Animal health is probably a bigger business than you think. The global market for animal health was around $34.5 billion in 2015. By 2025, it's expected to grow to more than $58 billion.

With this rapidly growing market, there are plenty of opportunities for investors. Zoetis(NYSE:ZTS), Merck (NYSE:MRK), and Eli Lilly (NYSE:LLY) stand out as the top animal-health companies to buy in 2017. Here's why.

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Zoetis

Zoetis ranks as the world's No. 1 animal-health company, with 2016 revenue of nearly $4.9 billion. The company's products include anti-infectives, vaccines, parasiticides, medicated feed additives, and other pharmaceuticals. Zoetis focuses on improving health for livestock (including cattle, swine, poultry, sheep, and fish) and companion animals (dogs, cats, and horses).

Although Zoetis' sales have increased only 7% since being spun off from Pfizer in 2013, its stock price has nearly doubled. That's partly because the company's earnings have increased by nearly 63% during the period. Just as important, though, is that analysts project continued annual earnings growth in double-digit percentages for the future.

While the economy doesn't appear to be in danger right now of slipping into a recession, Zoetis could be a good pick for weathering a downturn when it comes. Sales didn't slip much during the major recession of 2008 and 2009, probably because people still care for their pets and livestock regardless of what happens in the larger economy.

Zoetis' stock trades at more than 22 times expected earnings, which is a little pricey. The company pays a dividend, but its yield is a relatively low 0.7%. However, if you're looking for a pure-play investment in animal health, there's probably no better choice than Zoetis. Most other animal-health businesses are units within a large pharmaceutical companies.

Merck

One of those animal-health businesses within a large drugmaker is Merck. While Merck reported total revenue in 2016 of $39.8 billion, only $3.5 billion stemmed from its animal health segment. That was enough, though, for the company to rank as the second-largest animal-health business in terms of sales.

Merck's animal-health unit focuses on development and marketing of livestock, poultry, companion-animal, and aquaculture products. These products include antibiotics, vaccines, fertility medications, anti-inflammatory drugs, diabetes drugs for cats and dogs, and parasiticides.

One animal-health product especially helped Merck in the first quarter of 2017. Demand grew for its Bravecto line of products for killing fleas and ticks in dogs and cats. This helped Merck's animal-health business to post year-over-year growth of 13% during the quarter and come in second after diabetes drug Januvia in contribution to total revenue.

Although the company's animal-health business hasn't grown sales as impressively in recent years, Merck could be attractive to investors because of its core pharmaceuticals business. Cancer drug Keytruda is expected to drive sales higher in the coming years. In addition, Merck's dividend yield of 2.95% is a plus for long-term investors.