Transcript DISEASE, RECOVERY AND DIAGNOSIS

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
Objective
We will cover disease, recovery
and diagnosis in outline.
DISEASE
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.
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
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
RECOVERY
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.
Reasons for a failure to recover
DIAGNOSIS
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.
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
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.
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.
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.