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Infectious Diseases
Implications in a control programme

By Ray Butcher, Wylie & Partners

(Lecture presented on the Second International Companion Animal Welfare Conference, Bratislava, May 1998)


Table of Contents

Potential risks
Causes of infectious/transmissible diseases
Routes of transmission
    Oral - enteric diseases
    Inhalation - respiratory diseases
    Skin and mucous membranes
    Vectors
    Bites
    Direct contact
    Indirect contact
    Trans-placental
Other factors
Predisposing factors
Zoonoses
Infectious viral diseases - Dogs
Infectious diseases - Cats
Ectoparasites
Endoparasites
Risks from zoonoses
Risks in a colony
Clinical assessment
Clinical assessment - the Reality
Action after assessment
Prophylaxis
Cost implications

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Potential risks
 
Clearly the potential risks of infectious disease are to both:
  • Humans - Zoonoses,  and
  • To other animals within the colony/kennel

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Causes of infectious/transmissible diseases
 
As veterinarians we all recognise the main groups of infectious agents that are important in dogs and cats. These are:
  • Viruses
  • Bacteria
  • Fungi
  • Endoparasites
  • Ectoparasites
As we progress down the list, the organisms tend to be larger and so more easily identified. This clearly has implications in the specific diagnosis of the disease.
 
Ectoparasites such as fleas, ticks and lice may be readily identified by the naked eye. Some mites require the examination of skin scrapings by low power microscopy.
 
Adult roundworms and tapeworms can also be recognised by eye, though the eggs require microscopy.
 
Bacteria are much smaller and can be identified by visualising colonies grown on specific nutrient media, or by examining specifically stained smears under high power microscopy.
 
Viruses are generally tiny intracellular organisms consisting principally of nucleic acid. They can generally only be grown in tissue culture, and some may be visualised using an electron microscope. Clinically a specific diagnosis of a viral disease is made generally by identification of specific changes in histopathology or by serology.
 
Financial constraints may mean that many such tests are not practical, and so often one may need to rely on clinical examination and assessment.

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Routes of transmission
 
Before one can even consider programmes to limit the spread of disease (whether to humans or to other animals) we must clearly have a good knowledge of the epidemiology of the disease.
 
How are disease agents spread:
  • Oral - enteric diseases
  • Inhalation - respiratory diseases
  • Skin and mucous membranes
  • Vectors
  • Bites
  • Direct contact
  • Indirect contact
  • Trans-placental
 
Oral - enteric diseases
 
The infectious organism (e.g. canine parvovirus) passes out in the diarrhoea or vomit of the affected animal. This results in contamination of the environment and transmission occurs when a second animal takes this in by mouth. The organisms can generally survive for long periods outside the host.
 
Scrupulous hygiene is required to prevent spread - isolation and disinfection of cages is required. The surfaces must be thoroughly clean before disinfection, so this reflects very much on kennel construction.
 
The infectious agent can be transmitted accidentally on the hands and clothes of kennel staff when attending to multiple animals.
 
 
Inhalation - respiratory diseases
 
Respiratory diseases (e.g. Cat Flu) are generally spread by aerosol transmission. The affected cat will cough or sneeze and the virus inhaled by a second cat. Generally these agents do not survive long periods outside the body as they are susceptible to desiccation. The spread of the disease is therefore favoured by warm, humid conditions with poor ventilation and many animals sharing the same air space.
 
 
Skin and mucous membranes
 
Hookworm larvae can penetrate the skin. This is generally more of a problem in the tropics, but can be seen in some poor greyhound kennels in the UK where the dogs are kept in damp conditions.
 
 
Vectors
 
The disease organism is transmitted by biting vectors such as ticks, fleas or mosquitoes. This is an essential part of the life cycle of the disease organism. Control is often aimed at controlling the life cycle of the vector.
 
 
Bites
 
Rabies is transmitted when an animal bites, the virus being implanted within the body tissues.
 
Cat bites can also cause serious infections, both in humans and other cats. These are commonly as a result of infection by Pasteurella.
 
FIV is also transmitted by bites from infected cats.
 
High density and overcrowding will increase the number of fights and hence the potential for problems.
 
 
Direct contact
 
Many ectoparasites (Sarcops) require prolonged direct contact between animals. Overcrowding will clearly increase the risk of transmission.
 
 
Indirect contact
 
Some organisms can be transmitted indirectly via the contamination of objects. The spores of the fungus responsible for ringworm can be transmitted via combs, brushes or even rubbing against door posts.
 
 
Trans-placental
 
FeLV can be transmitted in a number of ways, including the vertical transmission from mother to offspring.
 
In dogs, larval stages of Toxocara canis are encysted in the muscles of the bitch. Some of these are stimulated to continue their development by the hormones of pregnancy, and these will cross the placenta to infect the puppies within the uterus.

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Other factors
 
The development of a disease is often dependent on other factors apart from the infective organism.
  • Nutrition
A good balanced diet containing adequate levels of essential vitamins and minerals is an important part of general health. Poor nutrition will clearly predispose to disease.
  • Intercurrent disease
An animal weakened by one disease may become more susceptible to another. Skin damaged by chronic parasitism may be more easily infected with ringworm fungus.
  • Immune suppression
Infection of cats with FeLV of FIV will result in immunosuppression. These cats often then succumb to additional infections of Infectious anaemia or FIP.
 
In the dog, Demodex infestation may result in immunosuppression.

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Predisposing factors
 
Within a colony of animals, whether as a feral population or in a kennel/cattery, the following predisposing factors are therefore important:
  • Overcrowding
  • Poor nutrition
  • Poor housing
  • Poor sanitation
We will discuss the implications of these later.

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Zoonoses
 
Perhaps before going further, it is worth mentioning the sort of diseases that are important. The incidence and relative importance of these will vary in different geographic locations, temperate and tropical climates etc. The following are examples.
  • Rabies
  • Toxoplasma
  • Salmonella
  • Dermatomycosis
  • Sarcops
  • Cheyletiella
  • Toxocara
  • Cat scratch fever

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Infectious viral diseases - Dogs
  • Distemper
  • Parvovirus
  • Canine Hepatitis
  • Leptospirosis

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Infectious diseases - Cats
  • Panleucopaenia
  • Calicivirus/rhinotracheitis virus
  • Infectious peritonitis
  • Feline Leukaemia virus
  • Feline immunodeficiency virus
  • Infectious anaemia

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Ectoparasites
  • Fleas
  • Lice
  • Ticks
  • Mites - Sarcops, Cheyletiella, Notoedries, ear mites, Demodex

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Endoparasites
  • Roundworms
  • Tapeworms

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Risks from zoonoses
 
Zoonotic diseases clearly pose a threat to all the population, and so the control of strays is an important measure to improve human health.
 
Clearly the risks are greater to those that have greater contact i.e. the "feeders", wardens or kennel staff. It is essential that these people are educated about the potential hazards and that they adopt a procedure that reduces the risks to a minimum.
 
The control programme must therefore balance the welfare requirements of the animal with the risks to the handlers.
 
The main areas of risk are:
  • Catching and handling
Apart from direct physical injury, diseases such as Rabies and Cat Scratch fever are potential hazards. Humane safety equipment is available for trapping and handling animals, and examples are given in the WSPA document.
 
It is important to instruct the handlers in the correct use of these, and indeed why they are important. It is also important that any equipment is maintained in good condition.
 
Improper use or faulty equipment may not protect the handier, or may cause unnecessary suffering to the animal.
  • Personal hygiene
We have already seen how enteric diseases are transmitted. Salmonella can be transmitted in this way to humans and so careful personal hygiene and wearing washable protective clothing is important.
  • Special Groups
Young children are especially considered to be at risk regarding Toxocara canis.
 
Pregnant women are regarded as especially at risk from Toxoplasma.

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Risks in a colony
  • Susceptibility of existing colony to new organism > epidemic
  • Susceptibility of new animals to endemic problems

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Clinical assessment
 
It is clearly important that the health status of animals in the colony is assessed on a regular basis, as well as that of all new animals likely to be introduced. The assessment can be at a number of levels:
  • Direct clinical examination
It may be possible to make a diagnosis on the strength of a clinical examination. A puppy with vomiting, diarrhoea, a cough and a mucopurulent oculo-nasal discharge is likely to be suffering from Distemper.
 
Fleas and lice are clearly visible to the naked eye. Ear mites can be seen with a standard otoscope.
  • Screening tests - simple and cheap
If a microscope is available, skin scrapings will help identify Sarcops, Cheyletiella, Demodex and ringworm.
 
Some species of Microsporum can also be identified by using an ultraviolet lamp (Woods Lamp) when the hyphae fluoresce.
 
The microscopic examination of blood smears may identify the inclusions within the red blood cells seen in FIA.
  • Screening tests - more expensive
More sophisticated (and hence more expensive) tests may help to identify specific bacteria.
 
Stains/cultures/dyes
 
Test kits are available for serological tests for viruses: FeLV, FIV.
 
Some viral diseases require histopathlogical examination for confirmation.
  • Screening tests - difficulty of interpretation
Even if money is not a problem, screening for some diseases remains a real problem. FIP remains a major problem in welfare colonies of cats in the UK and USA. It is a relatively common virus and interpretation of blood titres can be a problem.

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Clinical assessment - the Reality
 
All these techniques are potentially available to you, but in reality financial constraints will mean that probably most of the assessment will be made on the basis of a clinical examination, perhaps with some basic laboratory tests in some cases.
 
It is therefore essential that:
  • It is a veterinarian that makes this assessment,
  • and that you, with your special knowledge of local problems and financial constraints design the protocol that is most appropriate and practical.

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Action after assessment
 
Following the assessment, the protocol must outline the possible agreed actions. These might be:
  • Add to existing colony
  • Quarantine
  • Treat
  • Euthanasia
Clearly not all these options will be popular, and so it is important that all interested groups are involved in the discussions on designing the programme, though the veterinarians must take the lead.

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Prophylaxis
 
In any situation, prevention is better than cure. The following measures may be considered as part of the programme, though clearly at a cost:
  • Good general hygiene, sanitation
  • Improved nutrition
  • Regular treatments for ectoparasites
  • Routine worming
  • Vaccination

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Cost implications
 
Sadly any discussion about disease control has to be balanced by consideration of the cost:
  • How much?  and
  • What is available?
The costs involved in this particular aspect relate to:
  • Veterinary assessment/diagnosis
  • Kennel design and maintenance
  • Prophylaxis
  • Education
Perhaps this should be considered alongside the costs of failure to control these diseases.

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This lecture has been published first (in 1998) on the Internet Site of the Bulgarian Animal Defence League (Bulgaria, Registered Charity No. U-108063587) with the express permission of the Organisers of the Second International Companion Animal Welfare Conference. BADL would like to thank Ray Butcher for giving the opportunity to publish his lecture on the BADL's Website.
 
We just reprint this lecture in full. All copyrights are of their respective owners - Ray Butcher, for the text of the lecture, and BADL - for the online publication.

The AnimalawBgTeam


Published in the IWNS.org Website on July 20, 2002
©1998, by Ray Butcher, Wylie & Partners, for the text of the lecture
©1998, by BADL, for the first online publication
©2002, by IWNS.org, for this online version