Published May, 2014

Q.I have two geldings, a dark bay and a chestnut, that were turned out together for about two months. When I brought them back to the barn, the dark bay was covered in what seemed like hundreds of deer ticks. The chestnut maybe had five ticks. Why would there be such a difference in tick numbers if they were in the same pasture?

KELLY STEPHENS LEXINGTON, KY.

A.There can be distinct differences in blood-feeding arthropods’ (such as ticks and horse flies) attraction to individual horses. Some differences are inborn among these bugs; others are acquired after previous feeding exposure. Researchers are still learning which factors are important and how to use this knowledge to produce better repellents and protective chemicals for humans, horses, and livestock.

The factors that attract these bloodfeeders vary with the pest. For example, vision is important to horse flies. They see a dark moving shape in the distance and fly toward it. As they get closer, their senses check for other attractions, such as warmth, carbon dioxide, or specific skin odors. Often, dark-colored animals are more susceptible to attack from biting flies than light ones in the same herd because they are easier to see.

Ticks have limited vision so they rely on other senses. Hungry ticks climb vegetation and wait for a passing host. Ground vibrations from animal movement or carbon dioxide indicate a host is approaching. In theory, all horses pastured in the same field should pick up similar numbers of ticks. But once on a horse, the ticks might stay and feed or reject the animal and drop off without a blood meal. Over time, ticks continue to accumulate on “acceptable” animals; few will stay on those that have some sort of resistance mechanism.

Animals’ responses or resistance mechanisms to tick bites appear to be a very important factor in whether ticks remain on an animal. Ticks inject dozens of chemicals as they feed. Animals with strong immune responses to these materials are not good hosts. Their physiological reactions interfere with the ticks’ ability to feed so the ticks drop off without engorging. Differences in skin chemistry or oiliness, or accessibility of capillaries also may interfere with the feeding process. In some cases, strong reactions that result in itching and irritation may increase grooming activity, which reduces tick numbers.

It’s clear that some animals are more attractive to blood-feeding pests than others.

Continued research may help to develop more effective pest control tactics. In the meantime, protection efforts (e.g., sprays, clearing brushy pasture areas, etc.) can be focused around susceptible animals.

Lee Townsend, MS, PhD, is an entomologist in the University of Kentucky College of Agriculture.