Canine Hearing Management

Abstract

The United States military employs multipurpose canines as force multipliers. A newly developed baseline audiology program applicable to noise effects on the hearing threshold for these dogs has just been developed by the University of Cincinnati FETCHLAB using brainstem auditory evoked potentials to detect estimated threshold shifts in this population. Dogs that are routinely deployed are subject to consistent exposure to noise in the field. Few investigations have focused on the effects of transport noise on the auditory system in multipurpose dogs. The consequence of these dogs having a significant hearing threshold shift is a failure of the dog to properly respond to voice commands and to miss critical acoustic cues while on target. This chapter specifically discusses the baseline protocol for audiological testing of special operations’ multipurpose canines related to helicopter transport.

1. Introduction

The United States Military employs multipurpose canines as force multipliers. The primary breeds of dogs serving are the Belgian Malinois and the German Shepherds. A baseline audiology program has previously never been developed that is adequate to their needs as it applies to noise effects on canine hearing. Thus, there remains a need for criteria to be developed for canine auditory fitness. Presently, auditory fitness in dogs is judged by handler observation of canine behavior, including response to verbal commands, veterinary otoscopic examination, and ability to train [1].

Constant noise can have physiologic and psychological effects in several nonhuman species [2]. This investigation was focused specifically on the deleterious effects of environmental noise on the auditory system in dogs. Whether constant noise can affect dogs, particularly working dogs that are relied upon for their enhanced sensory capabilities (e.g., those used in special military operations or search and rescue), it is important to determine the conditions or environments that can potentially impair these sensory capabilities to adequately understand their impact on canine hearing. The most important frequencies for multipurpose canines to hear in practicality are in the human audible range of 20–20,000 Hz (even though dogs are very sensitive to higher frequencies past 20,000 Hz) since, operationally, it is paramount for the dogs to be able to take vocal commands from the handler and that higher frequencies attenuate rapidly in the field. This requirement is based on handler and veterinarian requests for the information (personal contact, unpublished).

Although, anatomically, the canine ear canal differs from humans and the canine cochlea differs anatomically (where dogs have a higher range of frequencies of hearing than humans), functional magnetic resonance imaging (fMRI) studies have shown analogies between human and canine auditory cortices and central auditory systems [1, 2, 3, 4].

As a result of the number of cases of congenital deafness in dogs, the veterinary and breeding communities have made an extensive effort to perform auditory screening between the ages of 5–8 weeks of age. The only acceptable audiological test for determining baseline hearing acuity is the brainstem auditory evoked response (BAER) test [5, 6, 7, 8, 9, 10, 11, 12]. We are using BAER testing for threshold estimation as a baseline for establishing current hearing threshold in dogs in the current protocol. Another test that can be used for baseline and routine follow-up testing is the distortion product otoacoustic emission (DPOAE) [10, 11, 13]. In addition, the auditory steady-state response (ASSR) has also been used to evaluate hearing in dogs [10] The Malinois breed is not one of those recorded on the list of breeds known to suffer from congenital deafness, although the German Shepard dog is on the list [14].

The BAER electrophysiological test is relatively objective in its output (waveforms); however, the establishment of which peak on the resultant waveforms is subjective with the possible exception of Wave-V and the subsequent trough (VT) of Wave-V. This routine technique that has been used with humans since 1967 [15] and slowly introduced into the animal industry since the 1980’s [5, 6, 7, 11, 13].

The comparison of evoked responses with behavioral hearing thresholds would be the norm when attempting to determine the normal hearing threshold of an animal [1]. In this situation, the subjects of this testing were dogs that were already kenneled for some time and had already been in previous flight training situations. The testing was conducted using an opportune time when flight training was underway.

Figure 1 shows the canine hearing threshold. Figure 2 is an example of a typical canine BAER waveform. Figure 3 shows hearing thresholds for tested subjects. Figure 4 shows examples of the BAER waveforms for a tested subject. Outside of congenital deafness, elevated hearing thresholds have been recorded in military working dogs (MWDs) during transport in trucks and helicopters, when exposed to gunfire and explosives, and commonly in working dog kennels (data from samples taken on military bases-unpublished). Most occupied military kennels may have peak noise at 100 dBA, which requires hearing protection of the handlers upon entering [17, 18, 19, 20]. The consequence of significant elevated thresholds is a failure of the dog to properly respond to voice commands and to miss critical acoustic cues while working, especially when working in gunshot or explosive noise (Personal correspondence) (Table 1).

Figure 1.

These are behavioral hearing threshold curves for various breeds of dogs [16], where 1 is generalized canine threshold, 2 is Poodle, 3 is Dachshund, 4 is Saint Bernard, and 5 is Chihuahua.

Figure 2.

A typical canine BAER trace taken in the right ear of a dog at 90 dB peSPL (54 dB nHL) using a broadband 100-microsecond click stimulus. Various peaks are marked as stops along the auditory pathway.

Figure 3.

Hearing thresholds based on BAER tests to estimate threshold shifts obtained pre- and post-30-minute and 60-minute military helicopter flights for seven multipurpose canines using a 100-microsecond click stimulus over ER-3 ear inserts.

Figure 4.

Example of a (A) baseline BAER threshold at 50 dB peSPL, (B) 30-minute postflight BAER threshold at 120 dB peSPL, and (C) 60-minute BAER threshold after helicopter flight noise exposure at 70 dB peSPL showing threshold recovery over time.

	Dog 1 RE	Dog 1 LE	Dog 2 RE	Dog 2 LE	Dog 3 RE	Dog 3 LE	Dog 4 RE	Dog 4 LE	Dog 5 RE	Dog 5 LE	Dog 6 RE	Dog 6 LE	Dog 7 RE	Dog 7LE	Average
Preflight	32	20	32	34	24	34	44	44	34	24	14	24	14	14	27.71
Post 30 minutes	50	64	54	64	64	54	34	24	64	54	84	84	59	64	58.35
Post 60 minutes									39	54	54	74	34	24	46.5

Table 1.

Descriptive statistics for all dog hearing thresholds in units of dB HL.

Routinely deployed dogs are subject to relatively consistent exposure to noise in the field during training and operations. Although hearing protection devices (HPDs) for canines exist commercially, those that we have tested do not sufficiently attenuate frequencies below 1000 Hz. These low frequencies are particularly important to attenuate for multipurpose dogs that are exposed to machinery, helicopter flights, certain military operations, and explosives [21, 22, 23]. Currently, when multipurpose canines are transported in helicopters, each handler uses his/her own method for ear protection, and in many cases, no hearing protection device is used. The Army Research Office has awarded tasking to develop both over-the-ear (snood) and in-ear electronic HPDs. The dogs tested in this project were undergoing routine flight training and were not wearing any hearing protection except for one dog who wore a snood to see what BAER thresholds would result in after using an HPD.

This chapter will specifically discuss our newly accepted baseline protocol to be used for the audiological testing of military working dogs and the impacts of helicopter transport noise as an example on working canine hearing threshold. This protocol has now been accepted by the Army Special Operations Command and is in use at multiple bases where multipurpose canines are trained and housed.

2. Methods

The BAER test provides an electrophysiological measure of neural responses (from the cranial nerve VIII and lower brainstem auditory nuclei) to auditory stimuli through the use of surface or subdermal electrodes. The technique is a widely used objective measure of auditory system function in humans and has also been used extensively in the auditory assessment of dogs [4].

Typically, a BAER test involves five waves occurring within 6–15 milliseconds following the evoking stimulus [3]. The fourth and fifth waves in the sequence will, occasionally merge into a single broad wave or a wave IV/V complex making identification of all the five waves difficult to define. Our experience with puppy screening and diagnostic testing in the FETCHLAB clinics is that we do see five waves most of the time. The second wave in the sequence (wave Il) is often of sufficiently small amplitude that it is masked by the background recording noise and, therefore, not readily identifiable [7, 8]. These variances in the morphology of BAER recordings are not considered unusual and likely result from an interaction between the selected electrode placement sites, acquisition parameters, and electrical transmission characteristics of the various tissues interposed between the neural generators and the electrode recording site [8].

When BAER testing is used to quantify hearing threshold levels, the most commonly used interpretation metric involves the identification of the lowest stimulus intensity at which the fifth peak in the sequence, or wave V, can be identified (the wave V threshold). This is also known as the lowest observable response level (LORL) [17].

3. Baseline test procedure

This testing was accomplished under the University of Cincinnati IACUC protocol #07-12-19-01 and USAMRMC proposal number 18263008, award number A2-7467. This protocol was approved after annual review on July 18, 2021. The hearing evaluation was considered normal clinical testing. Moreover, advantage was taken of these multipurpose canines undergoing normal flight (helicopter) training. We simply conducted pre- and posthearing evaluations to obtain auditory data for this research. Given the limitations placed on us to have access to these dogs, we were only able to conduct limited further threshold BAER testing at later times postflight to observe changes in the postflight threshold shifts.

Seven [7] military working dogs (MWDs) ranging from 2 to 5 years of age were baseline tested at a military base veterinary clinic using the following procedure: a BAER threshold estimation test was run using a 100-microsecond click stimulus on an Intelligent Hearing Systems (IHS) unit. This test was conducted using the following parameters:

Polarity: rarefaction.

Rate: 31.1.

Sweeps: 500.

Stimulus intensities: 110 dB peSPL (74 dB nHL), 100 dB peSPL (64 dB nHL), 90 dB peSPL (54 dB nHL), 80 dB peSPL (44 dB nHL), 70 dB peSPL (34 dB nHL), 60 dB peSPL (24 dB nHL), 50 dB peSPL (14 dB nHL).

*There is a 34-dB conversation/calibration factor from dB peal sound pressure level (peSPL) to dB normal hearing level (nHL) on the particular IHS system used for this project.

Amplification: 100,000.

Low-pass filter: 1500 Hz.

High-pass filter: 100 Hz.

Stimulus: 100 microsecond click.

Based on the results of the baseline BAER, a Wave I–V latency intensity function was developed for each ear.

The BAER test analysis consisted of observing similar wave 1, III, and V latencies in two separate runs.

All the dogs were taken directly from their kennel and then pretested in the veterinary clinic in the kennel complex. Prior to testing, an otoscopic examination was conducted to ensure that no occlusion or the possibility that conductive issues were present. The tympanic membrane was viewed in all the dogs. The dogs then proceeded directly to their flight training where they were flown for 30 minutes in an H60 helicopter as usual. The handlers normally used their own means of protection for the dog such as simply folding the ears over, cotton, or nothing at all. In this case, the handlers elected not to use any form of hearing protection with the exception of one handler by choice. This flight time was shorter than normal although, depending on the mission, helicopter transit times vary greatly. At the end of flight, they were immediately brought back to the veterinary clinic and retested (postflight). Threshold estimations were noted. Dogs were sedated during BAER testing procedures with dexmedetomidine based off weight to minimize muscle artifact.

The procedure began with BAER testing each dog at an intensity of 76 dB hearing loss (HL) and increasing the stimulus intensity by 10 dB HL until all the waves were present and a second set of similar waveforms existed. Then, the intensity was lowered by 5 dB HL until similarity was no longer present. This, then, constituted the dog’s threshold for that test sequence. The same protocol was followed postflight, 30 and 60 minutes later.

4. Noise level in flight

The noise level within the helicopter was measured using a Bruel and Kjaer model 2270 sound level meter. The average noise level (LAeq) was measured over the entire 30-minute flight. An example LAeq from a 30-minute helicopter flight in this project was 107 dBA. An A-weighted decibel (dbA) is a scale for measuring loudness corresponding to hearing thresholds of the human ear.

5. Results

After 30-minute in-flight, a 30-minute postflight BAER threshold estimation was run upon touchdown and with a retest 60-minute postflight to estimate threshold shifts. Results were compared to baseline thresholds. A total of seven canines (all male) were included in this study. Decibels (dB) are defined in different reference units. The dB peSPL is a decibel in sound pressure level comparing the pressure of sound at the microphone of the sound level meter to the reference pressure of 0.0002n dynes/cm². The results are listed in dB nHL, which is a decibel in normalized hearing level when using electrophysiologic testing, such as a BAER test. A conversation factor of 36 dB can change the dB SPL value to dB nHL with the particular IHS machine used for this study. The average threshold of baseline BAER thresholds was 27.71 dB nHL, the average of 30-minute postflight BAER thresholds was 58.35 dB HL, and the average of 60-minute postflight BAER thresholds was 46.5 dB HL. BAER thresholds increased by an average of 30.64 dB HL after 30 minutes postflight, which is equivalent to a moderate-to-severe hearing loss. BAER thresholds decreased after 60 minutes postflight by 11.85 dB HL, which is equivalent to a mild hearing loss. Dog 4 used a Zeteo Tech, Inc. snood-type canine auditory protection system (CAPS) for hearing protection, which resulted in having better thresholds 30 minutes postflight than the baseline. Dog 4 was subjected to kennel noise before baseline testing and had suspected elevated thresholds greater than expected.

6. Discussion

It has been shown that threshold shifts can occur in humans following exposure to noise levels between 90 and 125 dB SPL (8). A temporary threshold shift (TTS) may include a temporary reduction in hearing acuity, which may become evident within minutes after exposure but is usually reversible in time (this time is variable across individuals).

The underlying pathophysiologic changes involve cell death among various sensory and support cells in the inner ear, resulting from an oxidative stress reaction due to long-term overstimulation [15]. Given the similarities of the typical mammalian auditory system and specifically the canine versus human auditory systems, it is not unreasonable to imply that the triggering causes and attributes of the noise-induced hearing loss would be similar if not identical to that of canines.

From a behavioral perspective, canine handlers reported that after touchdown during mission helicopter flights, their canines were not reacting to standard verbal commands, reducing the tactical effectiveness of the canines. Handlers reported that it appeared that the canine was either not listening, was seemingly disoriented, or not readily responding to verbal commands for various lengths of time, which seemed to be canine specific (personal communication). The dogs in this flight training did not show any disorientation but did not react in the usual manner to vocal command once exiting the helicopter. The flight time during this training was shorter than most mission flights. Given the average threshold shift shown by the test dogs in this study (approximately 30 dB nHL), it is reasonable to presuppose that the threshold shift played a significant role in this behavior.

Presently, the actual prevalence of hearing loss in MWDs is unreported or classified. Given the newly established baseline auditory testing for MWDs, these statistics will become available, thus allowing for longer service and care of the dogs.