Recent attention on the West Nile Encephalitis outbreak has brought the need for more education and awareness to the threat of insects transmitting diseases to both humans and the livestock we use for food. Unfortunately most people are fixated on the West Nile problem and continue to ignore the myriad of other pathogens that can be transmitted by insects and other arthropods in North America. Most of the pathogens that are transmitted by arthropod vectors are of low virulence and due to exposure most of us have developed an immunity to them long ago. Unfortunately, with the migration of much of the population into areas of the country that had until recently been wilderness, the increasing introduction of new species of both pathogens and vectors into native ecosystem, and an increase in global temperatures, that is changing much of the habitat, an ever increasing number of cases are being seen in hospitals nation-wide. In this article we will discuss just a few of these diseases.
Plague
Many well known insect borne diseases occur in North America, but in such small numbers that most of the population is blissfully unaware anything is happening. One such pathogen is Plague. Better known as The Black Plague this bacterial disease swept through Europe and Asia during the middle ages killing an estimated 34 million people. While such large outbreaks are no longer a significant threat, several small outbreaks and single cases do appear annually. Most commonly transmitted by the bite of an infected flea, usually the Oriental Rat Flea. The largest known reservoir of Plague in North America is found in Prairie Dog towns around the Four Corners region. With 10-15 human cases in the U.S. and up to 3,000 human cases worldwide annually, Plague can manifest itself in any of three different forms.
Pneumonic plague occurs when Y. pestis infects the lungs. This type of plague can spread from person to person through the air. Transmission can take place if someone breathes in aerosolized bacteria, which could happen in a bioterrorist attack. Pneumonic plague is also spread by breathing in Y. pestis suspended in respiratory droplets from a person (or animal) with pneumonic plague. Becoming infected in this way usually requires direct and close contact with the ill person or animal. Pneumonic plague may also occur if a person with bubonic or septicemic plague is untreated and the bacteria spread to the lungs.
Bubonic plague is the most common form of plague. This occurs when an infected flea bites a person or when materials contaminated with Y. pestis enter through a break in a person’s skin. Patients develop swollen, tender lymph glands (called buboes) and fever, headache, chills, and weakness. Bubonic plague does not spread from person to person.
Septicemic plague occurs when plague bacteria multiply in the blood. It can be a complication of pneumonic or bubonic plague or it can occur by itself. When it occurs alone, it is caused in the same ways as bubonic plague; however, buboes do not develop. Patients have fever, chills, prostration, abdominal pain, shock, and bleeding into skin and other organs. Septicemic plague does not spread from person to person.
Mosquito Borne Diseases
The majority of the U.S. population is totally unaware of the majority of insect transmitted diseases. While West Nile is now a common household term, the more commonly occurring Jamestown Canyon, LaCrosse, St. Louis, Western Equine, Eastern Equine, and Venezuelan Encephalitis have never been heard of by most. These mosquito transmitted viruses are seen every year in the U.S., but due to the fact that most of the cases are asymptomatic or have very mild symptomology, coupled with the fact that most cases are in rural, often economically poor settings, these diseases get very little press. Though most cases are mild, acute cases begin with flu like symptoms that then can progress to inflammation of the brain. This can lead to coma or even death. Survivors of acute encephalitis often suffer varying degrees of brain damage.
Also receiving little press is the increasing occurrence of malaria that originates U.S. nationals that have picked up the disease while overseas and from infected persons that have recently migrated to the U.S. Several of these individuals have entered the country illegally, and therefore bypass needed healthcare due to fear of capture. This scenario is also being observed with several other diseases.
Dengue Fever also known as “break bone fever” is another mosquito transmitted disease that is of concern. Two versions of the disease can occur, with one version being a hemorrhagic fever. Dengue fever usually starts suddenly with a high fever, rash, severe headache, pain behind the eyes, and muscle and joint pain. The severity of the joint pain explains the name “breakbone fever”. Nausea, vomiting, and loss of appetite are common. A rash usually appears 3 to 4 days after the start of the fever. The illness can last up to 10 days, but complete recovery can take as long as a month. Older children and adults are usually sicker than young children.
Most dengue infections result in relatively mild illness, but some can progress to dengue hemorrhagic fever. With dengue hemorrhagic fever, the blood vessels start to leak and cause bleeding from the nose, mouth, and gums. Bruising can be a sign of bleeding inside the body. Without prompt treatment, the blood vessels can collapse, causing shock (dengue shock syndrome). Dengue hemorrhagic fever is fatal in about 5 percent of cases, mostly among children and young adults.
Chagas
One disease that is becoming more of a concern among my fellow entomologists is Chagas Disease also known as American Trypanosomiasis. This disease is transmitted by the >Kissing Bug=, which blood feeds on the human victims as they sleep, usually biting the host on the face. The bugs are found in houses made from materials such as mud, adobe, straw, and palm thatch in Central and South America. During the day, the bugs hide in crevices in the walls and roofs. During the night, when the inhabitants are sleeping, the bugs emerge. After they bite and ingest blood, they defecate on the person. The person can become infected if T. cruzi parasites in the bug feces enter the body through mucous membranes or breaks in the skin. The unsuspecting, sleeping person may accidentally scratch or rub the feces into the bite wound, eyes, or mouth.
The parasite causes damage to the cells of the heart. It is estimated that as many as 18 million people in Mexico, Central America, and South America have Chagas disease, most of whom do not know they are infected. Another 500,000 people in the U.S., many of whom are illegal aliens, are thought to have the disease. Since the insects that transmit this disease are also found throughout much of the U.S. the possibility of transmission does exist.
Though not common, human cases of Chagas are appearing more often in the U.S. with many more veterinary cases in dog and raccoon populations. These animal populations then act as a reservoir for the parasite. While it is commonly thought that better housing found in the U.S. will prevent widespread rates of infection, this is not entirely accurate. A recent infection of an infant that occurred in central Tennessee happened when the insect entered the patients’ home. The infants mother happened to observe the insect on the child, and having just watched a broadcast about Chagas on television, recognized the insect. The mother requested a parasite screening by the family physician that isolated the organism. The child was then able to get proper medical care. A check of wildlife and pet populations in the area around the residence found high infection levels among the animals. If the mother hadn’t seen the insect, the child could have been infected for years without proper treatment. Transmission can occur when someone is bitten on camping trips as well as in the home. The disease can also be transmitted through blood transfusions, organ transplants, to fetus during pregnancy, and by eating feces contaminated food.
Ehrlichiosis
The presence of Rocky Mountain Spotted Fever, Lyme Disease, and Tick Fever is fairly well known by many people throughout the U.S. Unfortunately these tick transmitted diseases are just the tip of the iceberg.
One tick borne disease becoming increasingly common is Ehrlichiosis. Human ehrlichiosis due to Ehrlichia chaffeensis was first described in 1987. The disease occurs primarily in the southeastern and south central regions of the country. To date six species of bacteria are known to cause Human Ehrlichiosis and are transmitted by three know tick vectors, the Lone Star Tick, Black Legged Tick, and the Western Black Legged Tick. Most victims of Human Ehrlichiosis have had underlying immunosuppressants, but this isn’t always the case.
In my home town in Tennessee an outbreak of Ehrlichiosis was detected in one of our more well know retirement communities. Researchers, including myself found that the majority of the victims had been exposed to the disease through tick bites that occurred while they were playing golf. The victims were generally older, but had been considered in good health. Patients with ehrlichiosis generally visit a physician in their first week of illness, following an incubation of about 5-10 days after the tick bite. Initial symptoms generally include fever, headache, malaise, and muscle aches. Other signs and symptoms may include nausea, vomiting, diarrhea, cough, joint pains, confusion, and occasionally rash. Ehrlichiosis can be a severe illness, especially if untreated, and as many as half of all patients require hospitalization.
Prevention
The best way to beat these diseases is to prevent the initial infection. This is often easier said than done. Several things can be done to limit your risk of exposure.
– Wear long sleeved shirts when possible
– Tuck pants into boots or socks
– Use repellents such as DEET (N,N-Diethyl-meta-Toluamide) or Permethrin
(Note: Permethrin should not be applied to the skin, only to clothing. Studies have shown that DEET products containing more than 7% DEET should not be applied to the skin.
– Wear light colored clothing when possible to aid spotting ticks
– Self check thoroughly when you return indoors
– Keep screens on windows and doors in good repair
– Caulk cracks and other entry point on homes
– Use mosquito netting when camping. Information on these and other arthropod transmitted diseases can by found through the US Centers for Disease Control web site.
With the proper education and a little preparation, the risk from insect transmitted diseases can be greatly reduced.