“Oh, that’s the spot.” Many of us companion dog parents say that when our pooches involuntarily start moving their legs when scratching their bellies. According to experts, that’s probably not the spot and may be an indicator of other medical conditions. This involuntary response to scratching is coined appropriately as the “scratch reflex”.
What is happening is that the nerves in a dog’s body are engaged by a scratch and send a message to the spinal cord, which then tells the dog’s leg to kick.
But, why does that reaction occur?
Way back in 1906, CS Sherrington of the University of Liverpool wrote an extensive research paper of experiments he performed using electrical stimulation on the bodies of dogs. He then mapped out the “receptive area”, where the stimulation evoked the hind leg response. He surmised the following reason for the scratch reflex:
“The reflex is presumably adapted to act against the parasitic life which makes the hairy coat a habitat. It is difficult to judge in what way and to what extent the scratching movement avails against the attacks of parasites. I have been quite unable to satisfy myself that the movement really combs the parasite out. It may be that the movement causes some reflex in the parasite which inhibits the insect from biting further. Also it seems probable that the scratching allays the irritant sensation in the attacked skin.”
The theory that – a dog’s hind leg involuntarily moves due to bugs or other irritants is a dog’s reaction to get them off for protection – prevails today.
Medical Conditions Associated with the Scratch Reflex
- Atopic Dermatitis
- Canine Scabies
- Environmental Allergies
- Flea Allergy
- Food Sensitivity
- Seasonal Allergies
Indeed, sometimes you may see the scratch reflex heightened during times your dog’s body is in distress from one of these or other medical conditions. It is similar to times when humans feel particularly sensitive to touch.
The Pinnal-Pedal Scratch Reflex
But, the belly is not the only spot that can trigger an involuntary leg scratching motion. Another spot is the ear that can provoke the involuntary kicking response. This is known more scientifically as the pinnal-pedal reflex.
Research has focused more around rubbing the earflap against the base of the ear to help zero in on a diagnosis of conditions like scabies and rule out the other possible causes of the scratch reflex.
In 2001, veterinary researchers at Colorado State University chronicled their experiment to diagnose scabies.
- 588 dogs with various and different skin diseases
- False positives were rare.
- Only 6.7% pruritic dogs (31 out of 463) without scabies had a positive pinnal-pedal reflex using the method described above.
- One of 78 dogs with otitis externa only reacted.
- 30% with pinnal otitis reacted
- 21.6% with both otitis externa and pinnal otitis had a positive pinnal-pedal reflex. These ratios were similar if dogs with diseases typically not considered as differential diagnoses for canine scabies were included.
- 93.8% – The specificity of the pinnal-pedal reflex for canine scabies.
Researchers at Cornell University backed up these results in 2012. They found that the pinnal-pedal reflex was positive in 78.4% of dogs confirmed via skin scrapings and presumed to be so (negative skin scrapings, but cured with antiparasitic medications). Only 1-12% of dogs with other pinnal diseases had a positive reflex reaction.
While this test is not perfect, it is definitely a useful, fast and noninvasive tool in a veterinarian’s arsenal.
Grush, Loren. “Why Does My Dog Kick When I Scratch His Belly?” Popular Science, 27 Oct. 2014, https://www.popsci.com/article/science/why-does-my-dog-kick-when-i-scratch-his-belly/.
Mueller, R.S., et al. “Value of the Pinnal-Pedal Reflex in the Diagnosis of Canine Scabies.” Veterinary Record, vol. 148, 19 May 2001, pp. 621–623, doi:10.1136/vr.148.20.621, https://pubmed.ncbi.nlm.nih.gov/11394797/.
Scott, Danny W., and Jr. William H. Miller. “A Retrospective Study of 350 Dogs Suspected of Having Scabies (1988-1998).” The Japanese Journal of Veterinary Dermatology, vol. 19, no. 1, 2013, pp. 3–9, doi:10.2736/jjvd.19.3, https://www.jstage.jst.go.jp/article/jjvd/19/1/19_3/_article/-char/en.
Sherrington, C.S. “Observations on the Scratch-Reflex in the Spinal Dog.” The Journal of Physiology, vol. 34, no. 1-2, 13 Mar. 1906, pp. 1–50, doi:10.1113/jphysiol.1906.sp001139, https://physoc.onlinelibrary.wiley.com/doi/abs/10.1113/jphysiol.1906.sp001139.