Ask the Infectious Disease Expert

This column, brought to you by Merck Animal Health, features insightful answers from leading minds.

“What’s the infectious disease outlook this foaling season?”

When working to protect foals, the infectious diseases to watch for fall into the 2 main categories of gastrointestinal and respiratory disease. Pay special attention to the following illnesses.

ROTAVIRUS

A new strain of equine rotavirus became an issue in Central Kentucky last year and has appeared again this year. It is particularly challenging from a testing standpoint, because not all labs are currently able to identify it on standard PCR panels. While rotavirus doesn’t generally cause much mortality, it can cause a tremendous amount of morbidity in young foals. It is a highly contagious, hearty, non-enveloped virus that’s difficult to kill, so it can cause challenging, widespread outbreaks on farms.

LAWSONIA INTRACELLULARIS

As foals reach the weanling age, watch for Lawsonia. While it is not necessarily a new disease, it’s becoming an issue in areas where practitioners may not have seen many cases in the past. Geographic pockets are emerging here in New York, for example, and more cases are being identified on specific farms. Anecdotal trends point to Lawsonia being an environmental concern that varies from year to year, so it’s an important disease to monitor.

RHODOCOCCUS EQUI

On the respiratory disease front, the major concern is R. equi pneumonia, which can cause significant morbidity and, while mortality is not common, substantial economic loss. Many of our infected patients are about 1 month old. There are limited treatment modalities, and practitioners are encountering more cases of resistant R. equi infection. Prevention is difficult, but we are starting to administer higher volumes of hyperimmune plasma against Rhodococcus. One study recommends giving 2 liters very early in age instead of 1, indicating that may be beneficial to help prevent infection. 1

Twice-daily temperature checks are the number one way to monitor for R. equi, as well as many other infectious diseases. To identify Rhodococcus early, rather than intensive ultrasound screening, I advocate monitoring for fever, as well as outward signs, such as a general quieting and lack of nursing. Ultrasound tends to lead to overtreatment with antibiotics, which promotes resistance of the R. equi bacteria. Many foals have subclinical R. equi infection that will resolve without treatment. Ultrasound results are most useful when combined with other factors, such as bloodwork, transtracheal wash results and temperature trends.

EIV, EHV-1 AND EHV-4

Foals are also susceptible to multiple respiratory viruses, such as equine influenza virus (EIV) and equine herpesvirus type-1 and type-4 (EHV-1 and EHV-4). These viruses can cause significant morbidity, especially in suckling and weanling aged foals. I recommend vaccinating foals as early as possible based on AAEP vaccination guidelines.

Again, routine temperature monitoring, ideally twice a day, is crucial. If time only allows for once-daily temperature checks, measure in the afternoon or evening. Horses with an infection tend to have biphasic fevers, so farms that measure temperature only in the morning, such as before turnout, may miss a lot of fever. To support early disease identification, I’m excited about the Bio- Thermo ® microchips from Merck Animal Health that measure temperatures accurately and rapidly by scanning the implanted chip.

In addition to daily temperature monitoring, farms would do well to step up their biosecurity plans to help limit the impact of infectious diseases. Being more mindful of new horses coming onto the farm is especially important during foaling season and on breeding farms with high traffic. If foals do become ill, I recommend screening for common infectious diseases to help guide treatment, implementing biosecurity measures and reviewing vaccination protocols. These diligent preventive measures and regular monitoring will help protect foals, one of the most vulnerable populations on the farm.

1. Flores-Ahlschwede, P., Kahn, S.K., Ahlschwede, S., Bordin, A.I. and Cohen, N.D. (2022), Transfusion with 2 litres of hyperimmune plasma is superior to transfusion of 1 litre for protecting foals against pneumonia attributed to Rhodococcus equi. Equine Vet Educ, 34: e67-e72. https://doi.org/10.1111/eve.13443.

ABOUT THE AUTHOR

Laura H. Javsicas, VMD, DACVIM (large animal), practices internal medicine at Rhinebeck Equine, LLP, in Rhinebeck, NY. She has a special interest in neonatology, ultrasonography, cardiology, gastrointestinal diseases, and emergency/critical care medicine.

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