DISEASE, RECOVERY AND DIAGNOSIS

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DISEASE, RECOVERY AND DIAGNOSIS

• PETER H. RUSSELL, BVSc, PhD, FRCPath, MRCVS
• Department of Pathology and Infectious Diseases,
  The Royal Veterinary College,
• Royal College Street,
• London NW1 OTU.
• E-mail Web site

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Objective

• We will cover disease, recovery and diagnosis in
  outline.

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DISEASE
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Virulence

• Virulent viruses cause disease in the susceptible
  host(s), avirulent viruses do not.
• Some viruses infect many species e.g. rabies
  whereas others are confined to one species e.g.
  feline peritonitis virus.
• Some viruses are avirulent in one species but
  cause severe disease in a second host e.g.
  HerpesB virus is harmless to monkeys but kills
  man.

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Stages of infection

• 1) Entry
• 2) Primary replication
• 3) Dissemination of virus, usually by the blood
  -incubation period, pyrexia
• 4) Secondary replication in target organs
  -disease
• 5) Excretion
• 6) Latency

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Stages of infection

• 1) Entry
• 2) Primary replication
• 3) Dissemination of virus, usually by the blood
  -incubation period, pyrexia
• 4) Secondary replication in target organs
  -disease
• 5) Excretion
• 6) Latency

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RECOVERY
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Mechanisms of recovery

• Non-specific immunity - inflammation with
  macrophages and interferon - active within hours.
• Virus-specific immunity neutralising antibody
  and cytotoxic T cells. This takes 4-5 days,
  unless the animal has been primed by vaccination
  when it takes 1-2 days.

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Reasons for a failure to recover
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DIAGNOSIS
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Suspicion from clinical signs

• If the disease is notifiable e.g. rabies the
  government must be informed if you suspect its
  presence e.g. a quarantine cat with behavioural
  changes.
• You should contact the diagnostic lab to insure
  that you are sending the correct samples in the
  correct container/medium. You should also sample
  enough animals and take the tissues known to
  contain the virus.

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Testing for virus

• Most viruses are diagnosed by finding antigen.
• For example. Feline leukaemia virus diagnosis
  involves finding antigen in blood, canine
  parvovirus involves antigen in faeces.
• Almost all tests for antigen now involve ELISA
  and take 15mins to 3hours to do. Test-kits
  should include a control ve and/or -ve to make
  sure the test is working correctly

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Testing for antibody after past disease or
vaccination

• Antibody detection also shows up the dog which
  has been infected or vaccinated in the past e.g.
  to confirm a rabies vaccine has worked before
  issue of an import certificate. This is also
  used to confirm the presence of a disease in herd
  when 10 or more animals are sampled.
  Sero-epidemiology is used to show which parts of
  the country contain diseased herds.

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SUMMARY

• Virulent viruses cause disease which is often
  confined to one species.
• Viruses spread from the infection site to the
  target organ e.g. rabies virus from bite wound to
  brain.
• Anaimals excrete virus which is transferred by
  contact, aerosol or inanimate objects e.g.
  clothing and drinking bowls
• Animals make innate and specific immune responses
  but these are not fast enough to control certain
  viruses. Vaccination helps speed up the specific
  immune response.

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SUMMARY (cont.)

• Some animals recover but the virus remains
  latent.
• Virus diagnosis involves the detection of virus
  antigen in a suspicious animal e.g. one with
  haemorrhages or skin ulcers. Isolation is often
  done to collect the virus for further laboratory
  characterisation e.g. serotyping.
• Antibody detection is used to identify animals
  or herds of animals with a slower less obvious
  disease or which have recovered.