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Infectious Diseases Implications in a control programme
By Ray Butcher, Wylie & Partners Table of Contents
Potential risks Potential risks Clearly the potential risks of infectious disease are to both:
[ top of page ] [ table of contents ] 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:
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. [ top of page ] [ table of contents ] 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 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. [ top of page ] [ table of contents ] Other factors The development of a disease is often dependent on other factors apart from the infective organism.
In the dog, Demodex infestation may result in immunosuppression. [ top of page ] [ table of contents ] Predisposing factors Within a colony of animals, whether as a feral population or in a kennel/cattery, the following predisposing factors are therefore important:
[ top of page ] [ table of contents ] 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.
[ top of page ] [ table of contents ] Infectious viral diseases - Dogs
[ top of page ] [ table of contents ] Infectious diseases - Cats
[ top of page ] [ table of contents ] Ectoparasites
[ top of page ] [ table of contents ] Endoparasites
[ top of page ] [ table of contents ] 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:
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.
Pregnant women are regarded as especially at risk from Toxoplasma. [ top of page ] [ table of contents ] Risks in a colony
[ top of page ] [ table of contents ] 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:
Fleas and lice are clearly visible to the naked eye. Ear mites can be seen with a standard otoscope.
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.
Stains/cultures/dyes Test kits are available for serological tests for viruses: FeLV, FIV. Some viral diseases require histopathlogical examination for confirmation.
[ top of page ] [ table of contents ] 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:
[ top of page ] [ table of contents ] Action after assessment Following the assessment, the protocol must outline the possible agreed actions. These might be:
[ top of page ] [ table of contents ] 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:
[ top of page ] [ table of contents ] Cost implications Sadly any discussion about disease control has to be balanced by consideration of the cost:
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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.
The AnimalawBgTeam
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