§ 507-d. Disability benefits. A county may elect to make the benefits provided herein available to the sheriff, undersheriff, deputy sheriffs who are engaged directly in criminal law enforcement activities, and correction officers who are in the employ of the county. Notwithstanding any provision of this chapter or any general or special law to the contrary, any condition of impairment of health caused by diseases of the heart, resulting in the disability or death to a member covered by this section, presently employed and who shall have sustained such disability while so employed, who successfully passed a physical examination on entry into service as a member covered by this section, which examination failed to disclose evidence of any disease or other impairment of the heart, shall be presumptive evidence that it was incurred in the performance and discharge of duty, unless the contrary can be proved by competent evidence.