Calf Health Spring 2023 – Information Leaflet

Calf Scour Infectious Agents

Neonatal Calf Diarrhoea, commonly called Calf Scour, is the most common cause of death in the neonatal calf worldwide. In the UK and Ireland up to 15% of calves are affected by scour, with around 3% of all calves born dying from the condition.

Scour is a multifactorial disease, meaning that there are many aspects of the calf’s environment and management which affect the level of scour on any farm.

The cause of scour is of course infection by one of the known gut pathogens. The names are well known to most farmers by now, but how important is it that we understand the differences between these bugs, and does it affect our control measures?

Let’s look at each of the 4 common infectious agents in turn, starting with the most common.

Cryptosporidium parvum

‘Crypto’, as it is widely known, is the most commonly identified cause of calf scour in the UK and Ireland. While we usually think of infections being caused by bacteria or viruses, Crypto is neither. It is a single-celled parasite called a Protozoa. This has implications in which medicines are effective in its control – unfortunately not very many!

The source of infection for calves is faeces, either from the adult cow, who may carry the parasite in her gut, or contaminated housing from previous calves. Calves are usually infected during the first week of life and scour follows an incubation period of 3 – 5 days. This means calves with Crypto will usually be seen to start scouring at about 1 week of age.

Research shows that up to 90% of calves will have been exposed to Crypto by the time they are weaned, therefore it is almost guaranteed that Crypto is present on your farm. The trick to preventing Crypto scour  is to ensure parasite numbers are kept low enough for the calf to be able to fight off.

Crypto has an unusual reproductive ability in that some of the eggs (oocysts) can hatch inside the calf causing a constant cycle of infection, so that the calf has no chance of getting clear. This accounts for the depressing picture in Crypto outbreaks where some calves become emaciated and weakened to the point of death, no matter how we try to help them. Other eggs pass out of the calf, contaminating the environment and infecting the next batch of newborns.

There are no medicines which ‘cure’ Crypto infections. Halofuginone and Paromycin reduce parasite multiplication inside the calf and so hopefully reduce the duration of the scour. They also reduce environmental contamination. These medicines do have side effects though and need to be used with care.

There are no vaccines against Crypto.

In a healthy well-fed calf, Crypto is unlikely to be fatal – in fact the signs may be quite mild. If, however, the calf’s immune system is under pressure for any reason, such as poor colostrum intake, inadequate milk feeding, cold environment, or an excessive pathogen challenge from dirty housing, then the resulting disease will be much more severe, and deaths will ensue.

Rotavirus and Coronavirus

These two viruses can be considered together. Rotavirus is by a long way the more common of the two, accounting for almost as many cases of scour as Crypto.

Rotavirus particles are hundreds of times smaller than a Crypto oocyst, but many of the same principles of infection apply. The source of infection is faeces, either from the cow, the environment or other calves. The virus is ingested and invades the cells of the gut. This causes the loss of fluid and greatly inhibits the efficient digestion of milk. This results in scour, the severity of which is related to the size and virulence of the infection and the immune defence the calf can mount. The most important component by far in this immune strength is the amount of good colostrum  the calf has suckled soon after birth.

Rotavirus typically causes a scour that starts from 5 days of age and in most cases has a yellow, ‘custardy’ appearance. By 14 days of age calves are much less susceptible to viral infection. Infected calves may suffer anything from a mild self-limiting scour to profuse diarrhoea resulting in dehydration, collapse and even death.

There are no drugs which are effective in treating the viral infection,  Rehydration therapy (either oral or intravenous),  and excellent nursing care, (heat lamps and clean, deep straw beds  are essential to facilitate recovery of the more severe cases.

In the last 25 years vaccination of the dry cow has proven to be beneficial in increasing the protection offered by the colostrum against both viruses. However, this may still be insufficient in the face of severe virus challenge or other managemental and nutritional problems which lower the calf’s defences.  It is however, an excellent place to start if a farm has a viral scour problem.

E.Coli (Entero.Toxigenic. E. Coli or E.T.E.C.)

E.coli is the only common bacterial cause of neonatal calf diarrhoea (Salmonella can cause diarrhoea in the young calf but requires specific management due to the potential severity of the infection and the risk to people)

There are many different types of E. coli. The one which causes scour is called E. coli K99 (you may have read this on your vaccine box) The number K99, (now confusingly changed to F5) refers to little Velcro-like hairs on the surface of the bacteria which, allow it to stick to the gut cells of the calf and multiply there. Bacteria which do not have these little hairs are washed through the gut and cannot cause an infection. Once attached to the intestinal surface these bacteria release a toxin which causes leakage of fluid into the gut resulting in diarrhoea, dehydration and acidosis

Yet again, the scour can be mild or severe, depending on all the usual factors. The severely affected calves are usually less than 48 hours old. This is a point of differentiation compared to Crypto or the Viruses, both of which are seen at around one week of age.

Only about 5% of diagnosed cases of scour are due to E.T.E.C. The combined scour vaccine offers protection against this pathogen, but vaccinated herds may still have outbreaks of ETEC scour.


While it is really helpful to know something about the scour pathogens, it is becoming increasingly apparent that in the more severe cases and outbreaks of disease, more than one of the bugs will be involved. One organism can invade and weaken the immune system enough to allow a second one to become involved. Together the damage they cause will be worse than anything they could achieve alone. Even so, a good on-farm system of vaccination, excellent colostrum management, good nutrition, warm calf housing and a dry bed should give the calf enough defensive abilities to safely navigate the risk period for scour.

On farms where scour is proving more difficult to manage we should consider the use of a bovine concentrated lactoserum (Locatim®). This is a prescription medicine available from your veterinary surgeon, it is not an antibiotic. A 60 ml oral dose contains a protective quota of antibodies against the causes of calf scour. When given to a new-born calf immediately after birth it boosts the specific immune protection against scour. The normal cow’s colostrum feed is given afterwards. This will protect the new-born calves in the face of an outbreak while the control measures outlined above take effect.