Deafness in Camelids

Discussion Topic Created:
Sunday, April 26, 2015
Deafness has become a well-recognized disorder in llamas and alpacas.
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The overall incidence of this condition in camelids is unknown, but an association between deafness and the presence of blue eyes and a white hair coat appears to exist. A relationship between deafness and hypo pigmentation exists across many species (humans, dogs, and cats) and is not unique to the llama and alpaca populations. Neither the mode of inheritance nor the gene defect(s) responsible for congenital deafness are known at this time.

Hearing loss can occur secondary to nerve conductive problems in the middle ear (Otis media) or due to sensorineural injury (inflammation or degeneration of the peripheral auditory receptors or cochlear nerve). Congenital hearing loss in most species is due to premature degeneration or abnormal development of sensorineural structures. The brain stem auditory evoked response (BAER) is a sensitive method to distinguish between conductive and sensorineural hearing loss. The BAER is a rapid, non-invasive evoked potential test that does not require patient attention or any type of behavioral response to the stimulus. Ear headphones or ear inserts are used and three small needle electrodes are placed subcutaneously over different areas of the scalp.

A stimulator generates clicks and the scalp electrodes record averaged electrical activity originating from ear and brain stem structures. A reproducible series of 5 waves are produced which represent sequential neural activity being relayed along the auditory pathway (Figure 1). Wave I is generated by inner ear auditory hair cells and the cochlear nerve. Relay areas in the brain stem generate waves II through V. A simple conduction problem in the middle ear will delay and attenuate waves I through V but will not usually result in their total absence. A severe sensorineural (inner ear) injury will result in absence of wave I, and all subsequent waves (II through V). This is called a "flat line" response and is seen commonly in animals with congenital deafness. This "all or none response" makes the BAER a very sensitive and reliable neurodiagnostic test for congenital deafness.

Initial work on assessing normal BAERs in camelids has allowed us to fine tune the BAER protocol for the llama and alpaca species. Due to unusual external ear canal anatomy in camelids and difficulties in preventing occlusion of ear inserts by ear canal wax and debris, the headphone method of stimulation was compared to the ear insert method for BAER measurement. Results indicated that the headphone method was more reliable, yielded reproducible results, was technically easier, and did not decrease the quality of the BAER waveforms. BAER measurements were done in both sedated and non-sedated crias and adults. General anesthesia was not necessary. Young crias could be done without sedation. Older crias and adults only occasionally required sedation. Sedation was rarely required if headphones were used instead of ear inserts. The equipment was also used to assess hearing in 14 sedated and non-sedated alpacas at a private breeding facility. Good quality BAERs were obtained on all individuals in this remote location and the headphone method was very well tolerated.

Clinical studies at OSU are in progress to assess the relative incidence of deafness in alpacas and llamas with various coat and eye colors. Phenotypic characteristics that are strongly associated with and/or predictive for deafness will be statistically evaluated. Data accumulated to date indicate that deafness is strongly associated with a solid blue eye color in conjunction with a white hair coat. There are rare exceptions. With this information, breeding studies of affected and related individuals, and candidate gene analyses, the genetic basis of this trait will be better understood.

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