What are common vector-borne diseases, and how do they commonly spread among vertebrate hosts?

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Let’s begin with a definition of “vectors.”  Vectors carry pathogens from one host to another.  Vectors are frequently invertebrate animals, typically arthropods.  (Arthropods have an exoskeleton, segmented bodies, and jointed appendages).  It should be noted, however, that vertebrates can also act as vectors. Common vertebrate vectors include foxes, raccoons, and skunks. These mammals can transmit rabies, but the majority of disease-causing organisms are arthropods (85%).  Arthropods spread their diseases primarily via stings and bites, and also by infestation of tissues, and through indirect disease transmission.  Mosquitos and ticks, blood-feeding arthropods, are the main culprits here; mosquitos can be vectors of malaria, and ticks can be vectors of lyme disease. Other vectors spread disease by literally carrying pathogens on their bodies from place to place.

Human acquisition of vector-borne diseases go through a three stages:  the presence of the pathologic agent which attaches itself to the arthropod vector, which then finds a human host.

Vertebrate hosts are the way in which most vector-borne diseases promulgate in nature; these hosts are called “zoonoses.”  (Humans, however, are the major hosts for diseases like dengue fever and malaria).  Animal hosts are often intermediaries; the animals serve as a reservoir until the vector can transfer to susceptible humans. How long a pathogen survives is completely dependent on the availability and suitability of hosts, whether that host is intermediary or attaches directly to the human host.  Environmental factors, especially temperature and humidity, also affect how successful a pathogen can become.  The human host’s own immune system, its strength or weakness, also plays a role in a pathogen’s success or failure.

Because of the pathogen’s dependency on temperature and humidity, vector-borne diseases are most common in the tropic and subtropics.  However, some colder climates are also ideal for pathogens, especially lyme disease, Rocky Mountain spotted fever, and viral encephalitis. 

There are different patterns which dictate how strong and wide-spread vector-borne diseases can be.  For example, parasitic and bacterial diseases, like Lyme diseases, have high incidence wrates but do not reach levels of epidemics. (Plagues, however, are the exception to this rule.)  Disease like Yellow fever, however, can quickly and commonly cause major epidemics.

Vector-borne diseases have gained momentum all over the world since the 1970s.  Some of the reasons for these new and strong outbreaks include resistance to drugs and insecticides.  For example, the mosquito which is a vector for yellow fever was thought to have been eradicated, but it has made a return. Other reasons have to do with the inability of health care systems to cope with the diseases, especially when they spread quickly. Vector-borne illnesses that have been problematic over the last forty years include “malaria, dengue, Yellow fever, louse-borne typhus, plague, leishmaniasis, sleeping sickness, West Nile encephalitis, Lyme disease, Japanese encephalitis, Rift Valley fever, and Crimean-Congo hemorrhagic fever.”

Because it is not tenable or desirable to wipe out the intermediary host, efforts continue to eradicate the arthropods which transmit the diseases.  Some relatively easy means of combating vector-borne illnesses include screens and bed nets. Wearing covering garments, boots, hats, and long sleeve also help, as does using insect repellent. 

Vaccines are also helping combat infections that lead to Yellow fever, tick-borne encephalitis, and even plague.  Attempts to stop transmission from vertebrate hosts are also being made. For example, in Europe and Canada, foxes are being vaccinated against rabies. 

We will likely never be rid of vector-borne diseases, but there are ways to reduce risk.

Source: Encyclopedia of Public Health, ©2002 Gale Cengage. All Rights Reserved

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