Some of the factors could be refined. For example, it is frequently posited that heartworm degree units for temperature likely influence prevalence rates [26]. Such units are typically measured from 14 degree Celsius (57 degrees Fahrenheit), below which it is difficult to have high heartworm transmission. We have not looked at temperature departures above 14 degree Celsius because our data are annual. Also, subtracting 14 from the annual temperatures would reparametrize the value of β0, but would not change the overall regression fit.