Dinâmica populacional e controle estratégico de Ornithonyssus sylviarum (Acari: Macronyssidae) em granjas comerciais de postura de Minas Gerais, Brasil

This study aimed to evaluate the population dynamics of Ornithonyssus. sylviarum, its sensibility in vitro to chemical acaricides and to evaluate the strategic chemical control in laying chicken farms of the Midwest region of Minas Gerais. The first determining the population dynamics of infestations was performed in two laying poultry farms between the months of August 2013 and August 2014. Monitoring the intensity of infestation by O. sylviarum was conducted every four weeks through the technique of visual examination of the pericloacal region. In each hen house 30 hens were analyzed and the number of mites was estimated by a score system of infestation ranging from 0 to 7. A descriptive analysis of the data, Spearman correlation and later analysis of survival and also a multinomial logistic model were performed. The descriptive analysis showed that O. sylviarum was present in both farms throughout the monitoring period, with an increase in the intensity of infestation from October to December 2013 and from May to July 2014. By Spearman correlation, the variable "scores" was the only significant (p 0.05) and the survival analysis demonstrated that the score 0 persisted for longer throughout the study and was different for the other scores (p < 0.0001). In a multinomial logistic model, the variable "humidity range" increased the risk (p < 0.01) of finding other scores in regarding to score 0. The variable "daily temperature range" was significant for almost all infestation scores (p 0.05) reducing the risk and the variable "poultry" had a significant effect (p < 0.05), but marginal for almost all the scores. The in vitro susceptibility of O. sylviarum to the three chemical acaricides was performed between January and March 2015 using an adaptation of the larvae package technique. The mortality of mites was assessed after 24 to 36 hours of exposure to different products concentrations. The DL50 and DL95 calculations were performed by Probit analysis using the concentrations in milligrams. The product 2 (cypermethrin + chlorpyrifos) was chosen and utilized in poultry farms for mite control. The second accompaniment of population dynamics concurrently with the strategic chemical control was carried out between January and December 2015 in the same laying farms. The monitoring of O. sylviarum population was performed similarly as described for the first study. The strategic control was performed from May to July and October to December, periods in which it was observed an increase on parasite population. Thus, pericloacal spraying with manual costal pump using the product 2 according to the manufacturer recommended concentration was performed adopting the cutoff point of score 3 as criterion for its implementation. Treatments were made with an interval of one week. A descriptive analysis of the data as well as survival analysis were performed after preliminary univariate assessments of the variables. The descriptive analysis showed that there was a reduction in the frequency of chickens with higher infestation scores and consequently an increased frequency of hens with lower infestation scores or uninfested at the start of the strategic control up to the end of the monitoring period. The occurrence of scores 3 considering the 12 months of the year was different (p 0.05, Pearsons Chi-Square test) comparing treating and not treating, with a significant reduction (p < 0.01, Pearsons Chi-Square test) from 80% to 20%. Moreover, the frequency of persistence of scores < 3 was significantly higher as compared to untreated (p < 0.001, Wilcoxon test) while the frequency of persistence of poultry infested with higher scores was significantly reduced (p < 0.0001, Wilcoxon test) in a shorter period of time when strategic control was performed. The strategic control was successful to significantly reduce the proportion of hens with higher infestation scores and to maintain this low ratio for a longer period below the threshold cause significant economic losses.