Rare and rarer

There has been a lot of scary information about mosquito-borne diseases repeated by employees and board members of BLSG. There are two serious diseases carried by mosquitoes in Vermont, and one killed two people in 2012. That fact alone deserves our attention, but how much should we be concerned about these diseases?

According to available CDC data (pre 2016), since West Nile virus first appeared in Vermont in 2011, eight people in the state have become ill from the virus, and we assume they contracted it from mosquito bites. Since 2011, only two people have gotten sick from Eastern equine encephalitis, both in 2012. Tragically, those two people died from the disease.

That is a total of 10 cases of mosquito-borne disease in Vermont. These two diseases are terrible, but they are both rare in the state.  Most healthy people who get infected with these diseases have flu-like symptoms and recover in a few weeks.

To put the risk of these diseases in perspective, they are both much rarer in Vermont than some other diseases that most of us never think about.  In Vermont, we are 10 to 100 times more likely to contract tuberculosis, Legionnaires’ disease, or Cryptosporidiosis (whatever that is) than to get eastern equine encephalitis.  We are four to 40 times more likely to get those potentially deadly diseases than to get West Nile virus.

The risk of contracting three uncommon diseases in Vermont compared to contracting two mosquito-borne diseases in Vermont. Mosquito-borne diseases are very rare in Vermont compared to these three diseases that most of us never think about. We are five to 100 times more likely to get tuberculosis, Legionnaires’ disease, or Cryptosporidiosis than to get either mosquito-borne disease.   Data from CDC. 

Knowing the true risks of mosquito-borne diseases in Vermont helps us make sound decisions about how to protect ourselves from them. Repeated and alarming rhetoric about these very rare diseases could cause more harm than good, especially when it encourages us to ignore the risks of regular doses of pesticides which are known to have potential health risks. Many people in the BLSG district are very concerned that the chances of being harmed by roadside spraying of toxic pesticides are far greater than the chances of ever getting a disease from a mosquito.

It is especially disturbing that BLSG continues to name Zika virus as a disease we should be concerned about in Vermont. No one in the US has ever gotten Zika virus from a mosquito north of Texas and Florida. You won’t learn that from the scary information at the BLSG website or their forays into the local newspapers.

Before we make decisions about whether to invite exposure to neurotoxins like malathion and permethrin we should understand the true risks of nuisance mosquitoes.

What’s in the CDC report for Vermonters?

Every week, the Centers for Disease Control and Prevention (CDC) releases a report on diseases, and last week’s report was about the rise in diseases spread by insects and ticks. This was a typically dry and technical report about trends between 2004 and 2016 in human cases of 16 diseases spread by ticks, mosquitoes, and fleas. The CDC also publicized this report at their Vital Signs site where the hype was cranked up by a dramatic video and some revealing graphs.

The story was picked up by many major news outlets (Chicago Tribune, New York TimesForbes, Boston Herald, CNBC) under scary headlines like “Diseases spread by ticks, mosquitoes and fleas more than tripled in the U.S.”

Should Vermonters be alarmed by this report?

For Vermonters, a primary message of the CDC report is: If you want to be concerned about a vector-borne disease in Vermont, be concerned about Lyme disease. For the last decade, the probability of contracting Lyme from a tick bite in Vermont has been at least 100 times greater than contracting any disease from a mosquito. Between 2004 and 2016, annual cases of Lyme in Vermont increased from 50 to 761, and the CDC suggests that additional unreported cases might increase those numbers tenfold. Vermonters should learn about Lyme disease and know how to avoid it.

There has not been a similar increase in the number of cases of mosquito borne diseases in Vermont. The first cases of West Nile virus (WNV) and eastern equine encephalitis (EEE) in Vermont were in 2011 and 2012 respectively. There have been a total of only two cases of EEE in Vermont and eight cases of WNV, and there is no indication that the number of cases is increasing.

The headlines highlighting a “tripling” of diseases in the US are accurate only if you include Puerto Rico. That’s because Puerto Rico tragically suffered some 40,000 cases of Zika in 2016. This spike is evident in the two graphs below.

Above: The big number of cases in 2016 is due to 40,000 cases of Zika in Puerto Rico. The rise elsewhere in the US is not so striking (Zika is not transmitted by mosquitoes in the US north of Florida and Texas).  Most of the cases included in the graph above are tick-borne, not mosquito-borne. Source.


Above: This is the same data as in the previous figure. Most of the cases included in the CDC report are tick-borne diseases (light blue). The outbreak of Zika in Puerto Rico in 2016 is represented by the large medium blue bar in 2016. Note the relatively small number of cases of mosquito-borne diseases in US states (dark blue) and the modest increase over time since 2004. Source.

The CDC report cites a 2017 study of local organizations, including insect control districts, that are responsible for controlling disease vectors. Many of these organizations are chronically underfunded, and most of them are lacking in at least one core competency required to effectively protect the public. The most common shortcoming was testing for insect resistance to the pesticides applied. Making reliable surveys of insects was the third most common shortcoming, and basing treatment decisions on those surveys came in second.

Above: The local organizations tasked with controlling disease vectors are lacking at least one core competency. Source.


Although cases of Lyme disease have seen a steady increase over 13 years, mosquito-borne diseases like WNV and EEE are more prone to short term outbreaks, often followed by many quiet years. Scientists are not able to predict the outbreak years, so monitoring for the presence of WNV and EEE in mosquitoes is critical. When the next big year happens in Vermont, we hope the Vermont Department of Health is up to the task.

BLSG doubles down on Zika scare tactics

There is some very useful information in yesterday’s Rutland Herald article by Will Mathis, Director of Operations of the BLSG Insect Control District. So it was disappointing to see the group is still misleading residents about the risk of contracting the Zika virus from mosquitoes in Vermont. The article mentions that insect borne diseases are part of their justification for controlling mosquitoes, and then adds “Vermont has now recorded its first case of the Zika virus.”

The Vermonter with Zika did not get it from a mosquito bite in Vermont or anywhere north of Florida or Texas. That Vermonter had travelled to an area where Zika is present. Zika virus and the primary mosquito vector of the disease (Aedes aegypti) are not present in Vermont (more here).

Vermont residents need to make important decisions about reducing the risks of mosquito borne diseases. Suggesting that Vermonters should be concerned about a disease that is not carried by mosquitoes here is counterproductive. The BLSG website also exaggerates the importance of Zika with a scary five-page “special report” about it. This seems like an irresponsible approach to public education.

Where is Zika?

There was a lot of news about Zika virus two years ago when 17 athletes, including four top seeded golfers and some big tennis stars, pulled out of the 2016 Rio Summer Olympic Games ostensibly over Zika fears. The Zika story fizzled after the games, although some athletes later tested positive for antibodies to other arboviruses.

In the US, Zika can be transmitted by mosquito bites, but according to the CDC, there is “no known Zika” from mosquitoes outside of Florida and Texas.

Zika has been present in mosquitoes in Florida and Texas, but not in any other state. Source.

Zika is thought to be transmitted by only two species of mosquitoes (Aedes aegypti and A. albopictus) which are rare or absent in New England. In 2016 and 2017, the Vermont Agency of Agriculture, Food & Markets monitored for A. albopictus and failed to find any. Populations of A. aegypti have apparently never been present in Vermont.

The Aedes mosquitoes which transmit Zika are either “very unlikely” or “unlikely” to live and reproduce in Vermont. Source

Although there is no reason for concern about exposure to mosquito-borne Zika in Vermont, The BLSG website has a page titled “Information Center for Zika, EEE, and WNV.” It is important to monitor Eastern Equine Encephalitis (EEE) and West Nile Virus (WNV) in Vermont, and we should all be aware of the level of risk associated with those arboviruses. On the BLSG website, more space is dedicated to Zika than to EEE and WNV combined. This is rather misleading to local residents trying to make decisions about staying safe from mosquito-borne diseases, and does not inspire confidence in other information provided by the BLSG district.

West Nile virus in Vermont

Avoiding mosquito-borne diseases like West Nile virus is a primary motivation for mosquito control. Since it was first confirmed in the US in 1999, West Nile virus has infected more than 46,000 people in the US and more than 2,000 people have died. Mosquitoes infected with West Nile virus have been found in every Vermont county in past years, so this threat should not be ignored. However, between 1999 and 2016, only 12 cases of West Nile virus were reported in people in Vermont (Figure 1).

Figure 1. Total number of cases (mild plus neuroinvasive) of West Nile virus in US states (1999-2016) ordered from most (left) to least (right). Source.

New England has a comparatively low per capita incidence of West Nile virus disease. Between 1999 and 2016, the chance each year of developing a serious disease from West Nile virus was zero in most Vermont counties, and less than 1 in 200,000 in Addison County (Figure 2).

Figure 2. Average annual incidence (1999-2016) of neuroinvasive disease caused by West Nile virus by county. Source.

These results suggest that monitoring the presence of West Nile virus in Vermont is important, but alarming rhetoric about the disease risk in Vermont might not be justified.