DALLAS — It is the worst year ever for West Nile virus in Texas, which has seen about half of the country’s deaths from the illness this year, health officials said Wednesday.

At least 43 people have died from the virus in Texas, and the state has recorded at least 510 cases of neuroinvasive West Nile, considered the most serious form of the illness because it affects the nervous system, according to the Department of State Health Services.

That is already worse than all of 2003, when Texas had 439 neuroinvasive cases and 40 deaths.

Officials say the numbers are expected to increase through mid-October. They say there are indications that August marked the peak, especially in the hard-hit North Texas area that includes Dallas and Fort Worth. But because symptoms can take two weeks to appear,reports of cases lags behind when people became infected.

‘‘Even if West Nile virus transmission were to stop today, we would continue to see reports of cases for several weeks,’’ said Dr. Lyle Petersen of the Centers for Disease Control and Prevention.

Federal officials say there have been at least 87 deaths nationwide reported to the CDC this year.

There are reporting delays as numbers from individual states are compiled, so that number includes only 35 deaths in Texas.

Nationwide there have been at least 1,069 neuroinvasive cases and 924 non-neuroinvasive West Nile cases. Combined, the total is the largest number reported to the CDC through the first week in September since West Nile was first detected in the United States in 1999, Petersen said.