Screening for strangles carriers

Serological screening cannot be relied on to prove a horse is not a strangles carrier, a recent report warns.

Asymptomatic carriers are probably the main way that strangles gains access to a previously unaffected yard. Most horses throw off the infection within a few months of an outbreak. But some remain infectious. They carry and excrete the bacteria without showing signs of disease.

These carriers usually retain the infection in the guttural pouches. With time, pus may solidify to form masses known as “chondroids”. These can harbour the bacteria and excrete them intermittently in the nasal secretions - acting as a source of infection for other horses.

The best way of identifying strangles carriers is by culturing a series of swabs from the nasopharynx, or by examining a guttural pouch wash for Streptococcus equi DNA (a PCR test). This can be time consuming and costly.

Blood samples are easier to obtain, and serological testing for Strep equi antibodies has become popular in clinical practice to screen for strangles carriers.

However, recent research suggests that such results should be interpreted with caution and cannot be relied on to be certain a horse is not a strangles carrier.

In a retrospective clinical study, Andy Durham and Jeremy Kemp-Symonds, compared results obtained by serological testing for antibodies against S equi antigens A and C, with the findings of microbiological sampling of the guttural pouches.

The study included 287 horses, that were examined as part of routine surveillance and quarantine procedures on arrival at a welfare charity quarantine unit. Guttural pouch washes were submitted for culture and PCR testing and blood samples were tested for the Strangles A/C ELISA.

(The Strangles A/C ELISA test gives results in terms of optical density (OD). An OD ≥0.5 to either antigen A or C is considered “positive”; OD≤0.2 “negative”; and OD 0.3-0.4 “borderline”.)

The investigators found no significant association between serological status and the presence of S equi in the guttural pouches. 

S equi was found in the guttural pouches of nine horses (3.1%). Of those, only one (11%) was seropositive using a cut-off of OD ≥0.5. A further two horses fell in the borderline category, leaving six horses with negative results.

So, in this study, over half of the horses with S equi in the guttural pouches, were negative on the Strangles A/C ELISA test.

Durham and Kemp-Symonds conclude: “It would appear that the only currently reliable means of determining S. equi carrier status is microbiological sampling rather than serological screening.

For more details, see:

Failure of serological testing for antigens A and C of Streptococcus equi subspecies equi to identify guttural pouch carriers.

Durham AE, Kemp Symonds J.

Equine Vet J. (2020)

 doi: 10.1111/evj.13276

Barley straw as an aid to weight loss

Feeding a mixture of barley straw and hay is a safe and cost-effective way of promoting weight loss in grazing ponies over winter, according to recent research. 

Obesity is a widespread problem in pleasure horses and ponies. In the past, it was accepted that horses and ponies would lose weight over the winter and then gain weight in the spring when grazing quality improved. Nowadays, however, horses are fed to maintain condition over the winter, and, at the same time, they may be doing less work. 

Obese horses and ponies are at higher risk of laminitis; but shedding that weight is often quite a challenge.

A study conducted by Dr Miranda Dosi and colleagues at the University of Edinburgh Royal School of Veterinary Studies and the Redwings horse sanctuary in Norfolk UK, looked at whether feeding a mixture of straw and hay could be used for reducing weight in grazing ponies over the winter. 

A report of the work is published in the Veterinary Record.

The study involved native type ponies maintained in groups at grass over winter. They were fed supplementary roughage – either hay alone, or an equal mixture of hay and barley straw.

The research team weighed the horses regularly during the four-month long study.

They found that, over the study period, all animals in the hay/straw-fed group lost weight. In the hay only group, three horses lost weight, but overall, horses in that group gained weight. 

One concern of feeding straw is that it might lead to digestive problems such as impaction. However, in this study, there were no reports of colic in either group. Neither were there any reports of laminitis. 

An advantage of the higher fibre barley straw is that it may prolong the time spent eating and may reduce behavioural problems such as aggression.

The researchers conclude “straw is a cost-effective and low-energy roughage, which may be a useful alternative to hay alone when trying to induce weight loss in grazing equids over winter.”

For more details, see:

Inducing weight loss in native ponies: is straw a viable alternative to hay?

Dosi MCM, Kirton R, Hallsworth S, Keen JA, Morgan RA.

Vet Rec. (2020)

doi: 10.1136/vr.105793

Testing for insulin dysregulation

Horses with Equine Metabolic Syndrome (EMS) are susceptible to laminitis associated with insulin dysregulation (ID). Advanced cases have markedly elevated resting insulin levels (Basal Insulin Concentration: BIC), and the diagnosis is straightforward. It is quick and easy to carry out a basal insulin test, only requiring a single blood sample. 

Unfortunately, it may not detect many early cases of ID which can have resting insulin levels within the normal reference range.

This may lead owners to conclude that changes to diet and management are not required.

There is a need for a simple test that can be used by ambulatory veterinarians at the owners’ premises to identify more of those suspect cases of EMS that are missed by the BIC test.

Researchers in the Netherlands investigated to see if using an Oral Sugar Test (OST) would be more useful than BIC for detecting ID.

Linda Van Den Wollenberg and colleagues, studied 90 adult horses, of various breeds, used for recreational purposes, and maintained at the owners’ premises in the Netherlands and Belgium. Their work is published in the Journal of Equine Veterinary Science.

The researchers collected blood samples before and 75 minutes after administering sugar solution (0.45g/kg of commercially available corn syrup.)

They compared the number of horses classified as "ID-positive", "ID-suspect", and "ID–not diagnosed" according to the basal insulin concentration (BIC) (>50mU/L; 20-50mu/L and

There was a significant difference in outcome of the results of the two tests. 

The oral sugar test proved to be more useful than the basal insulin concentration. BIC classified 3 horses as ID-positive and 6 horses as ID-suspect (a total of 9/90, 10%). In contrast, the OST classified 25 of the 90 horses (28%) as ID-positive. 

The researchers conclude that, under field conditions, the OST is preferred over the BIC for use as a screening test for ID.

They add that “In addition, our results support the assumption that obese horses and “easy keeper” breeds have a higher tendency of having ID.” They suggest: “further studies are required to assess the repeatability of the OST as we performed it, under different circumstances and within an individual.”

For more details, see:

Comparison of Two Diagnostic Methods to Detect Insulin Dysregulation in Horses Under Field Conditions.

Linda Van Den Wollenberg, Veerle Vandendriessche, Kees van Maanen, Guillaume H.M.Counotte

Journal of Equine Veterinary Science (2020) Vol 88, 102954

https://doi.org/10.1016/j.jevs.2020.102954