Dr. James Evermann
Lecture 4: Infectious Organisms

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Examples of Infectious Microorganisms (Figure 1)

There are 4 general types of organisms: 

  1. bacteria
  2. protozoa group that includes Rickettsieae, Chlamydia and Ehrlichieae. 
  3. viruses
  4. prions 

Bacteria usually grow in synthetic media and will grow on the surface of the skin and on mucosal surfaces. They will also grow on the surfaces of inanimate objects. It is this ability to grow on surfaces that allows a bacteria to be more contagious than any of the other agents. It has the ability to replicate on surfaces. 

Unlike bacteria, the protozoa group, viruses, and prions are referred to as obligate intracellular parasites. They need a living host cell in which to replicate. To further differentiate the four groups; bacteria have RNA and DNA as part of their nucleic acid. The protozoa group, Rickettsieae, Chlamydia and Ehrlichieae, are like bacteria in that they have RNA and DNA. Viruses have only DNA or RNA depending on the individual virus.  Prions have neither DNA or RNA and are quite unique as they are infectious protein; prion protein.

  Main Groups of Infectious Microorganisms

  1. Bacterial or prokaryotes - are single celled organisms without a true nucleus (A true nucleus is a characteristic that defines a eukaryotic cells). Bacteria cause disease by two basic mechanisms.  First, they set up a cascade of events in the body resulting in inflammation. This results in tissue disruption and can eventually result in shock and death. Inflammation is triggered by invasion of the tissues with bacteria and the production of toxins: toxins can be exotoxins or endotoxins. The degree of tissue invasion and the production of toxins are commonly referred to as virulence factors.

    Control of bacterial infections varies according to the agent or "type" of the bacteria. Good hygiene is very important. Antibiotics are one of the major controlling agents available to minimize the spread of bacteria. Antibiotics may be bacteriostatic or bactericidal. In some cases we can use vaccinations against bacteria. Vaccines against bacteria are referred to as bacterins. Bacterins can be either inactivated (killed), or they can be alive. The live vaccines are usually attenuated. What is meant by attenuated is that infection occurs but disease doesn't develop. 

  2. Protozoa or Eukaryotic organisms that have a true nucleus. A prominent characteristic of protozoa is that they invade blood cells, either red blood cells (RBC), or white blood cells (WBC).  This is an important feature because the white blood cells are directly involved in immunity. Then the events are very similar to bacteria; an inflammatory response, tissue disruption, shock, and death. Another feature, generally seen preceding shock, is anemia, due to protozoon's ability to infect red blood cells. This differs from viruses, which impair cell function. Many protozoal infections are spread by insects so you can decrease insect spread although controlling insects is very difficult. Good hygiene is also important and my be easier than controlling an insect vector. You can use antibiotics, the best ones cover a broad spectrum. There are killed vaccines available. The effectiveness is quite variable. Some are good and some not so good.  Rickettsia, Chlamydia, Ehrlichia are also in the protozoan group. They are smaller than bacteria and are obligate intracellular parasites. They resemble bacteria because they contain both DNA and RNA and multiply by binary fission.
  3. Viruses - Are obligate intracellular parasites. They contain either DNA or RNA, and a protein coat referred to as a capsid. Viruses are either enveloped or naked. Enveloped viruses have an outer envelope composed of lipid and carbohydrates. Viruses either slow down cell growth, kill cells, or cause alteration of function. For example in  upper respiratory tract infections viruses may impair ciliary function. Some viruses are immunosuppressive. This is a very important feature of viruses. Especially Herpes viruses and retroviruses cause immunosuppression and lead to secondary infections. So it is easy to see why viruses can precede bacterial infections and in some cases, protozoal infections. You can decrease insect spread for those viruses that are spread by insects. Good hygiene can also minimize spread. You can vaccinate against some viruses.  There are two types of vaccines: killed and  modified-live vaccines. Modified live vaccines are attenuated or altered to reduce the chance that they will cause disease. To reduce spread of viruses you can decrease animal density or decrease animal contacts. There are also anti-viral drugs. Right now, these are very, expensive and not generally used in agricultural animals. Antivirals are used in human medicine.  An example is Interferon.
  4. Prions  Proteinaceous infectious particles associated with transmissible degenerative neurologic diseases of humans and animals. Prions are neuropathogens; they have a predilection for neural tissue. There is altered tertiary protein structure. Prions go from a very organized protein structure to a very disorganized protein structure. This results in insoluble protein within the neurons accumulating, causing the neurons to misfire, resulting in  hyperesthesia, which is increased excitability. The animal usually goes down, starts paddling, often the owner will put them to sleep at that time. Once the clinical symptoms of prion disease are manifested, they are irreversible – there is no cure. This is important to recognize because there are animals out there for example: Suffolk breeds that have the prion proteins, but they have not yet begun the altering protein structure. So the animals are not showing clinical signs at this point in time. Once you see the signs it is most humane to put the animal down. You cull affected animals. The clinical signs cannot be reversed. A test was developed here at WSU to screen for genetic susceptibility to prion disease by looking for the prion protein. Currently the test is only available for scrapie. Researching are test for BSE and hopefully prion disease in humans, but not yet. As the scrapie test becomes available it is just a matter of time before we have the detection ability to look for prions in other species as well.

It is really important that you recognize the differences and similarities between groups of microorganisms. It makes a big difference in how infections are controlled.

There are several important agents of disease that do not fit into one of the four main categories.  Fungi are eukaryotic organisms without chlorophyll that have cell walls. Mycoplasma are prokarytes that lack cell walls and are highly pleomorphic and resistant to penicillin. They are generally regarded as the smallest of free-living organisms.

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Selected Pathogens of Livestock

An extensive list of bacteria will be presented. I don’t expect you to memorize them all. An example of what you are expected to know: Give two examples of clostridial infections and the type of disease that they cause. 

Bacteria:
Streptococcus species that affect the skin or cause abscesses

Staphylococcus species

Corynebacterium pseudotuberculosis (C. ovis): causes pseudotuberculosis, although "ovis" implies sheep this is primarily a disease of goats. It causes sheep caseous lymphadenitis. Caseous means cheese and lymphadenitis means inflammation of the lymph nodes. Affected goats have swollen lymph nodes around the neck, in the abdominal area, and in the inguinal area. The nodes abscess and eventually rupture. When they rupture they release large amounts of bacteria into the soil, and so then the soil acts as a reservoir for this particular bacteria. So you will see this in sheep and goats, primarily manifested as lymphadenitis in goats. When sheep are sheered if the skin is "nicked" and is not disinfected, these bacteria will invade the skin causing abscesses. 

Rhodococcus equi: is primarily an infection of horses. It is a soil bacteria that results in pneumonia, usually in young foals.

Listeria monocytogenes:  There was a recent major recall in a meat packing plant because of Listeria. Listeria may contaminate wet feed. Cattle and sheep are primarily susceptible to Listeria. The bacteria reach the brain and causes neurologic signs. Listeria can also cause abortion in both these species. It is a common food borne bacteria.  Listeria is zoonotic, primarily transmitted in cheeses and processed meat. It is a particularly bad bug, for the host animal, but and for humans.

Clostridium species:

Of these, probably the several that would be worthy of remembering would be C. botulinum, C. tetani, and C. perfringens. This whole group are spore formers. The hallmarks of the clostridial infections are gas filled tissues, edema (swelling) and hemorrhage into tissues.

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1) Clostridium chauvoei
2) C. septicum
3) C. haemolyticum
4) C. novyi type B
5) C. perfringens
6) C. tetani
7) C. botulinum
8) C. difficile

You should know the organism, the host, and the disease for C. perfringens, C. tetani, and C. botulinum. These are really important. The clostridiums can be further divided into:

There is a 7-way clostridial vaccine for cows and sheep. It normally has these agents in the bacterium. The 7- way bacterin contains chauvoei, septicum, novyi type B, haemolyticum, sordelli, and two types of perfringens, type C and type D. 

C. perfringens, cause hemorrhagic enterotoxemia and pulpy kidney disease in sheep. In sheep and young calves. Some bacterins contain killed bacteria and are referred to as toxoids. Bacterins can also contain antitoxins which are antibodies made against the agent. These are antibodies made through the agent. Bacterins are usually given subcutaneously. This is a good example of passive immunization. Recall that there are two types of immunity, passive and active. Passive immunity is short-lived, and active immunity is longer lived. These are premade antibodies that are given subcutaneously to animals, so they are short-lived.

Bacillus anthracis: Another spore forming bacteria is Bacillus anthraxis which causes Anthrax. This bacteria may be used in biological warfare. A few of these spores deposited in a body of water could affect a large population. 
There was a recent case of anthrax in cattle.  When the cow dies the spores remain infective in bones for long periods of time. Then other animals eat the bones and become infected.

Bacteroides nodosus: This causes contagious foot rot in sheep. The organism usually resides in the soil, and enters the hoof following a hoof injury resulting in disease.

Escherichia coli: are an important group. E. coli cause enteric or gastrointestinal (GI) infections. There are many types of E. coli. E. coli can cause diarrhea in calves, lambs, pigs, and humans. A virulence factor of E. coli is the presence of pilli and the presence of enterotoxins.

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Salmonella also cause gastrointestinal infections. Salmonellas can reside as a normal infection within the GI tract. If the animal gets stressed, infected with other bacteria, or over-treated with antibiotics Salmonella may cause disease. An example is the treatment of colicky horses with antibiotics in order to control overgrowth of certain bacteria. Salmonella is resident in the gut and starts to overgrow. There is a delicate balance of organisms in the normal gastrointestinal tract. Antibiotics can upset this balance of bacteria. 
There are many types of Salmonellas. Two that you should know about are S. typhimurium and S. dublin.                        S. typhimurium has a multiple host range meaning the bacterium can be transmitted from one host to another host. A common sign seen in cattle with S. typhimurium is enteritis and/or septicemia. Septicemia means bacteria and its endotoxin in the blood.  Bacteria only in the blood is called bacteremia.  
Salmonella dublin is very important as it may cause subclinical infection and then spread to heifers and young calves, resulting in a series of illnesses. Signs may include enteritis, septicemia, and meningitis, which is inflammation of the brain. Some animals will abort. Dublin is emerging as a major Salmonella biotype that you should be familiar with. 

Pasteurella groups represent the major respiratory pathogens of cattle, sheep, goats, and rabbits. Paturella is a normal resident of the upper respiratory tract. Pasturealla is a commensal organism that leads to opportunistic infections. Diseases caused by pasturellas is called pasturellosis. 
P. multocida
P. haemolytica

Hemophilius somnus is another important respiratory bacteria. It causes thromboembolic meningitis which is primarily a feedlot disease abbreviated TEME. Haemophilus somnus enters by way of the respiratory tract and causes primarily respiratory conditions in feedlot cattle. The organism can effect the brain causing downer cow syndrome. So there are two major signs; a respiratory disease, and brain disease.

Taylorella equigenitalis causes Contagious Equine Metritis. The organism is carried by stallions who have no clinical signs. However, in the mare it causes severe metritis (uterine infection). The disease occurs primarily in Kentucky and Tennessee. This is a reportable disease, which means that if you see this condition, you should report it to the veterinarian as soon as possible. The veterinarian should report it to the federal veterinarian. 

Review:What reportable disease has been discussed in this class?  It causes vesicles in the mouth of cows. It is Vesicular Stomatitis. Do you recall why Vesicular Stomatitis is reportable? Because Vesicular stomatitis resembles Foot and Mouth disease. 
 
Campylobacter species (vibrio)

Moraxella bovis causes pinkeye, primarily in cattle.

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FAQ

Question – What are the signs an animal clinically infected with a prion protein would show?
Answer – The first signs are hyperexcitability including head pressing, and abnormal behavior. Then they go down and then they start paddling. The signs may up to two weeks and appear painful.  For reference see "A" under prion disease.


Question – Are there any other illnesses that would cause an animal to act as if it had a prion disease?
Answer – Cattle and sheep infected with listeria show neurologic signs. Be on the lookout for prion diseases. Horses with equine encephalitis show neurologic signs but to date no prion diseases have been identified.  If a sheep shows hypoerexcitabiliity ocnsider prion diseases and rabies as possible causes.  


Question – Is there a cure for the human prion disease?
Answer – Prion disease in humans is called Creutzield-Jakob disease and is labeled as a new variant of Creutzield-Jakob disease. It is thought that this variant was acquired from people who had eaten BSE-tainted meat, probably as long as 8 to 10 years ago.  There is no cure for prion disease in people.

 
Question – Is it proven or is it a theory that consumption of BSE-tainted meat causes Creutzield-Jakob disease?
Answer – It is still theoretical at this point.


Question – Is the human scrapie disease considered zoonotic?
Answer – Yes, I guess you would have to call it a zoonotic disease. It spreads from animals to humans. There is no cure.

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