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Infectious disease concepts

This section provides information on the infectious disease timeline and tracing used in EAD responses. It is intended to provide background information on some principles that guide the activities of an EAD response.


Incubation, Latent and Infectious Periods

In any individual animal, the spectrum of disease over time can be described by the incubation period, latent period and infectious period. The temporal relationship between these periods can vary by agent and individual. For many infectious diseases, the latent period is shorter than the incubation period, resulting in shedding of the infectious agent before the animal develops clinical signs. In these instances, quarantining of clinically-affected individuals or groups of animals will not be sufficient to control the outbreak. This presents difficulties in disease control.


Timeline of disease in an individual animal


Tracing

Tracing records movement of animals or items that may have carried infection to and from the property. It aids in determining where the disease has spread from and to so that it can be contained. Rapid tracing of animals is critical in any emergency disease response. Effective tracing minimises the number of properties that may be affected due to possible stock movements.

Priorities for tracing are guided by knowledge of the incubation, latent and infectious period for the specific disease. A timeline is created to determine the period in which infection and transmission of the disease might have taken place:

  • A time window for exposure to the agent, based on the incubation period
  • A time window for spread to other individuals, based on the infectious period

Foot-and-mouth disease example

Following infection, incubation for FMD is 1-14 days, with the most likely being 2-5 days 1. The infectious period usually begins up to a day prior to or at the appearance of clinical signs. Virus can be detected in milk up to 4 days before the appearance of clinical signs. Virus excretion usually ceases about 4 to 5 days after the appearance of vesicles. The most likely period of infectiousness for cattle is between 1 and 5 days.

The oldest lesion present on the premises must be identified and aged as accurately as possible (keep in mind the accuracy to which lesions can be aged). This allows estimation of the date at which clinical signs first appeared on the premises, and back calculation of a time window for when exposure most likely occurred, and thereby prioritisation of back-traces to properties that were the most likely source.

With consideration of the infectious period, forward traces can be prioritised based on the period that susceptible in contact animals could most likely have been infected.

Timeline from UK FMD outbreak (2001)

Gibbens, J.C. et al. 2001. Descriptive epidemiology of the 2001 foot-and-mouth disease epidemic in Great Britain: the first five months. Vet. Rec. 149, 729-743

A dairy farm was visited on the 31st August and the farmer had noticed milk drop and lethargy in some animals ‘yesterday’. All the animals were examined and some lesions consistent with FMD were noted. The oldest lesions present were 4 days old (extensive fibrin deposition and smooth edges).

  • When are the earliest and latest possible dates of infection of animals held on this property?
  • What is the most likely time for transmission to have occurred from animals held on this property to those elsewhere?

Click on the answer below to see if you are correct.

Answer

FMD timeline

Backward trace to premises from which the infected property received animals, humans or items between 13th August and 26th August. Priority is given to movement between 22nd August and 25th August.

Forward trace to premises to which animals, humans or items were sent from 23rd August to 31st August with priority given to movement from 26th August to 31st August. The property would have been put in quarantine on the 31st following notification with no further animals, human and item movements.

Equine influenza example

The incubation period for equine influenza is commonly between 1 to 3 days but can be as long as 5 days with low dose exposure 2. Horses may shed the virus in nasal secretions 24 hours after infection and then for 5–7 days depending on dose and method of infection3.

Horses were first detected as infected at Centennial Parklands Equestrian Centre in Sydney on 24th August 2007. Back tracing led to an equestrian event near Maitland (NSW) held from 17-19 August 2007. If a single horse was assumed to have been infectious on arrival at the equestrian event at Maitland, what are the backwards and forwards tracing windows from this event?

Click on the answer below to see if you are correct.

Answer

All animals that attended the Maitland equestrian event would be traced. The earliest infection of a horse attending the event would be on the 5th August. The latest date a horse could have been infected at the event is on the 19th August.

It is now considered that the consignment containing infected horse(s) that introduced this outbreak into the quarantine station in Sydney arrived on 8 August 2007. The exact pathway of transmission from horses at the quarantine centre to horses at the equestrian event near Maitland on 17-19 August was thoroughly investigated, but could not be definitively identified (Callinan, I, 2008)


1 Alexandersen, S., Zhang, Z., Donaldson, A.I., Garland, A.J.M. (2003). The Pathogenesis and Diagnosis of Foot-and-Mouth Disease, Journal of Comparative Pathology, Volume 129, Issue 1, 2003, Pages 1-36, https://doi.org/10.1016/S0021-9975(03)00041-0.
2 MUMFORD, J. A., HANNANT, D. and JESSETT, D. M. (1990), Experimental infection of ponies with equine influenza (H3N8) viruses by intranasal inoculation or exposure to aerosols. Equine Veterinary Journal, 22: 93-98. doi:10.1111/j.2042-3306.1990.tb04217.x
3 Daly et al, (2011). Equine influenza: A review of an unpredictable virus, The Veterinary Journal, Volume 189, Issue 1, 2011, Pages 7-14, ISSN 1090-0233, https://doi.org/10.1016/j.tvjl.2010.06.026.