August 2004 Human Disease Transmission by Blood Feeding Insects and AIDS From the beginning of the HIV (human immunodeficiency virus)/AIDS (acquired immunodeficiency disease syndrome) epidemic, there has been concern about transmission of this virus by blood feeding insects. Many studies have been conducted by independent research facilities, the CDC (Center of Disease Control), and other public health agencies. These studies, even when done in areas of abnormally high incidence of AIDS and large populations of blood feeding insects, have shown no evidence of HIV transmission by insects. But before I explain why insects, specifically mosquitoes, don’t transmit HIV, let’s take a look at some of the human diseases that are.
Human disease agents can be parasites, protozoan, bacterial, viral, or rickettsial. Arthropod-borne, human diseases are usually transmitted from human to human by specific insects or ticks. In the broad sense, all disease agents are parasitic, using a host that provides nutrients and a habitat for their survival and reproduction...but all parasites are not disease agents. For example, the bacterial and protozoan fauna in the human digestive system is essential for the process of digestion, they are parasites but cause no disease (dis-ease) to the host, it is a symbiotic (mutually beneficial) relationship. The problem with “parasites” is when they cause tissue damage or release toxins into the host, causing disease.
There are many insects capable of transmitting disease to humans, mosquitoes are the most infamous. Malaria, the most common and deadly of all arthropod-borne diseases, is a protozoan that utilizes 2 hosts during its life cycle, Anopheline mosquitoes and humans. Malaria affects 250 million people worldwide and is the cause of 2 million deaths annually. Mosquitoes transmit about 28 viruses of major public health importance. Dengue, Yellow Fever, West Nile and several types of Encephalitis are just some of the diseases caused by these mosquito-borne viruses. Mosquitoes are also the vector for Filariasis, a disease caused by a parasitic roundworm that clogs the lymphatic system of its human host causing a condition known as Elephantiasis, resulting in gross disfigurement and eventually, incapacitating deformity.
Fleas are the vector of several diseases, the most notorious being the plague or “black death.” Plague is caused by Yersinia pestis, a bacterium that causes disease in man as well as in rats and other rodents. Plague has killed millions throughout history, over 25 million deaths were recorded in 14th century Europe, alone. Plague is still a problem today with over 5,000 cases reported annually, worldwide. Murine typhus, caused by Rickettsia mooseri, is also a disease transmitted by the bite of fleas.
Ticks are blood feeding ecto-parasites capable of transmitting a variety of disease agents. Lyme disease has recently become the human disease of greatest concern. Lyme disease was first discovered in Lyme, Connecticut, hence the name. The deer tick, the vector for Lyme disease, carries within it a bacterium called Borrelia bugdorferi. This bacteria enters the bloodstream of the animal or human that an infected tick feeds on. Lyme disease doesn’t cause a lot of mortality and is curable with antibiotics. But, if left untreated, this disease agent can cause chronic disease such as arthritis, heart, and other internal organ damage.
Human lice, Pediculus humanus, the body louse and Pediculus capitus, the head louse, can transmit rickettsial pathogens that cause epidemic typhus and trench fever. These lice are blood-sucking ecto-parasites specific to man. Lice were responsible for the epidemics of trench fever and epidemic typhus during World War I amongst the troops involved in trench warfare, on both sides.
There are many more human diseases transmitted by blood feeding insects, throughout the world. The one thing they all have in common is that the disease causing agents are capable of living with the vectoring host insect. These relationships, host-vector-parasite, have taken millions of years to develop. The AIDS virus has not been able to do this, survive within an arthropod host. Insect biting behavior is such, that when an insect bites, it does not inject its own or a previously bitten person’s or animal’s blood into the next person bitten. Instead, what the insect injects when it bites, is saliva that acts as an anticoagulant so it can feed effectively. Mosquito-borne diseases are transmitted by their saliva. The pathogens injected, are, in most instances, relatively harmless to mosquitoes.
The AIDS virus, HIV, can only live a short time outside the human body, it has evolved to live at a temperature like is in our body. Upon ingestion of HIV infected blood by a mosquito, the ambient temperature drops, but the real killer of the AIDS virus is the insect’s digestive system. The enzymes in the mosquito’s digestive system breaks down the virus as it would any other food particle it ingests. The virus is digested right along with the blood meal. Since the virus does not survive to reproduce and invade the salivary glands, the mechanism used to transmit infection, the injection of salvia, is not possible for the AIDS virus.
But aren’t mosquitoes flying hypodermic needles? The answer is no. A mosquito’s feeding apparatus is an extremely complicated structure, quite unlike a single bore syringe. The food canal and salivary canal of the mosquito are separate. Unlike a syringe, the mosquito delivers saliva through one passageway and draws blood from its host up another. As a result, the food canal is not flushed out like a used syringe. Blood meal flow is always unidirectional, into the mosquito’s stomach, never out. In short, mosquitoes are not “flying hypodermic needles.” A mosquito that injects saliva is not flushing out the remnants of its last blood meal. Supporting the research results, that insects are not able to transmit HIV, is the pattern of reported cases. If HIV was transmissible by insects, then there would be many more young children getting diagnosed with the disease.
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