This year, cases of West Nile virus are being diagnosed earlier than usual, in a warning sign that the United States could face a large outbreak of the mosquito-borne disease.
Cases usually don’t appear until July, experts say, but nine have already been recorded in 2024 across the United States.
Tennessee and Kansas both recorded their first cases in May, almost two months ahead of schedule. Arkansas, Mississippi, Maryland, Michigan and Arizona have also recorded human cases, with Colorado being the latest, reporting its first on Wednesday.
Mosquitoes carrying the disease are also being detected earlier than usual in states such as Michigan and Connecticut, where the insects arrived at least a month earlier than expected.
And they are being detected in record numbers in and around Las Vegas, worrying officials who say the greater number of infected mosquitoes increases the risk of more people becoming infected.
So far no deaths have been reported.
The map above shows areas where West Nile virus has been detected in mosquitoes (light blue) or people (dark blue) so far this year. The season normally begins in late July in much of the country, but officials are concerned about the early start this year.
About one in five patients develop symptoms of West Nile, which can include a rash, as shown in the image above. Less than one percent of them have a severe form of the disease.
In Las Vegas, officials typically find 6,000 mosquitoes in traps between April and June. But this year they have already found 24,000, or almost four times more than normal.
Of the mosquito ponds, or traps that caught insects, 169 tested positive for West Nile virus, meaning at least one mosquito was infected, breaking the record set in 2019.
Of the nine patients reported nationwide so far, five have suffered from a severe and life-threatening form of the disease.
Sounding the alarm about the emerging situation, Kansas health officials said: ‘Although cases of West Nile virus occur annually in (our state), the troubling trend this year is its unusually early onset.
“Normally, West Nile virus cases begin between late July and early August, but we have already received reports of two cases this year (in May).”
In Michigan, officials echoed: “This discovery (of infected mosquitoes) occurred earlier than in previous years, indicating the need for increased surveillance and preventive measures.”
And in Las Vegas, officials said: ‘This year is unprecedented. We have had the highest number of mosquito clusters test positive early in the transmission season.
‘It’s just late June or mid-late June. This will continue until October. That’s why we are quite worried.”
Vivek Raman, environmental health supervisor for the Southern Nevada Health District, added: “This is a huge number of mosquitoes and we have already identified a concerning number of them carrying West Nile virus.”
West Nile virus cases have been increasing in recent years: about 2,500 cases were recorded last year, more than double the 1,130 in 2022.
However, experts say these numbers are likely very low because most people who contract the virus are not tested for it.
Every year there are about 150 deaths from this disease.
West Nile virus is endemic in the United States and the first cases emerged in 1999, probably after it was brought from another part of the world where the virus was more prevalent.
People contract the disease through the bites of infected Asian tiger mosquitoes (shown by their black and white striped markings) that contract the virus from birds.
The above shows the number of cases reported per year from 1999 to 2023. Cases have remained stable in recent years.
About one in five people develop symptoms, including fever, headache, and a blotchy rash that covers the trunk, head, neck, fingers, and toes.
These can last from a few days to several weeks and patients are treated with fluids, bed rest, and over-the-counter pain relievers.
Of those with symptoms, less than one percent will develop a severe form of the disease. – called West Nile neuroinvasive disease (WNND), which can be fatal.
In this case, the virus infects the brain and causes encephalitis or inflammation of the organ, leading to seizures, disorientation and coma.
About three to 15 percent of patients with WNND die from the disease, and death is sometimes caused by the virus infecting and disabling the cells needed to send impulses to tell the body to breathe.
Young children, adults over 50 years of age, and those with weaker immune systems are at greater risk of this severe form of the disease.
Experts say mosquitoes are active earlier this year due to warmer, wetter weather in many parts of the United States.
They are expected to remain active until October, a period during which people are at increased risk of infection.
And there is also concern about other mosquito-borne diseases, such as dengue.
To protect against West Nile virus, authorities have advised people to use insect repellent and wear long-sleeved shirts and pants in the evenings and early mornings, when mosquitoes are most active.
People have also been advised to remove standing water near their home, as this can be a place where mosquitoes can breed.