By Lewis Krauskopf

NEW YORK (Reuters) - After starting 2008 with a horrendous slide in their share prices, U.S. health insurers will try to persuade investors that they are not mired in a downturn when they report first-quarter earnings starting next week.

The Morgan Stanley Healthcare Payor index , a broad gauge of health insurers, dropped 35 percent in the first quarter. This was the group's worst quarterly performance in more than 10 years, according to Stifel Nicolaus analyst Thomas Carroll.

The stock decline began after disappointing fourth-quarter results reported in January and February.

Then in March, WellPoint Inc (WLP.N: Quote, Profile, Research) compounded the malaise with a profit warning, citing high medical costs, weaker enrollment and a worsening economy.

Other health insurers also cut their earnings outlooks, but some believe that their problems were company-specific and contained.

"The question is: Has the disease spread, or is there only one really sick patient?" BMO Capital Markets analyst Dave Shove said.

Answers will start to come on Tuesday with results from UnitedHealth Group Inc (UNH.N: Quote, Profile, Research). Investors will want to see if the company, the largest U.S. health insurer by market value, also slashes its 2008 forecast.

WellPoint, the largest by membership, follows on April 23, with Aetna Inc (AET.N: Quote, Profile, Research) reporting on April 24.