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. Web siteWeb 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.