Thursday, March 09, 2006

Farm animal cannibalism strikes again?

Cannibalism is one of those practices that, at least as far as humans go isn't very high on the list of things that are socially acceptable. One of the numerous problems with cannibalistic practices is the transmission of diseases, after all what infects dinner is just as easily going to infect the cannibal. It should be reasonable common sense as a result not to feed a farmed animal the remains of their fellow animals. This would greatly aid the spread of an infectious microorganism through a herd and possibly even rapidly increase virulence (which is often directly correlated to the ease of transmission).

The case example of why this practice shouldn't be performed, with any animal, is the dramatic outbreak of bovine spongiform encephalopathy (BSE) in Britain. BSE was found to be spread by an infectious protein called a prion, which is predominantly found in the brain and spinal matter of the remains of cattle. Worse, these parts were frequently fed back to other cattle as a supplement to their feed, providing an easy method of transmission for the infectious prions. The worst part of the entire discovery was not just that other cattle could be infected in this manner, but the potential spread of the disease between beef from infected cattle and humans. This led to the culling and suffering of a large number of animals and an overall ban on British beef that lasted a considerable time.

The entire result was a heavy economic and consumer confidence toll on British farmers and on the entire beef industry that cost millions (billions?) of pounds. Predictably, the practice of feeding dead cattle back to other cattle was immediately halted. With such a brilliant example of why allowing cannibalism with farm animals is a silly, you would probably imagine that most places would ban the practice regardless of species. Unfortunately, an outbreak of a disease among pigs on South Island farms recently in New Zealand, may be linked to feeding pigs the remains of other pigs imported from overseas (emphasis in the quote below is mine and I shortened the length, indicated by the ….)

An animal health expert says a pig disease that has struck the heart of New Zealand's pork industry is almost certainly Post-weaning Multi-systemic Wasting Syndrome (PMWS).

PMWS is a highly contagious viral disease that kills young pigs up to 12 weeks old. It has forced the closure of about six pig farms in the North Island.

Vets have been investigating a disease outbreak that has so far killed about 1,000 piglets on eight to 10 farms near Christchurch in Canterbury.


He says it is possible that the Canterbury PMWS outbreak has been caused by infected imported pig meat being fed to pigs in foods scraps.

Feeding waste material to pigs has recently been banned again.

Obviously this is a severe disease and has resulted in a heavy economic loss for several farmers. The disappointing part is that lessons of the past from the emergence of BSE clearly weren’t heeded, especially if the outbreak is conclusively found to be linked to cannibalistic practices. Although the disease, called Post-weaning Multi-systemic Wasting Syndrome or PMWS is not suspected to be dangerous to humans, the outbreak still demonstrates important principals about how diseases may result from farming practices.

PMWS is caused by a virus called porcine circovirus 2 (PCV2), which was first identified in 1997 and is probably one of the most puzzling diseases around. PMWS is diagnosed not by using molecular techniques such as antibodies, but instead through the manifestation of certain symptoms that occur over the course of an infection. This is because antibodies to PCV2 can be detected amongst herds that do not show any overt signs of disease, indicating the organism is present but inexplicably not causing an infection. As a result, the following clinical symptoms are used to describe the disease: wasting (rapid loss of body weight), dyspnea, enlarged lymph nodes, diarrhea, pallor (loss of skin colour) and jaundice. Additionally, in some cases animals may exhibit coughing, pyrexia (a fancy term for fever), gastric ulceration (often leading to anemia), meningitis (an infection of the meninges surrounding the brain) and dropping dead (which is terminal).

How PCV2 causes such a wide array of symptoms and even how it initiates disease is the puzzling aspect about it. Experimental animal models have largely failed to reproduce the disease by using PCV2 by itself except in certain cases. Interestingly, more success has been attained when PCV2 was injected with other viruses, such as Porcine parvovirus. Regardless of the mechanisms that determine if the virus becomes infectious, when PCV2 does it targets some of the most vulnerable members of the herd, namely piglets aged between 7-15 weeks old. Possibly the most striking characteristic of PMWS is the overall mortality rates in infected animals, potentially getting as high as 40% and can even induce pregnant sows to abort their fetuses.

Dealing with the virus is equally difficult, because it tends to be widespread throughout the herd and circoviruses are notoriously difficult to kill with antiseptics and detergents. Containment and control requires more practical, such as reducing herd sizes, keeping animals separated by age and reducing animal density. As can be seen, the biology PVC2 and the linking of the outbreak to imported pig meat being used on the farms, raises an intriguing hypothesis as to the potential aetiology of the disease. Let us imagine a potential scenario from a land far, far away….

On a farm in an unknown land, a sick piggie called Mr. Pork wakes up to a pretty bad headache among other things…

Mr. Pork: Owww my head, I feel under the weather here. I know, I’ll go to farmer Brown for help! He’ll know what to do!

Farmer Brown: Why hi there porkie, how’s your day going?

Mr. Pork: I’m feeling a bit under the weather here and I may be getting ill!

Farmer Brown thinks to himself: I’d better do something about this before it spreads!

Farmer Brown: Well I have just the solution there Mr. Pork, we should go and visit Dr. Mallet. He’ll fix you all up!

Mr. Pork: Oh that sounds great, let’s go and –

Image hosting by Photobucket. Image originally from an article written by the onion*CLUNK*

Farmer Brown: Off to the sausage factory for you!

Later at the pork to sausage factory….

Worker 1: What are we going to do with all the left-over pig bits?

Worker 2: Let’s package them up as supplemental feed and send them to New Zealand.

Worker 3: Genius!

Anyway, returning from the realm of imagination, there may be solid indications that this is a possible origin of the PMWS outbreak in New Zealand. The suspected causative agent PCV2 has already been widely identified in New Zealand pig herds, and has caused outbreaks in the North Island previously. It is theoretically possible that a disease transmitted from infected pigs overseas, such as a different serotype of Porcine parvovirus that is not immunologically present here in New Zealand was in the imported meat. Once fed to the pigs on the farms in question, it encountered the ‘native’ PCV2 and potentially triggered the silently waiting virus to a full blown infectious disease.

Of course, this is not the only explanation but given the biology of the disease it’s a fairly compelling place to at least make a start. This is of course where one would expect the government to step, but unfortunately, it doesn’t actually appear such assistance is going to be quite what some farmers may expect:

Canterbury pig farmers at the centre of a disease alert have been given a clear message from the government that it will not step in with an eradication plan.

The Ministry of Agriculture and Forestry's Biosecurity Authority has advised the government that eradication of the suspected disease, PMWS or Post-weaning Multi-systemic Wasting Syndrome, is not viable.

Well, that almost sounds to me as if they are throwing in the towel before they even try to do anything. At the very least, by identifying why the outbreak occurred and potentially trying to contain or eliminate the cause authorities could do a fair amount of good. While I do suspect that elimination of PVC2 may be unfeasible although containment and other measures should at least be attempted. If there is a positive to be taken from the recent outbreaks, it’s that feeding pig remains back to other pigs has been re-banned. I guess you take your victories where you can get them.


Chae C. (2005). A review of porcine circovirus 2-associated syndromes and diseases. The Veterinary Journal, 169:326-336.

Darwich L., J. Segal and E. Mateu (2004). Pathogenesis of postweaning multisystemic wasting syndrome caused by Porcine circovirus 2: an immune riddle. Archives of Virology, 149:857-874.

Harding J.C.S. (2004). The clinical expression and emergence of porcine circovirus 2. Veterinary microbiology, 98:131-135.