For physicians, mosquitos are not just a summertime annoyance: They can be a point of occupational dread. The same is true of fleas, ticks, kissing bugs, sand flies, and the other invertebrates — or “vectors” — that spread disease.
Vector-borne illnesses are some of history’s biggest killers. The World Health Organization (WHO) says they account for 17 percent of infectious diseases each year and kill about 700,000 people worldwide. That puts them collectively in a class with bubonic plague, which ravaged Europe, Africa, and Asia in the 1300s; and malaria, which WHO ranks as one of the top five deadliest diseases.
While malaria has been mostly stamped out in the United States, vectors still spread biological mayhem here, and Texas’ warm climate makes it a prime breeding ground. Vector-borne illnesses like West Nile virus, murine typhus, Zika virus, chikungunya virus, dengue fever, and Chagas disease are or recently have been endemic in all or parts of the state. (See “Chasing the Problem,” page 23.)
The Texas Department of State Health Services (DSHS) keeps annual records on all reportable outbreaks for the vector-borne illnesses known to be in Texas. (See “Reported Cases of Major Vector-borne Illnesses in Texas,” page 24.)
But experts in this arena say testing for them can be difficult, and the state’s methods of disease surveillance remain inadequate, so these numbers provide only a partial picture. More importantly, Texas has all the conditions for vector-borne illnesses to get worse — an exploding population, fast-growing international trade, a warming climate, large pockets of poverty, and lots of the wrong kinds of insects. In addition, numerous roadblocks stymie the availability of new vaccines.
“Texas is probably the most vulnerable state in the union to these diseases,” said Peter J. Hotez, MD, head of the National School of Tropical Medicine at the Baylor College of Medicine in Houston.
In fact, a study released in May by the U.S. Centers for Disease Control and Prevention (CDC) found that the number of people each year who contract 16 insect-borne illnesses in the United States has more than tripled to 96,000 since 2004. The report blamed several factors for the increase, including warming weather and increased travel. The study also said the real number of cases was surely higher.
An underreported problem
The Texas Medical Association recognized the growing concern by backing several bills in the 2017 session of the Texas Legislature that would help control vectors and improve reporting. Meanwhile, TMA’s Committee on Infectious Diseases is working on education efforts to teach Texas physicians about vector-borne illnesses that are frequently misdiagnosed or missed altogether.
“When we’re in the height of an outbreak, like Zika in Dallas, we [physicians] tend to ask about it. But then we tend to relax. Somehow it needs to be standardized and incorporated into our practice,” said committee chair Jane Siegel, MD, a pediatric infectious disease specialist from Corpus Christi.
Vector-control and education — both for physicians and led by physicians — are the best ways to protect against these illnesses, said Scott Weaver, PhD, principal investigator for the Western Gulf Center of Excellence for Vector-Borne Diseases at The University of Texas Medical Branch at Galveston (UTMB Health).
Physicians “need to make sure they know what’s in the area and then educate their patients so they can protect themselves,” Dr. Weaver said. “Preventing exposure to the vectors is the most efficient thing to do.”
Many vector-borne illnesses are underreported, in part, because they are not commonly seen by many Texas physicians, leaving doctors with limited first-hand experience, says Ashley Howard, DO, a member of the South Texas Zika Task Force Team at Driscoll Children’s Hospital in Corpus Christi.
Also, the symptoms of these illnesses resemble those of more common ailments.
Dr. Howard, a pediatrician, said Zika virus is a good example. Its symptoms often look exactly like the flu — fever, joint pain, rash, non-purulent red eyes. She said even in those rare cases when patients are tested for Zika virus, physicians may be frustrated because the tests are not always accurate or easy to interpret.
“Zika virus testing can be very complex and it can be confounded by other past infections, such as flavaviruses” like dengue, she said.
A bigger issue is the passive way statistics are currently reported on these illnesses, contributing to silent outbreaks that can be devastating to both public health and to individual patients, says Baylor’s Dr. Hotez.
“It requires that the patient recognize that he or she is sick enough to see a health care provider. It then requires the health care provider to put two and two together to recognize that this might be one of these neglected tropical diseases and order the test,” he said. “And then testing is not as simple as a lab test from Quest Diagnostics … This requires the physician to pick up the phone and arrange testing, which is very hard to do when you’ve got a busy practice. So it requires all three stars to align before you can diagnose somebody.”
Assuming all this happens, the doctor then relays the diagnosis to county health officials who then relay it to DSHS. Dr. Hotez estimates this cumbersome process catches perhaps 1 percent of the actual cases. As a result, many disease outbreaks go unnoticed. For instance, he said, Houston had an epidemic of dengue fever in the early 2000s that was not recognized until after it had passed.
Dr. Hotez said a better approach would be to identify areas where outbreaks are likely and test broad groups of people in those areas for specific illnesses.
“Active surveillance means that [health officials] go into a community where you’re concerned that a disease might be happening and do sampling, to take blood samples from, say, all the patients who are coming into emergency rooms or seeing health care providers with a fever,” Dr. Hotez said. “Then you can get a true idea of the percentage of people who have this disease.”
In 2017, the Texas Legislature approved House Bill 2055, a measure supported by TMA that would have allowed health authorities to do more active surveillance of vector-borne diseases. However, lawmakers did not fund the measure, so Dr. Hotez said no new testing is possible.
Legislators in 2017 also passed Senate Bill 570, another TMA-backed measure that would have regulated the retention, storage, transportation, and disposal of used tires. The goal was to deprive mosquitoes of breeding grounds, but Gov. Greg Abbott vetoed the bill. However, the TMA-backed House Bill 3576, which helps DSHS track, study, and prevent the spread of Zika, became law.
As a key player in combatting vector-borne illnesses, DSHS’ biggest task is compiling data. Physicians and local labs report instances of the illnesses to local health authorities or to a DSHS regional office. They in turn report to DSHS in Austin, which compiles and publishes the numbers.
DSHS spokesman Chris Van Deusen says the agency doesn’t typically do more active surveillance or take other steps unless there’s a specific disease outbreak. For example, the agency currently tests pregnant women in nine counties along the U.S.-Mexico border where the local spread of Zika virus is considered most likely. The women are tested in each trimester of their pregnancy, and DSHS and local health departments follow up on any positive results.
“We’re not waiting for symptoms to present,” Mr. Van Deusen said. “Our recommendation is that those women be tested as part of their prenatal care.”
Concerns about the birth defects caused by Zika virus, he adds, have also commanded both state and federal funding for a full-throated public awareness campaign, including ads on TV, radio, and billboards; public meetings; and a specially designated website: TexasZika.org.
“[Zika virus] has been so much of our focus lately,” he said. “That’s where we’ve done a lot of public outreach and education, and provider education, as well.”
Surveillance of vector-borne illnesses is not just a Texas problem — it’s a nationwide problem, Dr. Weaver of UTMB Health says. He pointed out that several tick-borne illnesses have been discovered in the Midwest in just the past few years, including Heartland virus and Bourbon virus.
“In the U.S. we can do much better surveillance,” he said. “It’s just a matter of resources. There are a lot of vector-borne diseases where we don’t know their true distribution or we don’t even know they exist.”
There’s a vaccine for that?
Even with these surveillance issues, Texas physicians theoretically could combat vector-borne diseases using vaccines, Dr. Weaver adds. Vaccines exist for most of the ones that plague Texas, including Zika, dengue, chikungunya, and West Nile virus. In fact, Dr. Weaver helped develop some of them.
Unfortunately, these vaccines are stuck in a regulatory and commercial twilight zone that keeps them from reaching the public. Part of the problem, says Dr. Hotez, is the sporadic way vector-borne epidemics tend to work.
“We know Zika swept through the Caribbean in 2016, and it really ravaged Puerto Rico with large numbers of birth defects because of the consequence of it,” Dr. Hotez said. “Then, almost as quickly as it appeared in the Caribbean, it died out for reasons we still do not entirely understand. Possibly it was because much of the cohort of people became immune. The problem is that immunity doesn’t last forever, so there’s always the possibility it will come back.”
This uncertainty makes it very hard to predict when and where the next outbreak will occur. Dr. Weaver said it raises red flags for the large pharmaceutical companies that typically do the final testing of drugs and bring them to market.
As a result, it’s difficult to get these vaccines to the final “Phase 3” human testing demanded of U.S. regulators. This process, which can cost hundreds of millions of dollars, requires comparing people who’ve actually been vaccinated against those who received a placebo.
There are other obstacles. For instance, a vaccine that’s only needed sporadically is not likely to be profitable for pharmaceutical companies. Also, health insurance companies aren’t likely to cover them.
“So we’re stuck in this system where we have the initial stages of vaccine development done at universities, with funding from the National Institutes of Health,” Dr. Weaver said. “But ultimately a bigger pool of government money or big-pharma money has to bring those vaccines to the finish line.”
Dr. Hotez says Texas Children’s Hospital and Baylor College of Medicine in Houston came up with a way to help fill this need. They created the Texas Children’s Center for Vaccine Development specifically to develop neglected disease vaccines.
To date the center has successfully transitioned two vaccines — for hookworm and schistosomiasis — from discovery to clinical trials. Both will be used in developing countries, but not the U.S., where those diseases are much, much less prevalent.
Producing vaccines for neglected illnesses in the U.S. has been more challenging. Private funding sources, like the Gates Foundation, tend to focus on health issues overseas rather than in wealthy countries like the U.S.
“The interventions for these diseases really fall through the cracks,” Dr. Hotez said.
Filling in the gaps
A proper diagnosis of vector-borne illness also can fall through the cracks when physicians don’t recognize one, says Dr. Siegel, the Corpus Christi pediatric infectious disease specialist who chairs TMA’s Committee on Infectious Diseases. A big part of the problem, she says, is that these illnesses tend to concentrate in certain regions. But when cases emerge elsewhere, physicians often don’t realize what they’re seeing.
“People in Corpus Christi know murine typhus in their sleep,” Dr. Siegel said. “But people who haven’t seen it a lot have to ask for information about it.”
That’s becoming a bigger problem because more cases of murine typhus are being reported outside its traditional range in south and central Texas — one of the reasons TMA’s Committee on Infectious Diseases is developing educational modules to help physicians recognize murine typhus, as well as Chagas disease.
Dr. Siegel says most cases of vector-borne illness arrive in the U.S. from people who have visited or moved from foreign countries. Physicians need to do a better job of systematically asking patients about their travel habits, for instance, and considering the potential for vector-borne illnesses in everyday practice, not just during the height of an outbreak.
In the near term, Dr. Weaver says Texas’ best defense is mosquito control, but the state needs higher and more uniform standards at the local level. For instance, Harris County is well-known for its sophisticated mosquito-control efforts, but not all counties rise to that level.
Umair A. Shah, MD, executive director of Harris County Public Health (HCPH), agrees there are serious gaps in Texas’ local vector-control efforts. He pointed to a recent study by the CDC and the National Association of County and City Health Officials that found many counties did little or no surveillance for mosquitos during the 2016 Zika virus outbreak.
“There are communities across Texas that either do not have the resources or have not put the resources in for mosquito control in the way that they could,” said Dr. Shah, a member of TMA’s Committee on Infectious Diseases.
Dr. Shah said aggressive surveillance clearly gives Harris County a step up in detecting and stopping outbreaks. When those outbreaks occur, HCPH works with local physicians through the Harris County Medical Society and other channels, using special health alerts.
“That allows us to ensure that a lot of coordinated information is getting back and forth from the public health community to the health care community and vice versa. If the medical society learns something, they can bring it back to the attention of public health,” he said.
Dr. Hotez says efforts like these are important in controlling vector-borne illnesses. But more than anything, Texas physicians need to change how they approach these once-exotic diseases in the exam room. He pointed to the old medical school cliché that if you hear hoof beats, you should think horses, not zebras.
“These diseases are horses,” he said. “They aren’t zebras anymore.”
Chasing the Problem
Vector-borne illnesses are frequently misdiagnosed and often seriously underreported in Texas. So what are the worst diseases and where do they usually hit?
For Texans, vectors usually mean mosquitoes. Geographically, that means areas with more mosquitos — the Gulf Coast and South Texas — have the biggest threat. However, mosquitos are a statewide problem, and so are other vectors, so people in even the most arid corners of the state can get sick, says Scott Weaver, PhD, principal investigator for the Western Gulf Center of Excellence for Vector-Borne Diseases at The University of Texas Medical Branch at Galveston.
Of the vector-borne illnesses, Zika virus has dominated headlines in recent years, largely because it's hard to detect and causes terrible birth defects. However, West Nile virus is a bigger long-term threat to more people in Texas, Dr. Weaver says. Like Zika virus, many mosquito-borne disease outbreaks have a flash-in-the-pan quality to them. They flare up one year, create a lot of immunity in the infected population, and then seem to disappear or continue at much lower levels.
Dr. Weaver says this is because most of these illnesses circulate directly between humans and mosquitos. But West Nile virus circulates largely between birds and mosquitos. Birds have a much shorter lifespan than humans and faster turnover, so West Nile virus always has a fresh supply of animals to infect. That means West Nile virus is always present in the mosquito population — and always ready to infect humans.
West Nile virus
Symptoms: Most show no symptoms. Some have fever, headache, aches, joint pain, vomiting, diarrhea, rash. Serious cases have high fever, headache, neck stiffness, stupor, tremors, convulsions, muscle weakness, vision loss, numbness, paralysis.
Geographic spread: Statewide
Last large outbreak: 2012, focused on the Dallas area
Vector: Kissing bugs
Symptoms: In the acute stage, fever, swelling around site of the insect bite. In the chronic stage, patients often show no symptoms for years but later develop heart rhythm abnormalities, a dilated heart, and a dilated esophagus or colon.
Geographic spread: North, central, and south Texas
Last large outbreak: Ongoing
Symptoms: Fever, aches and pains, loss of appetite, nausea, vomiting, stomach pain, cough, rash.
Geographic spread: North, central, and south Texas
Last large outbreak: The number of Texas cases more than doubled between 2008 and 2016.
Symptoms: Many show no symptoms. Those who do experience fever, rash, headache, joint pain, red eyes, muscle pain.
Geographic spread: Statewide
Last large outbreak: 2016, when there were 315 reported cases, including six locally acquired in the lower Rio Grande Valley. In 2017, the number of reported cases dropped to 54.
Symptoms: Severe headache, severe eye pain, joint pain, muscle and bone pain, rash, low white cell count. Serious cases show severe abdominal pain, vomiting, red spots on skin, bleeding from nose and gums, vomiting blood, black stools, drowsiness, irritability, clammy skin, difficulty breathing.
Geographic spread: Gulf Coast and south Texas
Last large outbreak: Outbreaks in the U.S. date back to the 1700s; many involved tens of thousands of cases.
Symptoms: Fever, joint pain, headache, muscle pain, joint swelling, rash.
Geographic spread: Gulf Coast and south Texas
Last large outbreak: There were probably outbreaks in the 1800s throughout the southeastern U.S. involving tens of thousands.
Sources: Texas Department of State Health Services; Centers for Disease Control and Prevention; Western Gulf Center of Excellence for Vector-Borne Diseases at The University of Texas Medical Branch at Galveston
Reported Cases of Major Vector-borne Illnesses in Texas
The Baylor College of Medicine's National School of Tropical Medicine offers a diploma in tropical medicine that helps physicians learn how to recognize and treat a class of illnesses that are becoming more common in Texas. Physicians have three years to complete four classes, each a few weeks long, offered in face-to-face and web formats. Learn more at tma.tips/TropicalMedicine.
Tex Med. 2018;114(6):20-25