On The Acquisition Of Disease

What is it that moves an illness around?

Several factors make it possible for an infectious disease to take place in a new victim.

The requirements are: 1. A susceptible host, or victim. 2. Appropriate exposure. 3. An adequate "dose" of the infectious agent. And 4. Sufficient virulence of the infecting organism. In most cases, all of these are necessary for a clinically apparent infection to occur.

At a recent social gathering, someone asked why the AIDS virus, HIV, can't be transmitted by a mosquito bite. (Medical topics are fair game in any gathering!) This brings up the subject of transmission of an illness from one person to another, which covers almost all of the categories listed in the preceding paragraph. Take a simple case, like staphylococcal infections. A susceptible host is a preemie infant in the newborn nursery, the dosage needed is carried on the hands of a nurse, and the virulence of the bug is unquestioned (it lives in a hospital!).

Person to person direct transmission of disease requires nothing fancy in between the two. There is no "vector" necessary. Colds, influenza, tuberculosis, and measles are classic examples of infections which move with ease from person to person.

Transmission of many others is complicated by intervening agents, which are vectors for the particular disease. Often these conditions are moved from a non-ill host (the rats of bubonic plague fame) to one in which severe illness and perhaps death are the result. In plague, and in the closely related disease tularemia, the vector is a flea.

Prairie dogs, the wild "reservoir" for plague and tularemia in the Southwest US, are caught and played with by kids in the area. If the ever present fleas happen to land on and bite the child (adults should have better sense than to play with prairie dogs) either illness may occur. Neither the prairie dog nor the flea is "sick", but both can carry the virulent bacteria for a long time.

In the Northeast US, Lyme disease is endemic in the herds of deer which wander around the woods, healthy and cute. Ticks feed off these deer, and, not being terribly particular about their restaurant identity, they also get nourishment from humans. During the human dining experience, the Lyme agent (it's a borellia species of bacteria) is passed on to the new host, mankind, a very susceptible recipient. If the dose is big enough, Lyme disease occurs. Rocky Mountain Spotted Fever (a ricchetsial disease), which is by no means limited to the Rocky Mountains, is transmitted the same way as Lyme disease.

Mosquitoes are great vectors for viral and parasitic illnesses. In the former category fall yellow fever, dengue, West Nile virus, and several kinds of encephalitis-causing viruses. In these circumstances, the agent may be acquired when the mosquito feeds on a wild host (birds, monkeys, etc.) and then takes a drink from a human. The prize for parasitic transmission goes to the anopheles genus of mosquito responsible for bringing the world malaria.

This is where evolution shines - a complicated life cycle (malaria has several phases) adapting to the feeding habits of a vector. After the mosquito eats and in the process acquires the malaria parasite at a specific stage, it takes time out to digest its meal. During this period, the parasite is undergoing changes which will make it infective for the next individual the mosquito encounters. Ain't nature wonderful?

Mosquitoes and malaria have been around for a really long time, like millions of years. Given the short life spans of each, the evolution of the association which works to the benefit of both (the mosquito doesn't "get" malaria and the parasite moves undeterred from one stage to another more infective one) probably didn't take long. Maybe a century or two was all that was needed. The viral illnesses in which mosquitoes are a vector have also co-existed with their tiny transmitters for a long time. So what is it with HIV? Hasn't it been around a long time, too?

Probably a similar virus has indeed been around for a long time, in monkeys, among whom it was spread by, shall we say, reproductive activities. It was obviously not lethal for the monkeys, or we would have no monkeys by now! Humans acquired HIV before it was called HIV during the process of killing and butchering monkeys for food. This was direct transmission, from the blood of the animal to abrasions and cuts on the humans. No vector involved here.

And, in perpetuation of AIDS caused by HIV, no vector is necessary. It goes from one person to the next through exchange of some but not all bodily fluids. Nothing more is required for HIV to maintain its existence, so why bother with some time-consuming exercise by which it could gain an advantage (which it doesn't need)? There would have to be a mutual adaptation involving a potential vector like the mosquito, because at the moment, HIV can't survive in the little bitty pests.

The answer to the above party question is: "That's just the way it is!"