Types of Viral Infections

1. Unapparent infections—have no symptoms.

2. Acute infections--

   1. localized

   2. disseminated

   3. congenital

3. Persistent infections--

4. Transformation— Oncogenic process.

The virus starts in the environment, gets into the host, replicates, and then is released by the host and spreads to other individuals or into the environment.

IN the generalized infection—there are two types of spread of the virus—the most important is through the lymph and blood. The virus enters and implants in the local site of entry, then spreads through the lymph node byviremia . From the lymph node it spreads to a target organ such as the liver etc, and that is where you see the disease. They can infect other cells on the way, but you won’t see disease until they get to their preferred target.

Others spread through peripheral nerves. There are several that do this including the herpes virus. Here, the target organ is the CNS. This is true for the polio virus.

Centrifugal spread through the central nerves

Centripetal spread through the peripheral nerves.

IN congenital spread—when the embryo is infected, then you can see some serious damage in many places. The primary example is the rubella virus. The mother is infected first, the blood stream is the means to spread to the placenta and from there to the baby. By the third trimester, the baby is safer from infections. If the baby is born, then perhaps, it can have some disease, that can be chronic, acute or eventually heal up.

Babies can get cataracts—this can be the result of rubella infections.

Persistent infections—latent chronic or slow.

1. Latent infections—there is always an acute primary infection, the latent state is non-infectious, subsequent disease will be recurrent with reactivated virus. Some examples include Herpes, varicella-zoster, and the Epstein Barr, as Cytomegalovirus.

2. Chronic infections—have an acute primary phase, are usually infectious, and have chronic disease or recurrent disease. These include hepatitis B, Cytomegalovirus, Subacute sclerosing pan encephalitis (measles).

3. Slow—infections—don’t have an acute phase, they are infectious, have progressive disease, and include the Kuru virus, Creutzfeldt-Jakob, and retroviruses.

Herpes virus will enter through the skin, infect peripheral nerves, then travel up to the dorsal root ganglia. Here it can become latent and then later cause recurrent disease.

IN the case of zoster—you can get shingles—this is a recurrence of the chicken pox. You can not have a primary infection for zoster. It also travels to the dorsal root ganglia and then can come out again to cause subsequent disease. It is a painful disease and people commit suicide sometimes when they get it.

Persistent chronic diseases—usually infectious and patients usually shed some virus.

Prions—scrapie—occurs in sheep and goats.

Bovine spongiform encephalopathy—cattle (mad cow disease)

Creutzfeldt-Jakob disease—these people die (100%) and have a long incubation period and develop a tremor. This disease was acquired from the handling of the brains of infected people. People who processed the bodies , or ate them (cannibals!). once you stop eating people’s brains, you don’t have to worry about getting the disease so much.

Kuru—amyloid process stain with a red dye. Ti was found that this disease was similar to the Creutzfeldt-Jakob disease and scrapie with sheep etc.

Gestmann-Straussler syndrome—all human diseases.

Chronic wasting disease with spongiform encephalopathy

Prions—have no nucleic acids, they don’t seem to have a morphology that is well defined. They are protein only pathogens. They can be disinfected by roughly nothing but fire. UV, formaldehyde,protease's , heat are all ineffective at inactivating prions. They have a very long incubation period up to 20 years. There is no immune response at all to prions. The reason for the lack of immune response—the active form is just one point mutation from the natural protein—that one mutation is enough to kill you! Because it looks like a natural protein, it does not get attacked by the immune system.

PRP—prion protein—when it becomes infectious—it becomes PRSPC (scrapie)—normally proteins fold in a general pathway that includes some portions going one way and others another. When they become scrapie—they becomestretched out and become infectious. WE don’t really know exactly how this works yet. But it seems that they are aberrant folded.

Prions replicate by converting the normal form of the protein into the disease form.

A second hypothesis—they think that they produce polymers of the PRP. In reality we have no idea.

There have been four:

Kuru—causes loss of coordination followed by dementia. It was stopped with the end of cannibalism. It had an incubation period of years. IT was found in the highlands of New Guinea

Cretzfedt-Jakob disease—causes dementia followed by loss of coordination. There are three types of disease

1) SPORADIC—very rare.

2) inherited disease—sometimes the family will pass it on through the family.

3) iatrogenic form—through transplants that give the disease.

-growth hormone was a source. Typically, the incubation period is about 1 year but up to ten years

Gerstmann-Strassier-Scheinker disease—loss of coordination followed by dementia—inherited mutation in the PRP—

Fatal familial insomnia—affects the thalamus—causes it to shrink.

The infection is followed by the incubation period that can last up to 30 years. Death occurs about 4 months after the first symptoms appear.

Mad cow was first found in the cows by giving cows brains and goat for protein! That was their kind of protein shake. Six years ago, we started to see this in the UK. It would affect people too when they ate mad cows. It came to the conclusion that the disease may have come from the mad cows—this would be the firstinter species spread of a prion disease. IT killed McDonald’s in England.

Mad cow disease affects the brain stem. They devised a test that looks like a gel, and they take brain extracts. Using this system, they feel that they have matched the human disease to the cow form based on the sugar-content of the prion proteins.

Category: Microbiology Notes