**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 This requirement is based on handler and veterinarian requests for the information (personal contact, unpublished).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 [1–4].

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 [5–12]. 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 [5–7, 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).

#### **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.*

Most occupied military kennels may have peak noise at 100 dBA, which requires hearing protection of the handlers upon entering [17–20]. 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**).

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 [21–23]. 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.

#### **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.*

*Canine Hearing Management DOI: http://dx.doi.org/10.5772/intechopen.105515*

