<PAGE>   1


                                                                EXHIBIT 10.10(a)

                                HESKA CORPORATION

                              AMENDED AND RESTATED
                              EMPLOYMENT AGREEMENT
                                      WITH
                                ROBERT B. GRIEVE


     THIS AMENDED AND RESTATED EMPLOYMENT AGREEMENT ("Agreement") is entered
into effective as of February 23, 2000, by and between Heska Corporation,
formerly Paravax, Inc. ("Company") and Robert B. Grieve ("Employee"). Company
and Employee are collectively referred to herein as the "Parties."

     WHEREAS, effective as of January 1, 1994, Company and Employee entered into
an Employment Agreement, as amended by Amendment No. 1 to Employment Agreement
dated effective as of March 4, 1997 (together, the "Employment Agreement"); and

     WHEREAS, the Parties desire to restate the Employment Agreement for ease of
reference and to further amend said agreement for the purpose of making certain
provisions more consistent with current Company practice.

     NOW THEREFORE, in consideration of the foregoing, the Parties agree as
follows:

A.   The Employment Agreement is hereby restated and amended as set forth below:

     1. Employment. Company hereby employs Employee as its Vice Chairman of the
     Board of Directors and Chief Executive Officer, and Employee hereby accepts
     such employment.

     2. Duties and Responsibilities. Employee shall serve as Vice Chairman of
     the Board of Directors and Chief Executive Officer, with such duties and
     responsibilities as may be assigned to him from time to time by or under
     the direction of the Board of Directors of Company, and with such on-going
     daily duties and responsibilities as are typically entailed in such
     position. Employee shall devote his full time and energies to such duties.

     3. Compensation. Company shall pay Employee, as compensation for services
     rendered under this Agreement, a "base salary" per year, payable in
     accordance with the usual and customary payroll practices of Company. If
     for any reason during any given year, Employee does not work an entire
     year, other than normal vacations as provided hereunder, the compensation
     will be prorated to compensate only for the actual time worked.



<PAGE>   2


     4. Expenses. Company shall reimburse Employee for his reasonable
     out-of-pocket expenses incurred in connection with the business of Company,
     including travel away from the Company's facilities, upon presentation of
     appropriate written receipts and reports and subject to the customary
     practices of Company.

     5. Employee Benefits. During the term of his employment hereunder, Employee
     shall be entitled to receive the same benefits that the Board of Directors
     establishes generally for the officers and other employees of Company.
     These may include from time to time, medical insurance, life insurance,
     paid vacation time and medical disability insurance.

     6. Termination.

        (a) At-Will. This is an at-will employment agreement and does not bind
        either of the parties to any specific term or duration.

            (i) Employee is free to terminate employment with Company at any
            time, for any reason, or for no reason, and without any prior
            notice.

            (ii) Company is free to terminate the employment of Employee at any
            time, for any reason or for no reason, for cause or without cause,
            and without any prior notice.

        (b) Severance Pay.

            (i) Upon "Involuntary Termination" of his employment (as defined in
            subsections (ii) and (iii) below), Employee will be entitled to
            severance pay in the amount of one (1) year's "base salary" in not
            less than twelve equal monthly installments (subject to all
            applicable tax and other deductions), with the first installment
            being due 15 days after the date of termination and with the
            following installments due no later than monthly thereafter on
            Company's then regular payroll dates. Upon "Involuntary Termination"
            of his employment (as defined in subsections (ii) and (iii) below,
            Company also will pay (or reimburse Employee for) the health
            insurance premiums for the medical and dental insurance coverage
            maintained by Company at the time of termination for Employee and
            his eligible dependents for a period of one year after the date of
            termination, provided that, Company's obligations for such payments
            shall cease immediately if Employee becomes (and remains) covered
            under the medical and dental insurance plan of another employer with
            coverages and benefits comparable to those provided under Company's
            plans and the premiums for which are paid in full by such other
            employer. In

                                       2

<PAGE>   3


            the event such benefits provided by the other employer shall cease
            at any time during the one-year period following Employee's
            termination from Company, Company's obligations for payment or
            reimbursement of premiums under this subsection shall resume for the
            remainder of such one-year period. Upon request by the Company,
            Employee will sign a release of claims against Company and its
            affiliates as a condition of receiving severance pay, payment or
            reimbursement of insurance premiums, or any other benefits or
            payments provided in Section 6(b) of this Agreement. If Employee's
            employment shall end for any reason(s) other than those described in
            subsections (ii) and (iii), below, as "Involuntary Termination,"
            Employee shall only receive pay and benefits which Employee earned
            as of the date of termination.

            (ii) If the Employee is subjected to "Involuntary Termination" under
            this subsection (ii), Employee also shall be entitled to an
            additional twelve (12) months of vesting (calculated from the
            effective termination date, and not from the end of the severance
            pay period) under all stock option agreements, stock purchase
            agreements or other stock rights granted to Employee by Company
            prior to the effective termination date. "Involuntary termination"
            under this subsection (ii) shall mean any termination of Employee's
            employment by Company, other than a termination under one or more of
            the following circumstances:

                 (A) any termination by Company or Employee following Employee's
                 refusal to accept a reasonable transfer to a position with
                 comparable responsibility and salary with any affiliated
                 company that does not involve commuting more than fifty (50)
                 miles each way from his home in the Fort Collins, Colorado
                 area, unless such circumstances constitute a Change of Control
                 Event as defined in subsection (iii), below;

                 (B) Employee shall die, be adjudicated to be mentally
                 incompetent or become mentally or physically disabled to such
                 an extent that Employee is unable to perform his duties under
                 this Employment Agreement for a period of ninety (90)
                 consecutive days;

                 (C) Employee shall commit any material breach of his
                 obligations under this Agreement;

                 (D) Employee shall commit any material breach of any fiduciary
                 duty to Company;

                                       3

<PAGE>   4


                  (E) Employee shall be convicted of, or enter a plea of nolo
                  contendere to, any crime involving moral turpitude or
                  dishonesty, whether a felony or misdemeanor, or any crime
                  which reflects so negatively on Company to be detrimental to
                  Company's image or interests;

                  (F) Employee shall commit repeated insubordination or refusal
                  to comply with any reasonable request of the Board of
                  Directors of Company related to the scope or performance of
                  Employee's duties;

                  (G) Employee shall possess any illegal drug on Company
                  premises or Employee shall be under the influence of illegal
                  drugs or abusing prescription drugs or alcohol while on
                  Company business, attending Company-sponsored functions, or on
                  Company premises; or

                  (H) Employee shall conduct himself in a manner which, in the
                  good faith and reasonable determination of the Board of
                  Directors of Company demonstrates Employee's substantial
                  unfitness to serve.

            (iii) For purposes of this Section 6(b), "Involuntary Termination"
            also shall include a "Change of Control Event." If Employee's
            employment shall end as the result of such "Change of Control
            Event", Employee also shall be entitled to the immediate vesting as
            of the effective date of termination, under all stock option
            agreements, stock purchase agreements, or other stock rights granted
            to Employee by Company prior to the effective termination date. A
            "Change of Control Event" shall occur when, in connection with the
            merger, acquisition or sale of Company or substantially all of
            Company's assets with, into or to a previously unaffiliated third
            party,

                  (A) Employee declines to continue his employment with the
                  acquiring company within 30 days following the consummation of
                  such merger, acquisition, or sale;

                  (B) Employee is not offered or continued in a position of
                  employment with the acquiring entity with comparable pay,
                  benefits, title, and responsibility, for a period of at least
                  nine (9) months following the consummation of such merger,
                  acquisition, or sale, and Employee resigns as a result, with
                  such resignation being submitted no later than

                                       4

<PAGE>   5


                  ten (10) days after the date on which Employee is not offered
                  or continued in such position; or

                  (C) the acquiring company fails to assume or be bound by the
                  terms of this Agreement.

     7. Proprietary Information. Employee agrees that if he has not already done
     so, he will promptly execute Company's standard employee proprietary
     information and assignment of inventions agreement.

     8. Attorneys' Fees. If any legal action arises under this Agreement or by
     reason of any asserted breach of it, the prevailing party shall be entitled
     to recover all costs and expenses, including reasonable attorney's fees,
     incurred in enforcing or attempting to enforce any of the terms, covenants
     or conditions, including costs incurred prior to commencement of legal
     action, and all costs and expenses, including reasonable attorney's fees,
     incurred in any appeal from an action brought to enforce any of the terms,
     covenants or conditions. For purposes of this section, "prevailing party"
     includes without limitation, a party who agrees to dismiss a suit or
     proceeding upon the other's payment or performance of substantially the
     relief sought.

     9. Notices. Any notice to be given to Company under the terms of this
     Agreement shall be addressed to Company at the address of its principal
     place of business, and any notice to be given to Employee shall be
     addressed to him at his home address last shown on the records of Company,
     or to such other address as a party shall have given notice of hereunder.

     10. Miscellaneous. This Agreement shall be governed by the laws of the
     State of Colorado as applied to contracts between residents of that state
     to be performed wholly within that state. This Agreement is the entire
     agreement of the Parties with respect to the subject matter hereof and
     supersedes all prior understandings and agreements. This Agreement may be
     modified only by a writing signed by both Parties. This Agreement shall be
     binding upon and shall inure to the benefit of the successors and assigns
     of the Parties.

                                       5

<PAGE>   6


IN WITNESS WHEREOF, the Parties hereto have executed this Agreement the day and
year hereinabove written.


                                       HESKA CORPORATION


                                       By: /s/ Fred M. Schwarzer
                                           -------------------------------------

                                       Its: Chairman of the Board
                                            ------------------------------------


                                       EMPLOYEE


                                       Name: /s/ Robert B. Grieve
                                             -----------------------------------
                                             Robert B. Grieve

                                       6