Now his actions, or lack thereof, were easily explained. To give him credit, on the outside, he looked fine. However, obviously on the inside, he was far removed from OK. He was the classic example of a subclinical illness — no apparent clinical signs on appearance but the disease process is causing problems under the surface.

We as vets often refer to subclinical disease, but I became aware lately that we never explain this very well. Clinical disease is easy to spot as the animal is visibly suffering the effects.

Subclinical disease does exactly what it says on the tin — it has many damaging and costly effects, only without the obvious clinical signs. There are a few very common diseases that spring to mind that cause problems in their subclinical form.

IBR is a disease that we are all familiar with and the clinical signs have been well documented. A high temperature, clear nasal discharge, rapid breathing and a dramatic drop in milk yield make sick animals very easy to identify.

A sick animal will shed huge amounts of virus which all other animals in the herd will come in contact with.

Some of these will develop clinical signs but a lot of others will seem fine but will have a subclinical form of the disease. This can have devastating economic effects for the herd. Early embryonic death is quite common in an IBR outbreak, but this can go completely unnoticed until the cow “breaks” and comes back into heat well after she was presumed in calf.

At this time of year, a subtle drop in milk yield could be attributed to the heat, less than optimal grass or the expected drop post peak milk production. However, subclinical IBR can also be responsible and studies have shown that it can reduce a herd of cows’ milk yield by 2.6kg/cow/day.

In a 100-cow herd, this amounts to >€600 in just one week. This figure makes it easy to see why IBR vaccination is a no-brainer. In fact, vaccinating 100 cows twice in a year with IBR live vaccine would amount to much less than the €600 lost over one week’s milk.

Coccidiosis is another disease where the subclinical form can be even more serious than the clinical form. It’s very easy to spot a calf suffering from clinical coccidiosis — they will have a bloody scour, be very much off form and will often be forcing.

The subclinical form is much harder to spot and also, to diagnose. Subclinical coccidiosis occurs when the numbers of the parasite ingested are lower or when the calf has developed some immunity against the parasite.

Calves will have a slightly reduced appetite, a reduction in growth rate, a dull coat and a slight scour. Not only does it take time to spot a calf like this, but also, when you do, it can be easy to misdiagnose the condition as worms, summer scour syndrome or copper deficiency.

Faecal sampling will confirm a diagnosis of subclinical coccidosis but by the time it does, it can be too late. Mortality is usually low, but the loss of weight and the time taken to recover has a huge economic impact on a herd.

For example, if a Friesian heifer calf isn’t thriving for even a short period of time, she won’t hit her target weight for bulling next year and wont calve down at 24 months of age. At a conservative replacement cost of €2,000 per in-calf heifer, the cost of subclinical coccidosis becomes very apparent. A strategy for prevention should be devised with your vet to avoid this.

As far as most are concerned, mastitis is obvious — a swollen quarter that, when stripped out reveals discoloured and/or clotted milk. However, the subclinical form is much more common — this is what we are talking about when we refer to somatic cell count. SCC is basically subclinical mastitis — the body responds to a bacterial infection in the udder by using the immune system to send cells to combat the infection.

The affected quarter won’t be swollen and, as is often the case, the cow will self-cure and the only indication of the disease will be a rise in the bulk tank SCC. If the cow doesn’t self-cure, the SCC of the affected quarter will remain high and she will serve of a reservoir of infection to slowly affect the rest of the herd.

There are a number of studies that show the benefit of addressing SCC. One stat stands out — in a 100-cow herd, if SCC is reduced from 250,000 to 150,000, it will increase net farm profit by just over €8,000. A costly subclinical disease is an understatement.

Milk fever is a condition that is also worth mentioning. For every clinical case we treat, we presume that there are 10 more cows in the herd suffering from the subclinical form of the condition. Studies have shown that in some herds, up to 50pc of cows can have lower than desired blood calcium in the first 24 hours after calving.

These cows are more likely to have ketosis, retained cleanings, displaced stomachs, metritis and lower milk production. The cost of this subclinical disease is estimated at greater than €6,000 in a 100 cow herd.

We were soon counting the cost of our man with a subclinical hangover too. He failed to properly close the gate when 30 hyped-up weanlings ran into the yard to be tested. They promptly turned on their heels and ran straight out again.

Running after them yielded no result, save for moving our man’s subclinical condition to the clinical form. I don’t think he’ll be overdoing it on Sunday night for a long time to come.

Eamon O’Connell is a vet with Summerhill Vet Clinic, Nenagh, Co Tipperary