Monday, 15 May 2017 10:36

Calf-scour pathogens: Better the devil you know

Written by  Dr Gemma Chuck

Wet, wintery weather often coincides with busy calving periods and an abundance of young and vulnerable calves. A change in weather can exacerbate the environmental challenge of many calf-scour pathogens, resulting in an increase in morbidity at this time of the year.

This article discusses appropriate and timely diagnosis of calf-scour pathogens, allowing the best outcome for both the producer and the calf.

Why is diagnosis important?

There are many pathogens causing calf diarrhoea and these affect calves at different ages. Often multiple pathogens affect an individual calf at the same time.

Common bacteria affecting young calves include E.coli and Salmonella and only these bacterial pathogens will respond to antibiotics.

Viral pathogens, including rotavirus and coronavirus, and protozoa, such as cryptosporidia and coccidia, will not respond to antibiotics.

Therefore it is essential to obtain an accurate and representative diagnosis to avoid unnecessary and inappropriate treatment. Some causes of calf diarrhoea are preventable through vaccination. However, the vaccines are very specific to the pathogen.

For example, this means that a vaccine indicated for the prevention of rotavirus will be ineffective against Salmonella and vice versa.

Accurate diagnosis of calf-scour pathogens allows more effective and targeted treatment. Remember, calves are usually infected with multiple pathogens at the same time and these may require differing treatments and/or preventative strategies.

Removing the guess-work…

Diagnosis of calf-scour pathogens is usually based on faecal analysis or samples at necropsy.

When taking faecal samples, it is important to obtain samples from UNTREATED calves early on in the disease process.

Calves that have already been treated with antibiotics should not be sampled as the antibiotic treatment can interfere with test results.

It is essential that calves are sampled before any treatment and it is useful to keep some sterile sample pots for this purpose.

Despite many calves being infected with multiple pathogens, not all of the causative pathogens will be continually present in the faeces of a scouring calf.

As a result it is prudent to take samples from at least 3 (preferably 6) calves to help isolate all of the causative pathogens and avoid inconclusive results. If only one or two calves are sampled, then pathogens can be missed and calves may appear unresponsive to treatment.

Necropsies should be performed by veterinarians as soon as possible after the calf has died to improve the chance of obtaining representative samples.

Necropsies are best performed early on in the disease process and it may be necessary to perform a sacrificial necropsy to determine the underlying causes.

How to take a faecal sample:

  1. Pre-label the sterile sample pot with a calf ID, age of calf, your name and the date.
  2. Adequate restraint of the calf is required to obtain a sample. An assistant may be necessary.
  3. Wearing disposable gloves, insert your index finger into the rectum of the calf and hold there. Patience is a virtue. Don’t be tempted to wiggle your finger around- this can cause damage to the lining of the rectum and blood may appear in the sample as a direct result of this.
  4. With your other gloved hand, hold the open sample pot below the level of the anus, to catch the liquid faeces as it passes. Careful- it can be explosive!
  5. Once the sample has been collected, secure the lid tightly and place in a zip lock bag. This will make you popular with your vet.
  6. Either drop off the samples to your vet clinic or store in the fridge for no more than 24 hours.
  7. Care should be taken when handling faecal samples as some of the calf-scour pathogens are zoonotic, meaning they can cause disease in humans.

What happens next?

Samples can be tested in-clinic or sent away to an external laboratory.

The in-clinic tests can provide a good indication of which pathogens are involved but their major limitation is that they do not test for Salmonella.

These tests will help determine viral scour pathogens, E.coli and cryptosporidia and can be done immediately upon collection of the samples.

However, if there is an outbreak of calf scours involving the sudden death of multiple calves, it is wise to get the samples submitted to an external laboratory, as Salmonella may be suspected.

The turn-around time is slower with external laboratory analysis but all the pathogens are tested for, including different Salmonella species.

Various vaccines exist to aid in the prevention of Salmonella and an accurate diagnosis is important for future stock. External laboratories will also test the bacterial pathogens for resistance to different antibiotics.

Regardless of the diagnostic tests used, it is important to record disease events such numbers of sick  and dead calves so that your veterinarian can obtain an accurate history during a disease outbreak.  

Dr Gemma Chuck is a veterinary advisor at Apiam Animal Health.

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