Epidemiological investigation of bovine tuberculosis causes of herd breakdowns and persistence in Spain

Abstract

In Spain, herd prevalence has decreased from 11.1% on 1986 to 1.3% on 2011._x000D_ Despite this reduction on the prevalence, in the last years there has been just a_x000D_ moderate decline and the current bTB epidemiological situation poses a serious_x000D_ challenge towards the achievement of a national official tuberculosis free (OTF)_x000D_ status. In the country, approximately between 50 and 60% of the total bTB positive_x000D_ herds detected each year, are new positive herds, and the rest of the positives are_x000D_ herds that persist from the previous year. There is not a clear knowledge of the_x000D_ epidemiological circumstances that lead to the introduction or persistence of the_x000D_ infection. Therefore, the aim of this PhD was to improve the understanding of bTB_x000D_ epidemiology in Spain by assessing the most likely causes of infection and factors_x000D_ related with bTB persistent herds._x000D_ To estimate the most likely causes of introduction of the infection on a herd we_x000D_ developed a methodology based on bTB routes of transmission between herds using_x000D_ decision trees. We considered seven routes as a possible cause of herd breakdown. In_x000D_ order to discriminate among them, probabilities were assigned within each decision_x000D_ tree based on expert opinion. Macros were used for data management and to_x000D_ calculate the probabilities of the different possible causes._x000D_ According to the results of the 816 studied breakdowns (detected mainly in 2010 and_x000D_ 2011) most of the cases could be resurgence of residual infections that had previously_x000D_ affected the herd (39%, n=316). New introductions of the infection were produced by_x000D_ contact with wildlife in 12% of cases (n=98), and by neighboring herds in 10% of them_x000D_ (n=85). In 28 % (n=225) of the breakdowns, the origin of the infection remained_x000D_ unknown. In 309 herds, the difference between the first and the second cause with_x000D_ the higher probability was small (i.e., less than 1) so, for these herds, first and second_x000D_ possible causes of infection could be considered. Within this group the most frequent_x000D_ first option was residual infection (69%, n=214) and the most frequent second_x000D_ options were sharing pastures with other herds (36%, n=112) and interaction with_x000D_ wildlife (30%, n=94). The results of the comparisons between our conclusions and the ones of the_x000D_ veterinary officers conducting the epidemiological investigation on the field_x000D_ evidenced a low agreement. The lowest agreement was in the case of “wildlife”._x000D_ Within the group of herds where the veterinary officer just considered one cause_x000D_ (i.e., 309 herds) in 124 Wildlife was considered as the most likely cause, while we just_x000D_ found evidences to suspect of this in 33 farms. These discrepancies could be related_x000D_ to differences on access to data and perception about the importance attributed to_x000D_ the different causes._x000D_ In order to assess factors related to bTB persistence we conducted a case‐control_x000D_ study and compared persistent versus transient bTB infected farms from southern_x000D_ Spain. Farms were matched by herd size and geographical location (county). A_x000D_ questionnaire by personal interview was carried on 150 herds (80 controls and 70_x000D_ cases) from Andalusia and Castilla La Mancha regions._x000D_ According to the results of this study, farms with large pasture areas and positive_x000D_ neighbors had more difficulties in eradicating the disease and therefore, were more_x000D_ likely to suffer a persistent bTB outbreak. The odds of bTB persistence were between_x000D_ 1.1 and 5.0 times higher in those herds that had the possibility of contact with cattle_x000D_ from a neighbor positive herd. The probability of bTB persistence was also associated_x000D_ with the area of the pasture. Farms with larger pasture areas had odds between 1.2_x000D_ and 12.7 times higher of having a persistent bTB episode than farms with smaller_x000D_ pasture areas. Management of positive animals such as making replacement from_x000D_ positive herds or not isolating positive animals and the presence of goats within the_x000D_ farm seem to have also an influence on bTB persistence.

Date of Award	17 Sept 2013
Original language	Undefined/Unknown
Supervisor	Alberto Allepuz Palau (Director) & Jordi Casal Fàbrega (Director)