Heat-stressed Metarhizium anisopliae: Viability (in vitro) and virulence (in vivo) assessments against the tick Rhipicephalus sanguineus

Authors: ALVES, F.M. - Federal University Of Goias; BERNARDO, C.C. - Federal University Of Goias; PAIXAO, F.R.S. - Federal University Of Goias; BARRETTO, L. P. - Federal University Of Goias; LUZ, C. - Federal University Of Goias; Humber, Richard; FERNANDES, E.K.K. - Federal University Of Goias

Submitted to: Parasitology Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2016
Publication Date: N/A
Citation: N/A

Interpretive Summary: The common dog tick, Rhipicephalus sanguineus, is a serious vector of important diseases for humans and animals. Two differently formulated commercial products with the insect pathogenic fungus Metarhizium anisopliae as the active ingredient have been available in Brazil for use against this tick, and the available acaricidal chemicals are losing their efficacy against these ticks. Temperature is known to be one of the more important factors limiting the possible utility of fungus-based products to control dog ticks (and many other tick species vectoring debilitating diseases in Brazil), so this study sought to clarify the effects of elevated temperatures and of heat shock on the growth, germination, and infectivity of the Metarhizium used in these commercial products in the hope of improving their usefulness against the target ticks. The fungus is afforded some significant protection against heat when it is formulated in various oils rather than in water. It was confirmed that at least one of these commercial products can be highly effective against dog ticks if it is prepared in mineral oil even though this particular use and mode of preparation is not officially registered yet with the Brazilian government.

Technical Abstract: The current study investigated the thermotolerance of Metarhizium anisopliae s.l. conidia from the commercial products Metarril® SP Organic and Metarril® WP. The efficacy of these M. anisopliae formulations against the tick Rhipicephalus sanguineus s.l. was studied in laboratory under optimum or heat-stress conditions. The products were prepared in water [Tween® 80, 0.01% (v/v)] or pure mineral oil. Conidia from Metarril® SP Organic suspended in water presented markedly delayed germination after heating to constant 40 ºC (for 2, 4 or 6 h) in comparison to conidia suspended in mineral oil. Metarril® SP Organic suspended in oil and exposed to daily cycles of heat stress (40 °C for 4 h and 25 °C for 19 h for 5 consecutive days) presented relative germination of conidia ranging from 92.8% to 87.2% from day 1 to day 5, respectively. Conversely, germination of conidia prepared in water ranged from 79.3% to 39.1% from day 1 to day 5, respectively. Culturability of Metarril® WP decreased from 96% when conidia were cultured for 30 min prior to heat exposure (40 ºC for 4 h) to 9% when conidia were cultured for 8 hours. Tick percent control was distinctly higher when engorged females were treated with oil suspensions rather than water suspensions, even when treated ticks were exposed to heat-stress regimen. Oil-based applications protected fungal conidia against heat stress. Although Metarril® is not registered for tick control, it may be useful for controlling R. sanguineus, especially if it is prepared in mineral oil.