El Niño Intensifying West Nile Season in California

This year has become particularly deadly for West Nile disease in California, marking the most severe toll in almost a decade. The situation has worsened as El Niño has ushered in more humid weather, even bringing rain to parts of the state recently.

Currently, West Nile infections have reached a staggering nine-year high, with 210 confirmed cases and 15 fatalities reported. Experts warn that numbers could continue to rise in the upcoming months.

Susanne Kluh, general manager of the Greater Los Angeles County Vector Control District, noted early signs of trouble earlier this year, observing increases in both temperatures and bird infections. In response, efforts have been ramped up to manage the Culex mosquitoes responsible for spreading the virus.

Managing mosquito populations poses a significant challenge. A single female mosquito can lay around 100 eggs, and in ideal conditions, those eggs can hatch within just a few days. Kluh pointed out that early rains in Southern California have been detrimental to mosquito control efforts, as they rejuvenate small pools of water that attract the insects.

According to her, the rain from a recent Sunday was particularly problematic, refilling previously dry areas that mosquitos favor for breeding. Since then, there’s been a noticeable increase in both Culex mosquitoes and the invasive Aedes aegypti mosquito, known to spread dengue fever.

Kluh expressed concern over rising mosquito populations in local areas, all while the number of infected birds remains significant. The native Culex mosquito is the primary carrier of West Nile virus in California, and warm weather allows both the mosquitoes and the virus to replicate and survive more easily.

Most people infected with West Nile don’t display symptoms, but about 20% do experience mild issues such as fever and headaches. In severe cases, the virus can inflict more serious conditions like encephalitis or meningitis, putting one out of 150 individuals at risk. Among this year’s statistics are 181 cases of neuroinvasive infections, including a recent incident involving a man from Wisconsin who collapsed due to complications associated with West Nile.

Currently, there’s no cure for West Nile, but supportive care is available for patients symptomatic of the disease.

Dr. Matt Willis, an epidemiologist and former Marin County Health Department officer, pointed out that by September, it is usual for West Nile cases to taper off. Yet, as of late September, the California Department of Public Health reported a surge in cases—64 new instances compared to just 25 the week before, with a significant portion coming from Los Angeles County.

“It’s concerning that this trend seems to be accelerating into September,” he remarked.

Both Willis and Kluh indicated that while El Niño is impacting the situation, it isn’t the sole cause of the heightened West Nile activity this season. The rise in infections began earlier in the year, as a low-immunity bird population combined with a heat wave in early March set the stage for this problematic overlap of issues.

Kluh elaborated that many birds had not been previously infected, rendering them susceptible to illness. The warm weather enabled mosquitoes to survive long enough to transmit the virus effectively.

The next few weeks will be crucial, according to Kluh, as the potential continued growth of mosquito populations hinges on whether nights and days cool down. If temperatures drop, the spread of the virus could slow, and the new group of mosquitoes that emerge from the rain may not be as effective at spreading the disease.

This isn’t the first time El Niño has been linked to an uptick in insect populations. During one of the wettest El Niño years in 1998, warnings were issued about a potential insect “boom” across much of the United States, with specific concerns about conditions in California promoting significant increases in tick, flea, and mosquito populations.

Dr. Willis cautioned that if El Niño leads to milder winter temperatures in California, this could create an ideal setting for both Culex and Aedes aegypti mosquitoes, which may subsequently escalate the transmission of both West Nile and dengue diseases.

On the flip side, Kluh noted that not every aspect of El Niño weather will benefit mosquito populations. Heavy rainfall could wash away collected water where mosquitoes lay their eggs, potentially reducing larvae survival rates.

Beyond mosquitos, experts are also monitoring the impact of El Niño on other insect populations. Janet Foley, a veterinarian and disease ecologist at UC Davis, has expressed interest in how wet conditions might favor certain tick species, such as the common deer tick.

Foley remarked that prolonged cool and moist conditions could lead to an increase in deer tick activity. However, excessive rainfall might harm tick populations, as it can encourage the growth of fungus detrimental to them. Moreover, since ticks have a multi-year life cycle, understanding whether populations swell and the diseases they carry, like Lyme disease, may take a significant amount of time.

Ultimately, Foley indicated that a key element in determining an increase in deer ticks or other tick species will depend on whether there’s enough rain to boost populations of the rodents that ticks depend upon for feeding.