicles transporting pigs or carcasses, feed or bedding originating from areas in which wild boars have had access, and the possibility that farm employees or other persons visiting the farm participated in hunting or were otherwise involved in activities linked to wild boars [ 21 , 22 ]. Outdoor-bred pigs and pigs with access to the outside of farm facilities could have contact with the wild boar population.…”

African Swine Fever Laboratory Diagnosis—Lessons Learned from Recent Animal Trials

“…The sample matrices described above are routine for veterinary practitioners or pathologists. However, if passive surveillance is the most important tool for early detection of ASF [ 3 , 4 , 7 ], alternative samples may be better suited, especially for carcasses. In the European Union, the Commission implementing decisions [ 38 ] direct the sampling toward fallen animals that occur in a farm.…”

“…This is particularly crucial because of the fact that the disease is associated with high lethality, but also moderate or even low morbidity and mortality [ 5 ]. The latter is linked to contagiosity that can be moderate in wild boar populations or larger domestic pig farms in the absence of parenteral transmission routes by competent vectors [ 6 , 7 , 8 ]. The animals to be sampled in passive surveillance are obviously sick or have died, so it can be assumed that a significant viral load is present in several organs and tissues [ 6 ].…”