Re: Compositional commonalities of Milk and Blood

Thanks for the question. At first, the thought of blood and milk being 
similar didn’t seem to gel with me. But, on second thought, there are some 
similarities. Blood is composed of water, proteins, other organic and 
inorganic compounds and the red and white blood cells. Milk is primarily 
composed of water, protein, sugar, fat, vitamins and minerals. Thus, both 
blood and milk are aqueous protein solutions that, in separate ways, provide 
the body with essential elements. 

The exact composition of milk (and blood for that matter) can vary according 
to genetics, age, health, diet and environment. Early after giving birth, 
milk contains antibodies that we usually associate with being in the 
bloodstream. Since a newborn baby or calf has yet to encounter foreign 
substances like bacteria and viruses, these antibodies can provide needed 
protection early in life. This is called passive immunity. 

This leads to your enquiry on Bovine Spongiform Encephalopathy (BSE). First, 
let’s define the causative agent of BSE. BSE is believed to be a prion 
disease. Prions, whose name is derived from "Proteinacious Infectious 
Particles," are proteins that are essentially identical to normal, healthy 
proteins found on the cell surfaces of primarily brain cells. The prions, in 
ways not yet fully understood, "convince" the normal proteins to change 
their shape to one that matches that of the prion. Stanley Prusiner received 
the Noble Prize for suggesting and investigating this radical new mode of 
operation for an infectious agent. The prion disease progresses as more 
proteins take the prion shape and normal shaped proteins decline. 

Prion diseases occur in nearly all animal species. Even yeast have prion-
like proteins. In humans, recent news has focused on Creutzfeldt-Jacob 
disease (CJ). This, too, is a prion disease. However, it is extremely rare 
(only about one in a million will get it) and many cases are genetically 
linked. A new form of CJ, called vCJ (v stands for variant) has recently 
been characterized. Some researchers have suggested a link between vCJ and 
the outbreak of BSE in Britain. However, clear evidence for a case of the 
BSE prion either causing or mutating to cause vCJ in humans has not been 
established (a good article to read on Prions in general and the link 
between BSE and vCJ is by Stanley Prusiner, Science 278(5336), 245-251). 
However, it is possible that the BSE prion could cross species and cause vCJ 
in humans. This week in the journal Nature, data has been presented to show 
that the yeast prions can change to recognize more than one species. 

So is milk from an infected cow a potential danger? Potentially, yes. 
Afterall, milk can contain proteins like antibodies. But the probability 
seems extraordinarly low. To the best of my knowledge, no study has 
concluded that milk contains the BSE prion or is conducive for the transfer 
of the prion as an infectious agent. However, some researchers continue to 
study milk as a possible mode of transmission. 

Prions target brain cells. Thus, logically, they tend to concentrate in 
brain tissue. So the absence of prion in milk is not totally surprising. 
Blood, however, circulates throughout the body and can pick up and carry 
prion to other parts of the body. Because prions are so much like normal 
cellular proteins, an immune response to prions may be absent or weak. Thus, 
eating brain tissues of an infected animal would give one the highest 
probability of infection. This was the case for the prion disease Kuru that 
infected a group of natives of New Guinea that practiced ritualistic 
cannibalism. But depending on the concentration of prion (which may relate 
to the progression of the disease), other parts of the body can be a vehicle 
for transmission.   

In short, the jury is still out on a link between BSE in cows and vCJ in 
humans. As for milk, I know of no study that has shown the transmission of 
BSE by drinking milk…but more studies are currently underway.