Now that the Zika epidemic of 2015-16 is in the rearview mirror, it might be useful to 1) take a brief look back at the Zika episode and what we learned from it and 2) use this information to try and predict what the future may hold.
Let me make the point here that it is very difficult to try and predict outbreaks of mosquito-borne diseases. That is not the purpose of this article. What I will focus on are the many factors that contribute to an outbreak, what PMPs need to be on the lookout for when possible, and what some of the steps and preparations are for dealing with such an event.
We will use the recent Zika outbreak in South America, the Caribbean and the United States as an example so my emphasis will be on container-breeding mosquitoes, specifically the yellow fever mosquito (Aedes aegypti) and the Asian tiger mosquito (Aedes albopictus) and the pathogens that these mosquitoes transmit: Zika virus, yellow fever virus, chikungunya virus and dengue viruses.
NECESSARY CONDITIONS? It takes much more than mosquitoes and viruses for outbreaks such as the recent Zika one to occur. Let’s take a look at what factors are involved.
A Virus. First, we must have a virus present. This may seem obvious but where do these viruses come from? Many of them originate in Asia or Africa, and a few come from South America. In these places, the viruses are transmitted from mosquitoes to monkeys that live in the jungle canopy and humans occasionally become infected.
Once a human is infected, the virus multiplies to large numbers in the blood stream for the next three to seven days. During this time, the person does not feel sick, and can travel anywhere in the world, hence transporting the virus. So, these particular viruses move around the world in infected humans. I remember standing in the ports of Miami, Fla., and Jamaica watching all the cruise ships pull in, wondering where they were coming from and how many potentially infected people might be on board!
Vectors. Wherever an infected person ends up, the right mosquito vectors must be present. In fact, of the 3,500 or so kinds of mosquitoes in the world, most of them don’t transmit pathogens to humans. For instance, only Anopheles mosquitoes transmit human malaria parasites but it is the Culex mosquitoes that transmit West Nile virus. Not only must the vectors be present, they must be so in large enough numbers to sustain virus transmission; just a few won’t do. This is part of the reason why mosquito control by PMPs is so important in preventing mosquito-borne disease.
Susceptible Population. The next thing we need is a large, susceptible human population, much as we had when Zika virus hit Puerto Rico in 2016. There must be adequate numbers of people who have not been infected with the virus previously and/or have not been vaccinated against it (note that in the cases of most mosquito-borne viruses, there isn’t a vaccine anyway). The reason this is so critical is once you have been infected with one of these viruses, you almost never get it again; your immune system protects you. So, if too many people in a particular area have been exposed to the virus before, there is really no place for it to go “live and multiply.”
Proper Environmental Conditions. Environmental conditions have a significant impact on vector-borne diseases. Basically, the warmer it is and the higher the humidity, the faster things happen. The mosquito life cycle, normally 10-14 days from egg to adult, can be completed in as little as five to seven days. Also, mosquitoes live longer under optimum environmental conditions, thereby increasing their chance to spread pathogens.
Socioeconomic Conditions. These mosquitoes, especially Ae. aegypti, prefer to breed in artificial and natural containers in backyards. Preferred sites are tires, bird baths, clogged gutters, tree holes, buckets, etc.; basically, anything that will hold water from a bottle cap to an unattended swimming pool. So, in socioeconomic conditions where there are lots of containers and trash, and people don’t maintain their yards, there will be higher populations of vectors. They also have the nasty habit of breeding in cisterns and other containers where people store water for drinking, cooking and laundry purposes. It is not unusual at all for every water-filled object in a site to be positive for mosquitoes.
PMPs have an essential role here in educating customers about conducive conditions, thoroughly inspecting for mosquito breeding sites, and removing them or treating them as necessary.
Public Health Conditions. Public health conditions in many countries are inadequate at best. Medical facilities may be limited and overcrowded, proper diagnoses may be problematic and the ability of the government to mount an effective response to a mosquito-borne disease outbreak may be difficult. Therefore, virus transmission goes unchecked, in the local population as well as tourists (if present), so the threat for a global outbreak increases.
Political Factors. Politics often can play a part in combatting (or not) vector-borne diseases. When a large outbreak occurs and the cases, and perhaps deaths, start piling up, it is often convenient to start assigning blame. Although health and government officials may be doing the best they can under the circumstances, they might lose their jobs.
Additionally, it is not uncommon for consultants and foreign health officials, who may be brought in to assist, to disagree over what the disease is, what control methods should be employed, and what news to release to the local public and the world. All of these factors must be taken into account when dealing with an outbreak or epidemic both at home and abroad.
Vector Control Capabilities. Vector control capabilities in many countries where these diseases occur are limited at best. Fogging machines, insecticides and surveillance equipment may prove too expensive, and expertise in vector control may be spotty. Sometimes, governments request assistance from foreign countries or international relief organizations but by the time the help arrives and control efforts are implemented, the situation may be out of hand.
Closer to home, when West Nile virus broke out in the United States in 1999, as well as when Zika virus arrived in 2015-16, it was clear that we were underprepared to deal with both of these events from a vector control aspect. A scientific report from the Centers for Disease Control and Prevention eventually confirmed this. A robust and well-trained private industry mosquito control force will contribute to our nation’s ability to control outbreaks in the future.
Impact of the Media. Finally, don’t forget the impact of the media, which can be beneficial or detrimental. Most if not all of us have experience dealing with the media either during an outbreak or during mosquito season. We don’t have space here for a class on “Dealing With The Media 101” but just be sure you always have your facts straight, that you package your message into three or four concise and clear statements, and that you try to avoid speculation and confusion.
I strongly recommend that if you haven’t done so, establish a working relationship with a local reporter, educate them about what you do and why, and build trust. This cultivation will pay great dividends down the road.
CURRENT THREATS? We already have mentioned yellow fever, chikungunya and dengue. Cases of the latter two regularly occur in the United States in small numbers but they are almost always “imported” cases; that is, infections that were acquired overseas but the clinical illness occurred in the United States. Yellow fever is rarely seen in the U.S. but large outbreaks continue to occur in Africa and to a lesser degree in South America so it continues to be a threat.
Mayaro Virus. Mayaro virus causes a disease similar to dengue fever. It is an acute, febrile illness lasting three to five days. Symptoms include headache, muscle pain, pain behind the eyes and a rash that may or may not be present. Other symptoms may include vomiting, diarrhea and severe joint pain that can persist for a year or more.
The virus is native to the humid forests of South America, where it exists in a cycle between monkeys and canopy-dwelling mosquitoes. A human working in the forest can become infected while cutting down trees, working in the canopy, picking coconuts or other activities. If this infected person then returns to a large urban area where Ae. aegypti mosquitoes are abundant, a large outbreak can quickly occur and spread rapidly. This was likely the scenario we saw with Zika virus in South America in 2015-16.
There is now solid scientific evidence that Mayaro virus has spread into Central America almost to, or perhaps into, Mexico. Additionally, in 2015 a case was confirmed in a school child in Haiti. So, Mayaro virus is on the move and has entered the Caribbean. Additionally, we have laboratory evidence that some of the mosquito species already present in the United States, e.g. Ae. aegypti and Ae. albopictus, can transmit the virus to humans. Finally, there is evidence of imported cases of Mayaro virus occurring in the United States and Europe so it should be considered a significant public health threat for those regions.
OTHER POSSIBILITIES. I will briefly mention three other mosquito-borne diseases of concern (although there are several others); Japanese encephalitis (JE), Venezuelan equine encephalomyelitis (VEE) and Rift Valley fever (RVF).
JE occurs in several countries in Southeast Asia. The animal reservoir is pigs and the virus is transmitted to humans by Culex mosquitoes. According to the World Health Organization, about 68,000 clinical cases occur per year, primarily in children and, although a vaccine is available, JE kills between 13,000-21,000 people annually.
VEE occurs primarily in Central and South America, where it has impacted thousands of humans and tens of thousands of equines. It is spread by several different species of mosquitoes.
In 1971, a large outbreak in Mexico eventually reached the United States, with many human cases and thousands of equines affected.
RVF occurs in Africa and parts of Asia. It affects domestic animals as well as humans and can have a significant economic impact. Some epidemics have killed more than 100,000 animals. It can be transmitted to humans through the blood, body fluids or tissues of infected animals as well as by several species of mosquitoes.
We have excellent vectors of JE, VEE and RVF in the United States and the viruses are just a plane ride away. We must remain vigilant!
THE FUTURE? In this age of world travel, viruses, vectors and victims (infected people) can be anywhere in the world within about 24 hours. So, vectors and pathogens will continue to invade new areas. Insecticide resistance continues to increase so new vector control tools such as traps, baits, insecticidal nets and spatial repellents are needed.
Genetic mutations, both in pathogens and vectors, may result in increased infectivity as well as enhanced severity of disease, much as we see some years with the influenza virus. Climate change is already resulting in vectors increasing their geographic range.
Private industry mosquito control, utilizing the principles of Integrated Mosquito Management, will continue to play a key role in enhancing quality of life, providing peace of mind and protecting public health. It is a discipline that is essential to the public’s health, and one of which that all PMPs who are engaged in can be proud.
The author is vice president, technical products and services, AP&G (Catchmaster), and past president, American Mosquito Control Association.