No easy solution for tackling ticks
Ticks pose a tremendous burden to livestock production. An estimated 80% of the world’s cattle are affected by ticks and tick-borne diseases, both of which cause significant production losses. In addition to the nuisance factor, ticks are known to transmit anaplamosis, an infectious blood disease in cattle caused by bacteria. Anaplasmosis can result in fever, weight loss and severe anemia. Ticks are effective vectors of this disease because they often are removed from a host through grooming and attach to another host, spreading the pathogen from one animal to the next.
Unfortunately, there isn’t a simple solution for controlling ticks in cattle herds. Acaricides—pesticides targeted to the arachnid subclass of ticks and mites—are effective in the short term but do not offer permanent control. While very useful, acaricides have a number of drawbacks. Over time, ticks have developed tolerance to acaricides, and some strains are resistant to all types of this pesticide. As stronger and additional acaricides are needed to counter resistant ticks, worries about public safety become more pressing. For some of the hard-hitting products, there is a significant withdrawal period, making their use untenable for milk cows.
Anti-tick vaccines are an alternative to acaricides. This method of tick control has a number of advantages. There are no withholding periods for vaccinated animals, so it can be used on dairy herds. Vaccine use poses no public-health safety concerns and does not contribute to development of acaricide resistance.
Unfortunately, there are no cheap, robust vaccines that are highly effective against ticks. A vaccine against the cattle tick, Boophilus microplus, confers partial long-term control but has little immediate effect on tick burdens. This vaccine works best on breeds with average tick resistance and can help keep the population below economic thresholds without acaricides.
In animals with low tick resistance, the vaccine alone does not give satisfactory control. A combination approach of vaccination with strategic use of acaricides is needed.
More research is warranted. New anti-tick vaccines hold great promise, and new acaricides are also being investigated. Even with these emerging treatments, control is greatest among cattle breeds that have high natural tick resistance. Host resistance is a heritable trait. Just as milk yield or growth can be selected for in breeding programs, so can tick resistance. This avenue of tick control has seemingly been under-appreciated.
Tropical breeds of cattle, such as those with Brahman genetics, typically have high natural tick resistance. They’ve experienced selection pressure from their environment for hundreds of generations. Creating a breeding program for such a trait, however, is quite complex. Tick resistance in these breeds does not come from one, or two or even three genes—it is a polygenetic trait developed over long periods of time. Because of this, there is not a simple trait, or even set of traits, to select for during breeding.
Additionally, the goal of such breeding programs could not only be tick resistance. Producers also want cattle with high production value. What’s the point of producing an animal that is impervious to ticks if it will neither produce milk nor grow well? This makes it even more complicated—even impractical—to cross temperate and tropical breeds to improve tick resistance.
There is an option for tick control that doesn’t rely on acaricides, doesn’t need to wait for effective anti-tick vaccines, maintains high production values and is achievable in an economically viable timeframe. The solution is to use major resistance genes. This provides heritable resistance from a gene that exhibits substantial protection, as opposed to the polygenetic protection seen in breeds long exposed to high tick burdens.
An example of heritable resistance is found in Belmont Adaptaur cattle, which were developed in Australia by crossing Hereford and shorthorn breeds. Animals were selected for increased resistance to the stresses of the tropics, particularly heat and cattle ticks. This relatively new breed didn’t have the evolutionary time to develop layers of genetic protection. Rather, when exposed to ticks, carriers of the protective gene acquired resistance early in life. That resistance proved to be stable, inheritable and lasted the lifetime of the animal.
Because only one gene is being selected for, this type of breeding program is more practical for cattle producers. Crucially, breeds that have these major resistance genes are very similar to temperate breeds favored for their high production values. The bottom line is that total tick resistance in cattle via heritable traits is potentially achievable and may be the key to effective long-term control.
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