Infectious Diseases 1

General Principles.
Infectious diseases are those caused by the invasion of a micro-organism into an animals body.
Non-infectious disease does not necessarily involve any other organism, for instance cat flu is infectious, but a broken leg or malnutrition is not. Infectious diseases include those caused by: a)Bacteria : e.g. Staphylococci, Streptococci, Bordetella, Pasteurella, Salmonella, Leptospirab)Viruses: e.g. Cat flu (2 viruses) Distemper, Infectious canine hepatitis, Rabies, Parvovirus c)Fungi: e.g. Ringworm (Microsporum, Trichophyton)d)Protozoa : e.g. Toxoplasma, Neospora Contagious Diseases are those diseases which are transmitted from animal to animal. Most of the important infectious diseases of domestic animals are also contagious, but not all, for instance tetanus is infectious but not contagious. Parasitic infestations (ectoparasites and endoparasites) are also contagious.

Causes of Disease

Infectious agents cause disease by disrupting the normal functioning of the body. This can be:
Physical- Viruses can destroy cells by taking over the internal machinery
Chemical- Bacteria can release toxins that cause damageImmune- Small numbers of organisms can cause damage by stimulating the immune system to turn on the host.Signs of disease relate to how the organism attacks the body and how the immune system responds. General signs include fever, pain, enlarged lymph nodes, inflammation, and are caused by activation of the immune system.Specific signs depend on where the organism attacks the body, so respiratory viruses cause sneezing, coughing, and alimentary infections cause vomiting and diarrhoea.

The Spread of Infection It is important to know how particular diseases spread so that steps can be taken to prevent this.Some diseases are ZOONOSES — they infect both animals and humans. The routes of infection are:

Direct

The animal spreads disease by direct contact with other animals. Organisms which are very fragile are usually spread by direct contact, for instance some viruses. The exchange may take place via body fluids (biting- saliva / blood, sexual activity) or via surface contact — scratching, grooming, kennelling together.b) Indirect The disease spreads via some aspect of the environment, including other animals used as hosts such as in the case of parasites.Types of indirect spread:

Environmental — spread via contamination of the air, surfaces such as parks and gardens, water sources.
Fomites- refers to bedding, food or water bowls, grooming equipment which can be a source of infection.
Non Biological Vectors are animals which transmit the infection but which are unaffected by it. Transport hosts simply move the micro-organism from one animal to another for example ticks carrying Lyme disease. Paratenic hosts carry the organism embedded in their tissues, so although they are not affected by it they must be eaten in order to infect the normal host.
Biological vectors are animals in which a part of the life cycle of an infectious organism takes place. For instance fleas act as intermediate hosts for the tapeworm Dipylidium caninum. Part of the tapeworm’s life cycle must occur in the flea so the flea is a biological vector.

Carriers

Carriers are animals which shed the disease organism but are not themselves ill. They may have apparently recovered from the disease and still be infectious or may have contracted the disease without showing any signs. Cat flu and feline leukaemia viruses often led to carrier states.

Prevention of infection

1) Avoid direct animal to animal contact. This is the easiest option to achieve, but is often inadequate as many diseases are indirectly spread. 2) Prevent indirect spread via hygiene, particularly between animals in kennel situations.

avoid contamination by appropriate cleaning or disposal of fomites.
use of separate feeding bowls and utensils
disinfect and clean areas contaminated by body secretions- blood, urine, saliva, faeces, pus, discharges. Dirt and secretions allow bacteria to avoid disinfection.
Ensure separation from other animals, particularly air space separation in the case of respiratory disease.
Barrier methods to prevent spread- changing / using disposable clothing after handling an infected animal.
Appropriate precautions in the case of zoonoses to prevent staff infection.

3) Vaccination of susceptible animals where possible. If enough animals are vaccinated then non-vaccinated animals are protected. This is the reason why owners can "get away" with not having their animal vaccinated. If too few are vaccinated, however, then suddenly only the vaccinated animals are protected. 4) Quarantine is used to isolate potentially infected animals. It can be used in any situation to protect other animals, and not just in imported animals, for instance new parrots should undergo 6 weeks quarantine before joining an aviary. Stress is a great cause of carriers suddenly becoming "clinical"

Zoonoses Zoonoses are diseases which can be transferred between animals and humans. It is therefore important for nurses to recognise the possibility of a zoonosis where appropriate.
Current important Zoonoses in the UK are:
1) Salmonella. Carriers are common and there are many types, infecting all domestic pets, including exotics such as reptiles.
2) Campylobacter. Enteritis causing bacterium found in dogs and cats and birds mainly.
3) Yersinia. Infects small domestic pets via contaminated food.
4) Leptospirosis. Can pass through intact mucous membranes.
5) Toxocara canis.
6) Ringworm. All varieties of ringworm which affect pets can affect humans.
7) Sarcoptic mange. Localised irritation.
8) Cheyletiella localised irritation.
9) Chlamydia (Clamydophila) psittaci. Caught from cage birds.
10) Toxoplasma. Very important in pregnant women.
11) Echinococcus granulosus
12) Lyme disease
Important with the relaxation of quarantine:Echinococcus multilocularis RabiesRickettsial tick borne diseases, Hence the need for treatment against ticks and tapeworm for animals returning to the UK.

Defence against disease The body defends itself against disease both physically and via the immune system.

Physical defences

Constant renewal of the skin and intestinal epithelia helps to prevent the entry of pathogens, or harmful organisms. Grooming behaviours remove potential contamination. The respiratory system uses mucus to trap infectious agents.Behaviours such as rejection of unfamiliar or rotten food helps to avoid infections.

The Immune system

The immune system consists of specific and non specific defences. Non specific systems are those such as neutrophils attacking bacterial invaders, and the production of interferon, an anti-viral substance. The immune system recognises things as foreign to the body by testing surfaces. The proteins and molecules on the on the surface of a body tissue or of a pathogen act as molecular fingerprints, and so the immune system can separate self from non-self. Specific defences recognise particular pathogens, which means they must have a memory of the pathogen. This memory can be acquired in two ways:

Passive Immunity

Passed on from the mother via Immunoglobulins , in the bloodstream and also via the first milk or Colostrum. Reflects the immune status of the mother

Active / acquired Immunity

Immunity which is caused by exposure to antigens — proteins found on the surface of foreign organisms. This is the basis of immunity due to vaccination, or which arises after an infection.

There are also two types of acquired immunity- Cell Mediated and Humoral. Cell mediated is due to "T" Lymphocytes which directly attack infected cells. Humoral immunity is due to immunoglobulins produced by "B" Lymhocytes. The immunoglobulins attack foreigners chemically, and also attract T lymphocytes.

Other influences on Immunity
a) Genetics. Some animals catch some diseases more easily than others.
b) Age. Young animals and very old animals tend to be the most susceptible
c) Multiple diseases, for instance intestinal parasites can make parvovirus enteritis much worse.
d) Nutrition. Better nutrition leads to a stronger immune system.
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