NORTH DAKOTA (KXNET) -- North Dakota now has 55 cases of the West Nile Virus, as the state nears its highest total in seven years.

Twenty-two of the people who have the virus are at least 60 years old, since those who are older are more likely to have other conditions that put their immune system at risk.

Most people who contract the virus are asymptomatic, but those who do get sick typically have aches and a fever, which can progress into neurological symptoms. However, 80% of people remain asymptomatic, according to the Centers for Disease Control and Prevention (CDC).

The virus typically gets to North Dakota as birds fly into the state from the south.

"There's a bird flyway zone that comes from down south, straight up through Nebraska and into the Dakotas," says Amanda Bakken, West Nile Surveillance Coordinator for the Department of Health and Human Services. "Birds migrate in the spring and back in the fall, and those birds bring the disease with them."

Mosquitos then feed on the birds, which is how they transmit the virus the virus to mammals, including people.

The West Nile virus first came to the United States in 1999, and to North Dakota in 2002. Since then, there have been four years in which at least 100 cases were reported. This includes:

* 137 cases in 2006

* 369 cases in 2007

* 127 cases in 2013

* 204 cases in 2018

Bakken says in the early 2000s, free testing was offered around the state until a case was reported in every county, which inflated some of the peaks. In other words, more cases were detected, especially in individuals with milder symptoms.

Nowadays, even when people do have symptoms, they don't always report right away.

"It may be several days to a week before they go to see a doctor. Doctor orders a blood draw. You have a period of time between when they see their doctor and the test results come back and then they get reported to us and investigate," she said. "So sometimes, it can be at best, maybe three days after someone goes in to see a provider before we get notification of results."

There is no definite explanation for the spikes that seem to happen every five to seven years in the state, Bakken says, but this is something that continues to be studied.

"They've tried to do different weather modeling to see if it's something to do with that," she said. "Birds as well. Because we did mention they amplify the virus. Certainly weather conditions played a massive part of it. Just, you know, a lot of times, we actually see an increase when it's hot and dry because mosquitoes and birds are congregating more closely together and then they're having that contact with humans."

A weird statistic found on the HHS West Nile Virus dashboard is the pattern of increasing numbers of cases for older individuals, disrupted only by those from 50-59 years old, who have fewer reported cases than those 30-39 and 40-49.

Bakken says there isn't a clear reason for this, though it could be that people in that age range don't report being sick as often.

"Sixty is kind of a low cut off, if you think about it," Bakken said. "But generally, when we start to have people getting more into their sixties and above, you know, your immune system, it may not be working quite as well as younger people for a multitude of reasons. They may have comorbidities. So you start to have, you know, heart disease, diabetes, blood pressure issues, cancer, all sorts of different things that can negatively impact your immune system, which can weaken it. So then when you're exposed to a virus like West Nile, that can cause more severe symptoms because your immune system is kind of fighting multiple battles."

HHS serves in an advisory role when cases are detected, making sure that local providers continue testing for the virus. Other precautions, such as mosquito spraying, happen at the local level.

Bakken emphasized that the recommendations for what people should do remain the same regardless of the number of cases there are: use insect repellent, wear long-sleeve shirts and pants, and limit outdoor activity at night.

The number of cases typically peaks in late August and early September.