In general, about 1-3% of the tick population carries R. rickettsii, even in areas where the majority of human cases are reported. Therefore, the risk of exposure to a tick carrying R. rickettsii is low.The disease is spread by the American dog tick (Dermacentor variabilis), Rocky Mountain wood tick (D. andersoni), brown dog tick (Rhipicephalus sanguineus), and Amblyomma sculptum. Not all of these are of equal importance, and most are restricted to certain geographic areas. The two major vectors of R. rickettsii in the United States are the American dog tick and the Rocky Mountain wood tick. American dog ticks are widely distributed east of the Rocky Mountains and they also occur in limited areas along the Pacific Coast. Dogs and medium-sized mammals are the preferred hosts of an adult American dog tick, although it feeds readily on other large mammals, including human beings. This tick is the most commonly identified species responsible for transmitting R. rickettsii to humans. Rocky Mountain wood ticks (D. andersoni) are found in the Rocky Mountain states and in southwestern Canada. The lifecycle of this tick may require up to three years for its completion. The adult ticks feed primarily on large mammals. The larvae and nymphs feed on small rodents.Other tick species have been shown to be naturally infected with R. rickettsii or serve as experimental vectors in the laboratory. These species are likely to play only a minor role in the ecology of R. rickettsii.
In cardiology, ventricular dyssynchrony is a difference in the timing, or lack of synchrony, of contractions in different ventricles in the heart. Large differences in timing of contractions can reduce cardiac efficiency and is correlated with heart failure. Types of dyssynchrony
Three chief presentations of dyssynchrony can occur:
Atrioventricular (AV) dyssynchrony occurs when there is an unfavorable difference in timing between atrial and ventricular contractions. Interventricular dyssynchrony occurs when there is a difference in timing between right ventricular (RV) and left ventricular (LV) Systole. Intraventricular dyssynchrony occurs when the timing in a sequence of activations and contractions of segments of the LV wall becomes abnormal. In all three types, changes in timing lead to changes in the dynamic behavior of the myocardial tissues, leading to mechanical dyssynchrony. All three presentations allow distinct and easily reproducible electrical signatures as illustrated by left and right bundle branch blocks, hemiblocks, etc. The concise measurement of the time and morphology of the QRS interval allows the interventional ability to manipulate this interval with biventricular pacemakers. It is important to distinguish ventricular dyssynchrony from ventricular dyssynergy. Dyssynergy refers to changes in relative strength of contractions, whereas dyssynchrony is a change in relative timing of contractions. Once again the terms inotropy and chronotropy apply to the pathology under examination. Biventricular pacing strongly favors chronotropy over inotropy. Diagnosis
Echocardiography and tissue Doppler echocardiography are both needed to fully diagnose the different types of ventricular dyssynchrony. Treatment
Recent studies suggest that cardiac resynchronization therapy can reduce the incidence of ventricular dyssynchrony and thus increase cardiac efficiency.