UK Equine Showcase Presents Young Horse Research

The UK Equine Showcase held Jan. 20 focused on young horse heath, including infectious disease and deworming.

Photo: Anne M. Eberhardt/The Horse

The UK Equine Showcase held Jan. 20 focused on young horse heath; here is some research presented during the course:

Common Infectious Diseases of the Young Horse 

Young horses are more susceptible to infectious diseases because of the nature of the equine placenta: No maternal antibody is transferred directly to the foal in utero, according to David Horohov, PhD, William Robert Mills chair and professor at the University of Kentucky Gluck Equine Research Center. 

A foal’s cell mediated immune system is known to be competent, but naïve, said Horhov. Foals are born with an immature immune system that has to develop to produce antibodies on its own over time. However, the initial ingestion of colostrum (the mare’s antibody-rich first milk) allows for passive transfer of immunoglobulins, which provide almost immediate immunity against various infections, all dependent upon quantity and quality of the maternal antibodies. 
“But, foals do not approach adult levels prior to about 3 months old,” Horohov said.
In most cases, foals born without adequate amount of colostral antibodies from the mare are uniquely susceptible to infectious agents, he explained. A failure of passive transfer is not always obvious since the foal does not exhibit any clinical signs until weeks later. Thus, Horohov recommends breeders keep an eye on the neonatal foal and make sure he obtains healthy antibody levels within the first 12 hours.
“Besides adequate colostrum, vaccination of the mare prior to pregnancy is probably the best protection against viral and bacterial infections in foals, until it’s capable to develop antibodies on its own later in life,” Horohov said. “Foal mortality is a significant problem for the equine industry, and infectious disease plays a major role in this matter.”
According to a study performed in Central Kentucky, the primary causes of foal mortality from birth to 9 months old in two-thirds of all foals were from infectious diseases such as septicemia (bloodstream infection), gastrointestinal disease, and/or respiratory disease. The majority of sepsis cases and deaths occurring in the youngest foal, according to Horohov, suggest that the illness is likely to be related to birth. Septicemia is a systemic disease associated with the presence of bacteria in the blood spreading to the central nervous system, other organs, bones, and joints. Bacteria are normally present in the foal’s environment, but occasionally they invade the foal and take advantage of his immature immune system. Some of the dominant bacteria involved in foal septicemia are Escherichia coli (32%), Salmonella (11%), Klebsiella (7%), Enterococcus (2%), and Actinobacillus (7%). 
“Foal diarrhea is one of the most common illnesses and probably the most difficult to deal with,” Horohov said. 
The causative agents for foal diarrhea range from bacteria to viruses to parasites. He said there are probably be a lot of cases that go undiagnosed. Agents causing foal diarrhea include rotavirus, Clostridium, Salmonella, and the parasite Parascaris equorum. While most foals carry clostridial bacteria, it is yet unknown why some foals develop severe diarrheal disease. The role their immune system plays in preventing this disease is unknown. 
Respiratory Disease
Signs of foal respiratory disease vary from the occasional snotty nose to bronchopneumonia depending on the severity of the respiratory problem, Horohov explained. The agents involved are typically viruses such as equine herpesvirus, (EHV-1, EHV-4, EHV-2), equine adenovirus, equine rhinovirus, and equine influenza. These infections result in acute febrile infection and typically resolve after a few days. Bacterial agents such as Rhodococcus equi, Streptococcus equi, Actinobacillus, Salmonella, E. coli, and Klebsiella are typically more severe and last longer. These infections require therapeutic intervention and can be life-threatening if not treated promptly.
One of the main causes of respiratory disease in foals is the hardy bacterium Rhodococcus equi, which commonly causes pneumonia. Half the causes of foal mortality from 1 to 6 months old are associated with bacterial agents such as Rhodococcus equi, Salmonella, and Streptococcus equi spp., whereas one-third are associated with viruses. Foals younger than 6 months old are susceptible to Rhodoccocus equi, and foals younger than 3 months are at even higher risk. 
“The tricky part is that they might have become exposed and infected before 2 weeks old and the initial clinical signs of disease typically manifest several weeks later. The peak in disease in young foals likely occurs when maternal antibodies derived from the mare decline,” Horohov said. 
According to Horohov, numerous studies have demonstrated foals are deficient in their production of interferon-gamma (IFNγ) in comparison to the mature horse. Researchers have proposed that foals are born with an inherent inability to mount a Th1-based cell mediated immune response that might contribute to their susceptibility to intracellular pathogens such as Rhodococcus equi
“We do not yet fully understand the underlying mechanism responsible for this deficiency, however,” he said.
 Horohov and his colleagues previously examined whether common immunostimulants could accelerate the young foal's ability to produce IFNγ to better fight infections. Resistance to this disease appears to be associated to IFNγ production. 
“We were able to measure increased levels of IFNγ after 14 days, whereas earlier on in life they seemed incapable to respond to the stimulants,” Horohov said. 
This indicates that a foal’s ability to produce IFNγ increases with age. He said the risk for infectious disease in the foal is likely to be a combination of immune status, environmental factors, and farm management. All factors play a significant role, and prevention might be the best choice to decrease the risk of infectious disease. 
Vaccination Strategies and Immunity 
Many serious infectious diseases occur early in life and vaccination, along with management measures, remains the primary method for effective infectious disease control. 
Amanda Adams, PhD, assistant professor at the University of Kentucky Gluck Equine Research Center, said young horses should be vaccinated to help prevent disease, induce immunity, and reduce disease severity and spread.
The goal of any vaccination program is to induce immunity, which is a state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. The three types of immunity are natural acquired immunity (induced by infection), artificially acquired immunity (induced by vaccination), and passive acquired immunity (provided to the foal via colostrum). 
Vaccinating the Foal in the Face of Maternal Antibody
The neonatal foal is born with a naive immune system, but inherits immediate protection through the vaccinated mare’s colostrum, Adams said. The mare’s initial maternal antibodies decline over time, subsequently leaving the foal uniquely susceptible to a variety of infectious diseases. Depending on the quantity and quality of maternal antibodies the mare passes to her foal, the duration of protection varies widely. 
“Make sure not to interfere in the face of maternal antibodies or leave the foal unprotected after the waning of material antibodies,” Adams said.  “Foals need to develop adequate immunity against viral and bacterial infections in the environment. This makes it somewhat a challenge to pinpoint the timing of the foal’s first vaccination, since you will need to know the duration of the maternally derived antibodies in order to start a foal vaccination regime.”
According to Adams, vaccination of foals can be associated with a number of difficulties due to their limited cell-mediated immune response to the currently available vaccines. She said researchers have observed in numerous studies that if maternal antibodies are still circulating at a high level in the foal, they can block the foal's response to multiple types of equine influenza vaccines. 
It was shown in a 2001 study that 3-month-old foals failed to show increases in antibody titers against either influenza or tetanus subisotypes in response to two doses of vaccines. They generally needed one to three additional booster doses of vaccine to achieve titers similar to those achieved by yearlings after two doses. All three groups of inactivated, live, or vectored recombinant vaccines, however, failed to overcome maternal interference of antibody production, Adams said.  
“They are simply immunologically unresponsive until later in life,” she said.  
It has been proposed that mares during pregnancy produce factors that inhibit cell-mediated responses in order to prevent fetal rejection, Adams said, eventually causing delayed cell-mediated responses in the foal. However, the underlying mechanisms responsible need further investigation.
Weaning is a considerably stressful experience, both physically and mentally. Results of several studies in other species emphasize weaning’s impact on the effect of lowering cell-mediated immunity; however, little is known about this in foals. Adams recently performed a study in which cellular immunity was measured following abrupt weaning in foals and showed a significant reduction in cytokine production, which is important for fighting pathogens. 
“Vaccination in the face of weaning stress may not be the best timing to induce an immune response. But, further studies are needed to determine the right time,” Adams said.  “Always minimize the weaning-associated stress which might help limit the impact that weaning has on the immune response.”
A vaccination regime for the foal depends on the mare’s vaccination status, the foal’s age, geographic location, and foals’ and weanlings’ exposure levels. Adams advised to consult a veterinarian for a tailor-made vaccination strategy, since foal immunity is based on both exposure and protection. 
Core vaccinations for foals include Eastern and Western encephalitis (EEE and WEE) viruses, West Nile virus, tetanus, and rabies. If foals are exposed to other horses, they should also receive vaccination against equine herpesvirus types 1 and 4 as well as equine influenza virus (EIV).
When vaccinating against EIV, maternal antibodies can persist until 6 months old and prevent immune responses in foals vaccinated prior to reaching that age, so it is especially important to wait before administering the vaccine to foals. many veterinarians recommend foals begin vaccinations at 3 to 4 months old, followed by one to two boosters at four-week intervals. 
Mares should receive a booster vaccine one to two months prior to parturition, which induces antibody responses that are then passed on the foal via colostrum. Typically this includes vaccinations for tetanus, encephalomyelitis viruses, influenza virus, and rhinopneumonitis virus, with additional vaccinations for Streptococcus equi, Potomac horse fever, and in some circumstances botulism, depending on exposure level and geography, among other factors. 
Deworming Strategies 
The foal and the juvenile horse belong to the age groups that should be given the most attention in terms of parasite control. These groups are particularly susceptible to parasitic infection, because unlike older horses they do not yet have the advantage of acquired immunity to some types of parasites. Martin Nielsen, DVM, PhD, Dipl. EVPC, assistant professor at the University of Kentucky’s Gluck Equine Research Center, recommends designing a parasite control program for foals and young horses with the goal of avoiding parasitic disease, reducing the rate of drug resistance development, and allowing horses to develop acquired immunity. 
“In order to design a reliable parasite control program, we need knowledge about two key issues—know your drugs and your parasites,” Nielsen said.  “There might be huge differences from farm to farm when it comes to drug efficacy, and it should be born in mind that different parasites are present simultaneously. To avoid parasitic disease with manifestations such as stunted growth, diarrhea and colic, you will need to achieve information on the efficacy of drugs used on your farm. We still want to have efficient drugs available in the future for the ‘wormy’ foal.”
According to Nielsen, the cornerstone in parasitology is running fecal egg counts as part of an up-to-date parasite control program. On this basis, he recommends foal owners have a few egg counts performed to exhibit the exposure level of ascarids and strongyles.  
“It’s useful information to know when ascarids are actually kicked out and the strongyles are taking over as well as to monitor the ongoing efficacy of the drugs 14 days after the initial treatment of the foals,” Nielsen said.  
So what happens during the first year of the horse’s life? There is a distinct timeline in terms of which parasites a horse is exposed to as he grows up, Nielsen said. 
He added that breeders should pay particular attention to the ascarid parasites (large roundworms), as they are considered the most pathogenic in foals. 
Strongyloides westeri (threadworm), a mildly pathogenic parasite, is uniquely capable of reproduction in the environment, whereas most other parasites reproduce in the horse, Nielsen said. Parasite infection is transmitted in three possible ways: lactogenic transmission (larvae are passed from mother to foal through the milk), the fecal-oral route (infective larvae are ingested while grazing), and the transdermal route (the larvae penetrate the skin of the horse and enter the blood stream).  
“We rarely see any parasitic disease associated with this parasite, and it occurs almost entirely in the very young foal. This suggests that the foal builds up a strong immunity a few weeks after birth,” Nielsen said.  “The good news is that we don’t have any signs of drug resistance. The tradition has been to treat mares prior to parturition, but it’s questionable whether treatment prior to foaling is needed if mares are well maintained.”
Parascaris equorum (large roundworm) is the number one important parasite in foals less than six months of age with a prevalence of 80% to 100%,” Nielsen said. 
The infective eggs release their larvae in the small intestine, and the larvae subsequently migrate quite extensively within the body. They travel to the liver and lungs where they might cause airway symptoms.
“It has been suggested that this extensive migration could explain the strong immunity, which apparently kicks in at about 6 months of age,” he said.  “Take a fecal sample about the time of weaning. This will provide you with information on when these ascarids disappear and the strongyles take over.”
This is important because it affects dewormer choice, he added. Large numbers of roundworms can lead to intestinal impaction, which is associated with a reserved prognosis for survival. 
“Timing and correct choice of drugs, therefore, should be the key focus in any ascarid control program,” said Nielsen. “Frequent treatment with four to six intervals has been the tradition leading to widespread resistance to ivermectin and moxidectin. Foals 6 months old and older have started encountering strongyle parasites, which can be divided into two main groups of the 100% prevalent small strongyles (cyathostomins) and the large strongyles. 
“The strongyle parasites will accompany the horse for the rest of its life,” he continued. “The large strongyles are considered far the most pathogenic parasite, but they have become very rare in managed horse populations.”
Younger horses tend to have larger worm burdens and shed more strongyle eggs than the mature horse, but horses at all ages harbor cyathostomins, Nielsen noted. 
“This could suggest that the immunity to strongyles is fairly limited,” he said. “We see widespread evidence of resistance to benzimadoles, emerging levels of pyrantel resistance, as well as beginning signs of ivermectin and moxidectin resistance. The good news is that we still have no signs of resistance in large strongyles,” Nielsen said.  
The fourth parasite foals encounter is the ubiquitous tapeworm, also known as Anoplocephala perfoliata (flatworm). Usually it occurs during the same time period as the strongyles. Tapeworms are flat, segmented worms transmitted to the horse by ingesting an intermediate host--oribatid mites—infected with the tapeworm. 
“Because this parasite is uniquely dependent on the oribatid mite, only horses located in areas where these mites are present on pasture are at risk for tapeworms,” Nielsen said. 
Farm prevalences often vary in the range of 20-80%. The parasites stay in the horse’s intestine attached to the wall and might cause impactions, intussusceptions (where the intestine telescopes back onto itself), or spasmodic colic, but infection often goes unnoticed.
 “So far, there are neither established treatment traditions for this parasite nor reports of drug resistance.  We don’t yet have efficient methods available to measure drug resistance in tapeworms,” Nielsen said. “I would recommend an initial tape worm treatment from about 12 months of age, and it should be considered in combination with strongyle treatment based on a modified egg or a serum antibody measurement (ELISA) spring and summer.”
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