NOVEMBER
2003

 

 

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

 

 

 

 

11-13-03

Do Fleas Sneeze When They Catch Colds?

In the system that scientists use to classify living things, insects and humans are members of the animal kingdom. That is because we are similar to insects in a number of ways. For instance, we both eat, need oxygen to breathe, and are able to smell and see, more or less.

There are also differences between insects and humans. We are warm-blooded, have two legs and have the ability to think. Insects are cold-blooded, have six legs and don't think. Although some people could also be accused of the latter!

Both insects and humans fall victim to disease. In both animals, microorganisms can be the agent that causes disease. However, disease-causing organisms are specific to either humans or insects. To date, no organisms have been identified that will cause disease in both humans and insects.

We all know what happens when humans catch a "bug." Which, by the way, is a slang use of the term bug. Bug correctly should be used as a name for insects of the Order Hemiptera. Those microorganisms that cause disease are really germs, not bugs!

There are several types of microorganisms that are pathogens in insects. The principal ones are viruses, bacteria, fungi, protozoans and nematodes. Like microorganisms that cause human disease, many of the insect pathogens are widespread and only under certain circumstances cause disease.

There are numerous viruses that infect insects. Symptoms of viral diseases in both insects and humans vary with the causal virus. In general, it takes up to five days for an insect to show symptoms of a viral disease. Like in humans, diseased insects become less active and feed less. In caterpillars that die of a viral disease, the integument becomes weak and easily torn. In general, the innards of the caterpillar are mostly liquid as a result of the disease.

Like humans who cough and sneeze, the infected caterpillars also spread disease organisms. However, the caterpillar does so in death. When a caterpillar is near death, it climbs to the top of plants. So, when it dies, the contents of the caterpillar rain down on leaves below, allowing other insects to catch the disease.

There are a limited number of bacteria that cause disease in insects, which shows that insects have an effective antibacterial system. One bacteria that causes widespread disease in insects is the well-known Bacillus thuringiensis. This bacteria is generally called Bt. This bacteria works by producing crystalline proteins that are toxic to insects.

Strains of Bt have been used as insecticides to kill several types of insects, including caterpillars of butterflies and moths, immature mosquitoes and black flies, and some plant-feeding beetles. Modern technology has allowed the genes that produce the toxin to be incorporated into plants, so that insects feeding on the plants will be killed.

Another strain of bacteria gets into the gut of scarab beetles, like the lawn pest called a white grub. The disease causes the abdomen of the grub to get pale in color. Hence, the condition is known as milky disease, and the bacteria as milky spore.

In general, we humans are happy when disease epidemics hit pest insects. However, there are also diseases that affect honey bees and silkworms. Since we raise these insects, we try to prevent such diseases.

You'll never hear a flea or bee sneeze, but diseases are just as big a problem for insects as humans. The difference is insects can't go to doctors when they are feeling a little under the weather.

 

Writer: Tom Turpin
Editor: Olivia Maddox