While the western United States is in the midst of its worst season yet for West Nile virus, no infected mosquitoes, birds or humans have been detected yet this season in Massachusetts, a sign the infection’s presence in the state may only be minor this year.

As of Tuesday there had been 182 human cases reported across the nation this year, with 176 occurring west of the Mississippi River, according to the federal Centers for Disease Control. Arizona has been the hardest hit with some 125 cases reported. Nationwide, four people have died this year from the infection, one each in Iowa and Texas and two in Arizona.

In the Northeast, only one human case has been reported, and that was in New York.

By July 1 of last year, infected mosquitoes had already been found in Westfield and infected birds had been found in Springfield and four other Massachusetts communities. Nevertheless, 2003 produced fewer human cases, 19, than in 2002, when 25 occurred.

Dr. Bela T. Matyas, medical director of the Massachusetts Department of Public Health’s epidemiology department, hopes that the absence of the virus so far this year in the state will mean 2004 produces even fewer cases than 2003.

“Hopefully, it indicates that this virus has a cycle and that we’re on the downward trend. Obviously the disease is new in Massachusetts and there’s a lot we don’t know about it,” he said.

Last year in the United States, West Nile virus killed 264 people and sickened nearly 10,000 others. The virus first appeared in the United States in New York City in 1999 and has gradually spread west since then.

The infection can be passed to humans when a mosquito bites an infected animal and then bites a human. Late August through September is considered the peak season for the human infections. About one in 150 people infected will become seriously ill, with some victims developing potentially deadly encephalitis or meningitis.

“Many people become mildly ill with flu-like symptoms or headache, and never realize they have in fact been exposed to West Nile. But people over the age of 50 are at the greatest risk of developing severe illness,” said Dr. Deborah Hoadley of Baystate Medical Center’s Infectious Disease Division and the Massachusetts Tick-Borne Advisory Committee.

Birds are the most common reservoirs for the virus through the winter. Infected mosquitoes pass the virus to birds in the late summer and then new generations of mosquitoes pick it up from birds still harboring the virus in the spring.

Infected crows have been the best indicator that the virus is present in the area, since they die nearly 100 percent of the time from the infection. However, other species of bird can live through the infection. If a bird does not die when initially infected, it becomes immune for life once the illness passes, and it cannot be a carrier again.

Many epidemiologists believe this will create a cycle for the disease. As birds either die or become immune in an area, the number of human cases will decline. However, when enough birds without immunity are born, then the virus has a large enough pool of potential carriers to widely spread again, with a corresponding increase in the number of human cases.

Matyas said Western Massachusetts may offer more opportunities for the spread of the virus right now.

“In Springfield and the western part of the state, there hasn’t been as much activity as in the eastern part of the state. Because it’s newer in terms of being affected, there may be proportionately more activity there soon because of fewer immune birds in the region,” he said.