Tickborne Disease Doubled in 13 Years: CDC's Count of U.S. Vectorborne Illness

Lyme disease infects about 300,000 Americans a year — eight to ten times the number reported. That gap runs through CDC's count of diseases spread by the bites of ticks, mosquitoes and fleas. From 2004 to 2016, 642,602 cases of 16 such diseases were reported, a number that substantially understates the real toll. Even so, it shows reports of tickborne disease more than doubling, from over 22,000 in 2004 to over 48,000 in 2016.

Two different problems

The total hides two distinct trends:

Fleas added 89 cases of plague.

Reported tickborne disease cases by state and territory, 2004–2016. Credit: CDC.

Why ticks creep and mosquitoes explode

The difference is rooted in the vectors' biology:

• Ticks live long and feed rarely. Ixodes scapularis, an important carrier of the Lyme bacterium, may feed once a year — offset by its wide choice of hosts and a single tick's ability to carry several pathogens. Ticks travel only as far as their animal hosts, and humans are usually dead-end hosts. So tickborne disease rarely erupts suddenly. The area at risk for Lyme disease, meanwhile, has been expanding. Ticks are especially hard to control, which puts more weight on personal protection.
• Mosquitoes move more, and females feed every 48–72 hours. Aedes aegypti passes dengue, Zika and chikungunya directly from person to person after about a week's incubation in the mosquito, producing explosive epidemics.
• West Nile virus is one of the few purely animal-cycle pathogens with epidemic potential: people are at risk only from mosquitoes that have fed on infected birds. Birds carried it across the country after it arrived in New York City in 1999.

Having the right mosquito is not enough on its own. Ae. aegypti may now live in up to 38 states, yet local spread of dengue, chikungunya and Zika from infected travelers has been rare. About 1,500 malaria cases are imported each year without local transmission, though Anopheles mosquitoes live across much of the country. Still, the limited local spread of dengue and Zika in Florida and Texas shows the country is open to outbreaks of exotic diseases.

Reported mosquitoborne disease cases by state and territory, 2004–2016. Credit: CDC.

New threats keep arriving

Nine vectorborne human diseases were reported in the United States and its territories for the first time during 2004–2016. Newly found or newly recognized pathogens since 2004 include:

• Heartland and Bourbon viruses — two previously unknown, life-threatening tickborne viruses, both from the Midwest;
• Borrelia miyamotoi, a tickborne relapsing fever agent first described in Japan and now found widely in the United States, and Borrelia mayonii, discovered in the upper Midwest;
• the tickborne spotted fever bacteria Rickettsia parkeri and Rickettsia species 364D, and Ehrlichia muris eauclairensis, all found to cause human disease;
• chikungunya and Zika, introduced to Puerto Rico in 2014 and 2015.

Zika shows the danger. For 60 years after it was found in a monkey in Uganda, it was seldom reported in people. In 2016 there were more than 36,000 cases in Puerto Rico, limited local spread in Florida and Texas, and nearly 5,000 cases among travelers — and the birth defects of the 2015–2017 epidemic in the Americas came as a surprise.

How much goes unseen

A case is counted only if someone seeks care, a clinician orders the right test, and it is reported. Many arbovirus infections cause few symptoms: an estimated 30–70 milder West Nile cases occur for every neuroinvasive case reported. On that basis, 2016 should have seen 39,300 to 91,700 non-neuroinvasive cases; only 840 — 1–2 percent — were reported.

What is missing

• A vaccine: the Food and Drug Administration has approved one for only yellow fever. Vaccines for Lyme disease, dengue, chikungunya and Zika are under intense research.
• Elimination is often impossible, because many of these pathogens live in wild animals such as rodents and birds. Insecticide resistance is widespread and growing.
• Nearly all vector control is locally funded and run, and in a national survey, 84 percent of 1,083 local mosquito control organizations lacked at least one of five core competencies.

CDC's response has two parts: advancing innovation and discovery, and rebuilding the comprehensive vector control programs that have eroded over time — with stronger surveillance, more sensitive and specific tests that can be used locally, and better reporting in states and localities.

Sources

Based on "Vital Signs: Trends in Reported Vectorborne Disease Cases — United States and Territories, 2004–2016," by Ronald Rosenberg and colleagues, CDC Division of Vector-Borne Diseases, Morbidity and Mortality Weekly Report (posted as an Early Release on May 1, 2018), Centers for Disease Control and Prevention; a work of the United States government in the public domain. Data: National Notifiable Diseases Surveillance System.