When most people speak of reservoirs, they are discussing impoundments or collections of water, usually kept behind a dam that spans a river or valley. Most generally these reservoirs serve a host of purposes including flood control, drought protection, recreation, municipal water supply, hydropower, and industrial. While environmental scientists are interested in this type of reservoir, as are boaters and sportsmen, there are other types of reservoirs that are of great interests to public health professionals, especially epidemiologists.
A natural reservoir often referred to as a nidus, which is Latin for nest, is the long-term home of a pathogen of an infectious agent. Most often these hosts are not negatively affected by carrying the pathogen or as in the case of influenza, it is carried as a subclinical infection and thereby asymptomatic. Many infectious disease reservoirs are currently unknown, although suspect species have been identified including a certain type of bat with Ebola. It is only after the natural reservoirs have been discovered that the lifecycle of the pathogen can be understood and effective prevention and control measures can be instituted. Sadly, much of this is often ignored for economic or political reasons, as in the case of Ebola.
Some common examples of natural reservoirs and the associated pathogen’s include Field mice for Hantaviruses and in some areas Lassa fever, and Marmots, Prarie dogs, and black rats for Bubonic plague. Armadillos and Opossums are known reservoirs for Chagas disease as well as Leprosy and some species of Leishmaniasis. Bats are known reservoirs of Rabies, Ebola virus, Severe Acute Respiratory Syndrome (SARS). Reservoirs of human impacting pathogens are not relegated to mammals.
Some invertebrates, in particular, those that bite, are well-known reservoirs for pathogens, including Rocky Mountain Spotted Fever, schistosomiasis, Lyme disease, Malaria, and Tularemia. The reservoirs of other, less understood pathogens remain unknown.
Why doesn’t the reservoir species get the disease and die? The reservoir species may carry an active viral load of the pathogen, the absence of clinical signs is characteristic, indicating that the animal itself remains unaffected by the pathogen. This is due to natural immunity built up over generations. While one species may be immune to a pathogen, another species, one that has never experienced contact with the pathogen will become quickly infected. This is why bats that live deep in the Congo forest may have never come into contact with humans until fairly recently when humans were forced to go further afield to find food.
The journey from a reservoir to human may not be direct routes, such as a human eating a bat or a Prairie dog fleabite. Transmission to human may involve a vector.
One example is Dengue Fever; the reservoir is a primate, but the vector is a mosquito. Japanese encephalitis reservoirs are pigs and some wild birds, but it is the mosquito that acts as vector to humans. Still, other viruses double-down and use both direct and vectored infection of humans, for example, Rabies, which can be caused by the bite of a Bat, or infected animal such as a Cat, Coyote, Dog, Fox, Horse, Raccoon, Sheep, Skunk or Wolf. There are even recorded cases of a human vector; one person infected with rabies bit another and infected the second person.
Surveillance is an epidemiological practice by which the spread of a disease is investigated and monitored to establish patterns of progression. With surveillance, epidemiologists and other public health professionals can work to minimize the harm caused by outbreaks, epidemics, and pandemics and help inform physicians and health workers about the factors that caused the outbreak, including the reservoir and vectors if known. A key part of this disease surveillance is the practice of disease case reporting to a central body, for example, the Centers for Disease Control and Prevention (CDC), or the World Health Organization (WHO). However, efforts by those agencies, as well as state or local level health scientists are subject to the whims of two factors that are on the outskirts of disease investigation and health science, namely politics and economics.
Testing for a disease can be expensive, and often distinguishing between two likely diseases can be prohibitively difficult in many second and third world countries. One standard means of determining if a person has had a particular disease is to test for the presence of antibodies that are particular to this disease. In the case of H5N1 (Influenza A virus subtype H5N1), a highly pathogenic causative agent of H5N1 (Avian influenza). The perplexity is due to the existence of a low pathogenic H5N1 strain in wild birds in North America that a human could conceivably have antibodies against. Thus it would be extremely difficult to distinguish between antibodies produced by this strain and antibodies produced by the pathogenic H5N1 variety. These difficulties are common, especially for emerging infections and zoonotic, and make it difficult to determine how widely a disease may have spread and who may be susceptible.
For this reason, precautions must be taken to help protect you from these infections. This begins with an understanding of how the disease spreads. As we are heading into flu season in the Northeastern United States, I will talk a bit about everyone’s favorite illness, the flu. I might mention that neither my wife or I have had influenza in many years as we follow the prevention steps I will speak of momentarily.
Flu viruses have been thought to spread from person to person primarily through large-particle respiratory droplet transmission (e.g., when an infected person coughs or sneezes near a susceptible person). Transmission via large-particle droplets requires close contact as these droplets generally travels only short distances through the air. Airborne transmission of small particles in the vicinity of an infectious individual may also occur, however, airborne transmission over longer distances, such as from one hospital room to another is thought not to occur. Perhaps the biggest concern to those wanting to remain flu-free is indirect contact transmission via hand transfer of influenza virus from virus-contaminated surfaces or objects to mucosal surfaces of the face (nose, mouth). All respiratory secretions and bodily fluids, including diarrheal stools, of patients with influenza are considered to be potentially infectious. However, detection of influenza virus in blood from influenza-infected patients is very uncommon.
Preventing transmission of influenza virus and other infectious agents requires a multi-faceted approach. The spread of influenza virus can occur among family members, coworkers, or the community contacts. The core prevention strategies include getting the influenza vaccine for your area, washing hands frequently, avoiding sick persons whenever possible, and when not possible, be very aware of infection prevention measures including asking them to cover their nose and mouth with a tissue when they cough or sneeze, and immediately throw the tissue in the trash after use. Do not allow non-infected persons to handle anything the sick person has touched until it has been cleaned and disinfected as they may be contaminated with flu viruses. If you must handle something a sick person has used, wash your hands often with soap and water immediately. If soap and water are not available, use an alcohol-based hand cleaner. Do not touch your eyes, nose and mouth.
If you get the flu, you should try to keep it from spreading. Limit contact with others as much as possible to keep from infecting them. If you are sick with flu symptoms, the Centers for Disease Control and Prevention (CDC) recommends that you stay home for at least 24 hours after your fever is gone except to get medical care or for other necessities. If you have the flu, check with your physician to see if he recommends an antiviral medication. Antivirals can shorten the duration you are sick and can lessen more serious complications.
In the end, you can chose to avoid being a data point on the surveillance spread sheet by getting a flu vaccine (unless medically prohibited) and taking the outlined precautions.