By Stephen A. Berger, MD
Even during “normal times,” summer activities expose Americans to a long list of viruses, bacteria, fungi, and parasites. Few, if any, are prevented by face masks, hand-washing, and social distancing. Inevitably, camping, hiking, and even barbecues expose us to mosquitoes and ticks. As of 2020, a total of 68 human diseases are known to be transmitted by these tiny animals, 26 of the diseases occur in the United States, and many are potentially fatal. A differential diagnosis toolkit can play a vital role for physicians this summer by providing point-of-care diagnostic support. The following information is an overview of some of the most common summertime diseases.
The most important mosquito-related disease in the U.S. is West Nile fever (WNF). Just as COVID-19 arrived to our country on an airplane from China, WNF was first reported in the region of a New York City airport in 1999. After 21 years, the disease is currently endemic to 46 states. Over 2,000 cases—including 150 fatal cases—are reported each year. There is no specific treatment for WNF and no vaccine. One day to two weeks following the bite of an infected mosquito, patients develop fever, headache, and muscle pain. A flat, red rash and eye inflammation are often present as well. In most cases, the patient recovers completely within one week; however, fewer than 1% develop meningitis or encephalitis, which can result in death.
Fifteen years after the appearance of WNF, Zika became a household word to Americans. A total of 5,102 cases of Zika virus infection were reported in the United States in 2016. Although early cases were in travelers who had acquired the infection overseas, these numbers also included 218 infections from mosquito bites in Florida and six in Texas. During the next year, an additional 48 cases in the U.S. were acquired through sexual contact. As of 2017, 98 children with microcephaly due to maternal Zika virus infection had been born in our country.
Two to 15 days following a Zika-infected mosquito bite, patients develop a mild illness characterized by pain, conjunctivitis, and an itchy, red rash that begins on the face and spreads to the rest of the skin’s surface. Many patients also complain of pain behind the eyes, and in the muscles and joints. There are few complications, and patients recover completely within seven to 10 days.
Conversely, infection in a pregnant woman can be devastating. The chance that a pregnant woman with Zika virus infection will pass the disease on to her unborn child is approximately 26%, and 35% of fetuses infected will be born with severe brain abnormalities. As with WNF, neither specific drugs nor vaccines are available for Zika virus infection.
Lyme disease was first reported in Lyme, Connecticut in 1975. It is the most important tick-borne infection in the United States, with 30,000 to 43,000 cases yearly. Fewer than 20 cases per year end in death. Lyme disease is caused by a bacterium and transmitted by ticks. Cases are reported by most states, with the majority occurring in the Northeast, from June to August. Symptoms of Lyme disease usually appear seven to 14 days following a tick bite, but can appear as long as 180 days afterward; it is important for clinicians to note that only 25% of patients will actually recall the tick bite. Most patients develop fever, fatigue, and pain in the muscles or joints. A characteristic rash is present in 75% of cases, resembling one or more “targets” consisting of concentric rings. If not treated, a variable percentage of patients will develop more serious complications, including severe arthritis that may destroy joints, meningitis, or myocarditis. A wide range of effective antibiotics are available for treatment and prevention of Lyme disease following tick bites.
Some spotted fevers are also transmitted through the bites of ticks. Disease incidence has been increasing in recent years, with 4,515 cases reported in 2019. Most cases occur from April to September and in the Atlantic coast states, and only four to 10 fatal cases are reported annually. In some regions, over 60% of ticks carry the disease. Two to 14 days following a tick bite, patients will complain of fever, severe headache, and muscle pain—often with vomiting and abdominal tenderness. Over 80% of patients develop an overt rash covering the entire skin’s surface. Several antibiotic agents are effective against the bacteria that cause spotted fevers.
With the help of decision support tools, clinicians can differentiate between vector-borne diseases during a season where more people are at risk. A decision support system uses two or more items of patient data to generate case-specific advice. In practical terms, the output of such systems is used to arrive at a specific diagnosis. Although none of these systems can “make” a diagnosis, they are capable of generating a list of diseases that are compatible with a patient’s epidemiological background, signs, symptoms, and laboratory findings.
The databases that drive decision support systems may consist of a body of literature, experience with previous patients (i.e., “case-based systems”), or both. The system that I work with ranks the statistical likelihood of listed items based on the known incidence of every disease (in every individual country) and the known incidence of each sign or symptom (in every individual disease). Clinicians then review this list and select items that might be either excluded or confirmed through further testing.
Stephen A. Berger is cofounder and medical director of GIDEON, the Global Infectious Disease and Epidemiology Online Network. He is affiliated with the Tel Aviv Medical Center, where he has served as director of both geographic medicine and clinical microbiology, and he also holds an appointment as emeritus associate professor of medicine at the School of Medicine.