Wednesday, July 29, 2009

Infectivity - badgers.

When the word 'tuberculosis, is mentioned it is simplistic to suppose that infectivety is similar between species. In our posting below, we quoted the remarks of VLA personnel on the results of camelid postmortems thus:
.. the predominant distribution of tuberculous lesions in alpacas which they had postmortemed as "within the lungs and associated thoracic lymph nodes", which they say, is "similar to most TB cases diagnosed in British South American camelids." And go on to describe " a heavy concentration of thoracic mycobacterial infection" which indicates that the respiratory route is the most likely means of transmission between these animals in close contact. This effect they say "may well be enhanced when some of the lesions show pulmonary cavitation".
So how does tuberculosis in camelids compare with the disease in badgers or cattle? Buried in the labyrinth of information on this subject is a passage from earlier work, which describes relative infectivety in badgers and cattle;
Within an infected social group of badgers, only a limited proportion of them may be infected, and even fewer individuals may be excreting the bacteria due to the presence of open lesions leaking infected material. The badger can remain clinically normal for many years despite having severe lesions that produce infected material and will often excrete intermittently. The lungs and kidneys are most frequently affected, but infected bite wounds can result in new infections and can themselves excrete large quantities of infected pus into the environment.
And then the important bit:
Tuberculous lesions in badgers differ from those of humans and cattle in that there is little cellular reaction but massive numbers of tubercle bacilli are present that can contaminate the surroundings. Infected lactating sows are known to pass infection to their cubs, and this may be a major means of spreading infection within the sett. Disease does not spread rapidly between social groups unless there is a dispersal of the badgers or there are males fighting to protect their territories.
(Stuart F.A., and Wilesmith J.W. 1988. Tb in badgers: a review. Rev. Sci. Tech,,7(4),929-935.)
From that we assume the authors support local AHO postmortem results in that cattle lesions are generally pretty non-infectious even when open. And that in 'field' conditions (as opposed to shooting zillions of bacteria up the noses of young calves), cattle to cattle transmission is overstated, as we reported here from work done in Ireland.

But badger lesions are highly infectious without appearing large and open and will shed intermittently throughout the animals' life, eventually overwhelming the system and leading to them becoming 'super excreters'. Camelid lesions would appear to be both large, open - and highly charged.

The observations on badgers and cattle by Stuart and Wilesmith was published in 1988 Ten years later, the National Badger Survey, published in 1997 showed that the UK badger population had increased by 77% in the previous decade. No current figure is available for population density, and thus the opportunity for more 'territorial fighting'.

1 comment:

Andrew Proud said...

You have made some exceedingly good points here, Matthew; I agree entirely that very few TB reactors display a pathology which suggests that they are likely to be infecting other members of the herd. May I add a comment on the terms “pulmonary cavitation” and “open lesions”?
Originally the two terms were synonymous, referring to a common finding in humans when lung lesions coalesced and eroded into bronchioles so that large quantities of tuberculous pus were coughed up. This was very rare in cattle but in Maffspeak “open lesion” came to mean any lesion in the lung no matter how small. I was intrigued to find that the Report on the Animal Health Services (predecessor of the CVO’s annual report) for 1966 attributed a small increase in the national percentage of reactors (.005%) to the increased number (20) of herds with “open cases.” However, the same trend in the following year (.004%) accompanied a decline (of 34) in the number of herds with open cases!