WESTBROOK — Idexx Laboratories Inc., which makes diagnostic products for veterinarians and tests for livestock and poultry, said fourth-quarter earnings rose 16 percent amid growth in sales in its main companion animal business.

The Westbrook-based company said quarterly net income totaled $43.4 million, or 78 cents a share, compared with $38.0 million, or 67 cents a share a year ago.

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Revenues rose to $319.5 million from $307.2 million a year ago.

While Idexx posted better-than-expected fourth-quarter earnings, its quarterly revenues were slightly lower than expected, analysts said. Shares of Idexx fell $2.83 percent to $96.83 in morning trading on Nasdaq.

Revenue in the companion animal, or pets, group totaled $263.5 million, up from $251.3 million a year ago. In the water-testing business, revenues rose slightly to $20.9 million from $20.0 million a year ago.

Sales in the livestock and poultry-diagnostics business fell 4 percent to $22.6 million, from $24.1 million, driven by lower sales of certain bovine tests due to a reduction of government testing programs in Europe.

For the full-year 2013, the company said it expects revenues to be in the range of $1.405 billion to $1.42 billion, a rise of 8.5 percent to 9.5 percent. Earnings per share are expected to be in the range of $3.47 to $3.57 a share, up from previous guidance of $3.37 a share to $3.47 a share. The increased earnings guidance is due to the federal research and development tax credit, the company said.