Introduction

Invasive Species Defined

What's Being Done

 National Invasive Species Information Center

Submodule 6: Microbes and Fungi -- the Pathogens

Human Pathogens

A comprehensive estimate of the monetary cost of exotic pathogens to Americans is not available, but it is certain to be considerable. Especially since the advent of oversees exploration in the 14th century, humans have been sharing diseases and passing them from continent to continent. Entire civilizations are thought to have been eradicated because of exotic, lethal pathogens that arrived on ships from far away lands.

Most new human pathogens arise by the introduction of organisms from outside the country, or as the result of genetic changes (mutations) in already existing pathogens; sometimes both occur. Existing pathogens can travel around the world in humans or other hosts, with the explosive increase in the human population contributing greatly to their spread. Other contributing factors in the spread of human pathogens are increased in human mobility, the expansion of trade and globalization, and a rise in the global movement of pathogen vectors (or animals such as insects, arthropods, and rodents that carry disease).

The introduction of a pathogen does not necessarily mean that it will become established its new environment, but a pathogen does not need to become established to take a considerable human toll. There are several factors that increase the probability that a new pathogen will become established in its new habitat including:

• The pathogens ability to remain latent in animals (insects, mammals, and arthropods), which act as disease reservoirs.
• Environmental conditions similar between the new habitat and the habitat of origin. This includes climate, and flora and fauna.
• Continued availability of susceptible human hosts, such as people not previously infected and immigrants.

Currently, there are several nonindigenous pathogens that pose a threat to Americans. Let’s look at some examples:

• West Nile virus (Flavivirus): West Nile appears to have become established in the U.S. with outbreaks occurring in the summer and early fall. Usually the virus is spread by the bite of an infected mosquito, but it can also be spread by transfusions, transplants, and from infected mother to child during pregnancy and breast-feeding. Fortunately, casually touching or kissing an infected person does not spread the disease. About 80% of those infected show no symptoms whatsoever; most of the remainder develop a mild flu-like illness that usually passes in a few days. However, about 1 person in 150 develops serious complications such as high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. The best way to avoid infection is to use insect repellent containing chemicals such as DEET (N, N-diethyl-m-toluamide) or oil of lemon eucalyptus [p-menthane 3,8-diol (PMD)], be certain screens on windows and doors are well maintained, and remove mosquito breeding grounds – sitting water.
• Avian Influenza (Orthomyxoviridae): “Bird flu”, as it is commonly known, does not usually infect humans. The H5N1 viral strain might become dangerous because, while it now mainly infects birds, mutations could arise that enable it to easily infect humans and be passed directly form one human to another. If this happens its spread throughout the human population could potentailly be rapid and widespread as there is little human immunity to this virus subtype. In addition, it appears that up to half of human infections lead to serious respiratory illness and other deadly complications. Unfortunately, since 1997, there have been over a hundred cases of this disease reported in Southeast and southern Asia as detected by laboratory tests. The World Health Organization (WHO) is monitoring the situation closely by following up on any reports of this illness in humans. Most cases in humans have been traced back to prior exposure to infected poultry, with rare cases of person-to-person transmission reported. While most persons infected with the virus have typical flu-like symptoms, some experience severe complications including respiratory distress. The U.S. government currently restricts all importation of birds and bird products from areas where known infections have occurred. There is no evidence that eating properly cooked poultry and eggs is dangerous.
• Human Immunodeficiency Virus (HIV): Type 1 and Type 2 of this virus in the family Retroviridae cause AIDS, or Acquired Immunodeficiency Syndrome. The virus severely lowers the number of immune cells called T-helper cells, which compromises or destroys the body’s ability to fight infections. In the early 1980’s, men in New York and Los Angeles began presenting with opportunistic infections, or infections normally not a problem for healthy humans. These infections included Kaposi’s sarcoma and Pneumocystis pneumonia. HIV is most commonly spread by having unprotected sex with an infected partner or by sharing needles for administering intravenous drugs with an infected person. Also, about one-third of infected, pregnant women will pass the infection to their babies. Early symptoms include initial flu-like symptoms followed by a short asymptomatic period when the virus is easily spread by unsuspecting carriers. The virus may lie latent in blood cells for up to 10 years. Once the virus begins to multiply again, T-cells begin to die. This leads to the appearance of more severe yet ambiguous symptoms such as weight loss, fevers and sweats, and lack of energy. AIDS is the advanced form of HIV infection described by the Centers for Disease Control (CDC) as less than 200 CD4+ T cells per cubic millimeter of blood and 26 additional clinical symptoms. There are several drugs available to treat HIV infections, with most designed to slow replication of the virus. These drugs have prolonged life for many HIV/AIDS sufferers, although for many they have serious side effects. There is no cure for HIV infection or AIDS, so the only method of prevention is to avoid infection entirely through behaviors such as protected sex and not sharing needles.