Vaccination and Internal Parasite Control for Horses

Throughout the United States, routine vaccination and internal parasite control are key components of preventive healthcare programs for horses. Core vaccines prevent life-threatening diseases with potential to affect all horses regardless of age, region, or use; risk-based vaccines target contagious diseases that can cause illness and affect performance in certain populations of horses. Vaccination and deworming programs are ideally overseen by a veterinarian and are often done in concert with other key parts of an annual wellness program that includes routine physical examination, dentistry, and husbandry consultation. Although this article focuses exclusively on vaccination and internal parasite control, veterinarians are encouraged to engage clients in multifaceted preventive care programs that promote the overall health and longevity of their patients.

Vaccination

The American Association of Equine Practitioners (AAEP) recommends that all horses be vaccinated with core vaccines against tetanus, encephalomyelitis, rabies, and West Nile virus infection (TABLE 1).¹ Vaccination with risk-based vaccines depends on a horse’s risk for acquiring the disease, being affected by the disease, and spreading the infection. Further information and detailed vaccine recommendations can be found at the AAEP Vaccine Guidelines.¹

Yearlings Through Adults

A “one-size-fits-all” vaccination program for adult horses does not exist. In addition to the core vaccines, risk-based vaccines are selected according to a horse’s likelihood of contracting a particular disease and should be evaluated against the risks and cost of vaccination. Regional disease prevalence can drive risk-based vaccination recommendations, as can travel plans and intended use of the horse.

Currently, U.S. Food and Drug Administration (FDA)–approved equine vaccines are available to prevent tetanus, encephalomyelitis (Eastern, Western, and Venezuelan equine encephalitis viruses), West Nile virus infection, rabies, rhinopneumonitis (equine herpesvirus [EHV] types 1 and 4), influenza, strangles (Streptococcus equi subspecies equi), Potomac horse fever, botulism, equine viral arteritis, anthrax, leptospirosis, snake envenomation, and rotavirus infection. Most vaccines are administered via the intramuscular route; intramuscular and intranasal vaccines are available for influenza, EHV infection, and strangles.¹

As for other species, horses require a primary vaccine series (generally 2 to 3 doses administered 4 to 5 weeks apart) to achieve a protective immune response. Maximum immunity is generally achieved 1 to 2 weeks after vaccine series completion. The primary series is routinely administered to horses in the first year of life but can also be administered to adult horses with unknown vaccine history. After receiving the initial vaccine series, most horses will require annual or semiannual booster vaccinations. In the southeastern United States, where mosquito vectors are present year-round, boosters for insect-borne arboviral diseases may be administered more frequently.

Minimal research has been conducted to establish protective antibody titers or other measurements that could be used to dictate frequency of revaccination of horses. Given the current attention in small animal practice to lengthening revaccination intervals for adults, clients may ask their equine veterinarians for this information. Titer testing is also frequently requested by owners of horses known to react to vaccination. At this time, no evidence supports lengthening vaccine intervals for the horse, and doing so may leave the animal at risk for serious diseases. For horses with serious, life-threatening reactions to vaccination, veterinarians are encouraged to consult the AAEP Guidelines for Serology in Horses with Adverse Events from Vaccination.²

Broodmares and Foals

Broodmares require specific vaccines during gestation to prevent abortion and provide antibody-rich colostrum to the newborn foal. Vaccine protocols for pregnant mares use killed vaccine products and generally include vaccination against EHV-1 and EHV-4 at gestation months 5, 7, and 9 as well as a single booster for all core and risk-based vaccines 4 to 6 weeks before foaling, which prevents EHV-associated abortion and promotes high concentrations of antibodies in colostrum.¹

Foals born to properly vaccinated mares that receive adequate colostrum (as determined by routine blood immunoglobulin G concentration testing at 24 hours of age) should begin the primary vaccination series at 6 months of age to ensure timely protection for the foal, while allowing adequate time for clearance of maternal antibodies from the foal’s bloodstream.¹ Maternal antibodies are protective in the first few months of life but can also interfere with the foal’s immune response to vaccines. If a mare was not vaccinated 4 to 6 weeks before foaling or if the foal did not receive colostrum, the vaccination series should begin when foals are 3 to 4 months of age.¹

Equine Core Vaccines

Tetanus

Tetanus is caused by a potent neurotoxin derived from anaerobic, spore-forming bacteria (Clostridium tetani) that live in the soil and can be found in the intestinal tract and feces of horses. C tetani spores survive in the environment for many years, resulting in an ever-present risk for exposure of horses and humans on equine facilities. Horses are at risk of contracting tetanus if they receive a laceration at any location (e.g., a puncture wound to the foot) or any surgical procedure (e.g., castration). Foals that failed to acquire immunity through passive transfer are at increased risk of contracting tetanus via the umbilicus. Also at risk are postpartum mares after dystocia or with retained fetal membranes. The neurotoxin causes progressive muscle rigidity and spasticity (“lock jaw”) that can result in recumbency and death. Mortality can be as high as 80% in affected horses.³ Tetanus can be prevented by an annual vaccination with tetanus toxoid.³

If a horse becomes injured and has not received a tetanus toxoid vaccine in the previous 6 months, a booster is recommended at the time of injury.³ If a horse has never received a tetanus toxoid vaccine and sustains an injury, vaccination with tetanus antitoxin is necessary.³ Tetanus antitoxin induces immediate passive protection that lasts approximately 3 weeks. Historically, tetanus antitoxin was associated with Theiler’s disease (hepatic disease and often failure). Recently, this syndrome has been attributed to equine parvovirus contained in equine biological products, and all FDA-licensed equine biological products are now tested for equine parvovirus before distribution.

Encephalomyelitis

Eastern equine encephalitis (EEE) and Western equine encephalitis (WEE) are insect-transmitted viral diseases that affect the central nervous system, resulting in lethargy, ataxia, blindness, recumbency, and seizures. The disease caused by these viruses is colloquially referred to as “sleeping sickness” because of the initial lethargy, depression, inappetence, and fever. For all of the encephalitides, the virus exists in bird and rodent reservoirs and is transmitted by mosquitoes. Although incidence of disease is low, mortality rates can be high; mortality rates for EEE approach 90% to 95%.⁴ The mortality rate for WEE is significantly less (around 30%), and very few cases have been reported in Florida.⁴ An additional encephalitis virus, Venezuelan equine encephalitis (VEE), occurs in South and Central America, is considered a risk-based vaccine, and is included in some of the multivalent equine encephalitis virus vaccines. Recent VEE activity in Mexico has increased the risk for horses residing along the southern U.S. border, and VEE vaccination of horses residing in these geographic regions is recommended.

West Nile Virus

West Nile virus, a mosquito-transmitted neurologic pathogen, emerged in the United States in 1999 and is now considered endemic to the United States. West Nile virus infection is not as lethal as EEE, but infected horses may require several days of hospitalization and intensive care to survive. The mortality rate for horses infected with West Nile virus is approximately 32% to 35%, and some recovered horses experience long-term neurologic deficits.⁵

Rabies

Rabies is an infrequent but fatal zoonotic neurologic disease that affects all mammals, including humans. Rabies virus is transmitted through the saliva of infected animals. Common wildlife reservoirs for rabies virus include raccoons, skunks, bats, and foxes. Although the incidence of rabies in horses is low, it is a serious public health concern due to the potential for a rabid animal to infect its human caregivers. Thus, vaccination of horses is prudent not only for the health of the horse but also for the safety of people working with the horse. All horses should be vaccinated for rabies annually.¹ Although the label instructions for foals do not recommend a booster for the primary series, it is prudent to consider doing so due to the difficulty of predicting the end of maternal antibody interference. For adult horses receiving the rabies vaccine for the first time, no primary series booster is required; only annual boosters are needed. There is no evidence to support that the vaccination interval can be lengthened (as it can be for small animals).

Risk-Based Vaccines

Risk-based FDA-approved vaccines are currently available for rhinopneumonitis (EHV-1 and EHV-4), influenza, strangles (S equi subspecies equi), Potomac horse fever, botulism, equine viral arteritis, anthrax, leptospirosis, snake envenomation, and rotavirus infection (TABLE 2).¹ Of these, influenza, EHV, and strangles are the most commonly administered and are described in this article. Although risk-based vaccines are indicated for certain horses only, they play a critical role in preventing contagious diseases capable of causing illness; loss of productivity; and, in some cases, loss of life.¹

Influenza

Equine influenza, caused by the orthomyxovirus equine influenza A type 2, is a common contagious respiratory disease. Equine influenza virus is transmitted by aerosolization and inhalation and spreads easily through contact with infected horses and/or contact with fomites (e.g., infected clothing, equipment, brushes, tack). The most susceptible population is young horses (< 3 years) and horses in stressful conditions (e.g., shows, crowding, shipment). Common clinical signs include serous nasal discharge, coughing, anorexia, and fever (39 °C to 41 °C [103 °F to 106 °F]). Coughing horses aerosolize the organism and can spread the virus as far as 150 feet, quickly infecting an entire barn. Infected horses can shed the virus for up to 14 days, during which time humans can inadvertently spread the virus to horses through contaminated hands and clothing. In addition, horses may asymptomatically shed the virus, remaining free of clinical signs while shedding infective virus into their environment.

The respiratory epithelium generally takes at least 21 days to regenerate after infection with equine influenza virus. Clients may be reluctant to provide adequate rest after illness, which can lead to development of bacterial infections or inflammatory airway disease that later affect performance. Candidates for influenza vaccination are young horses, horses that travel frequently, and horses exposed to many other horses. Influenza can be self-limiting, often requiring only supportive nursing care (e.g., anti-inflammatory medication, oral or intravenous fluids, potentially antibiotics to prevent secondary bacterial infection).

Rhinopneumonitis (EHV-1 and EHV-4 Infection)

Infection with EHV may lead to 3 clinical forms of disease: respiratory disease, abortion in pregnant mares, and neurologic disease. EHV-1 causes respiratory disease, abortions, and neurologic disease. EHV-4 causes respiratory disease and infrequent abortions. For all types of EHV, transmission occurs via the respiratory route; infective droplets are broadcasted by coughing and snorting horses. Virus can be shed in nasal secretions for 7 to 10 days and can be spread by contaminated hands, equipment, and people. Infection can also be transmitted via aborted tissues, fluids, and other tissues. Even mares that abort transmit infection by the respiratory route.

As with other species, horses infected with herpesvirus become lifelong carriers, and it is estimated that up to 90% of horses are latently infected with at least 1 strain of EHV.⁶ Latent infection in horses is generally subclinical, and virus is shed only intermittently, particularly during times of stress.

Vaccination for the respiratory form of EHV may not prevent the disease but it will decrease the frequency and severity of clinical signs as well as decrease virus shedding and transmission to other horses. Vaccination is recommended twice a year for at-risk horses (i.e., those that travel, compete, or are kept in mixed populations).¹ There is no vaccine currently available labeled to protect against the neurologic form of EHV.

• Respiratory disease: Clinical respiratory EHV infections most commonly affect young horses, usually weanlings and yearlings. However, outbreaks can be widespread, especially among horses in high-density and stressful environments. Most horses are infected as youngsters and maintain a latent infection over the course of their lives. Older horses play a role in outbreak situations because illness may recrudesce and/or the horse may be subclinically infected but shed virus. Clinical signs of respiratory disease include mild fever, coughing, and nasal discharge.
• Abortive disease: Abortion can occur from 2 weeks to several months after exposure to EHV-1 without mares having shown clinical signs.⁶ Most EHV abortions occur in late gestation (> 7 months) but can very occasionally occur as early as 4 months.⁶ Abortions with premature placental separation (“red-bag” abortions) are commonly associated with EHV infection.
• Neurologic disease: The neurologic form of EHV is referred to as equine herpes myeloencephalopathy and is associated with outbreaks of infectious neurologic disease at boarding facilities, racetracks, and horse shows. Most horses experience respiratory signs and transient fever for 1 to 2 weeks before neurologic signs develop. Onset of neurologic signs may be triggered by stress (e.g., shipping or surgery). The virus will attack the spinal cord and brainstem. The clinical signs are commonly hind-end weakness, urine dribbling, incoordination, toe dragging, dog sitting, and fecal incontinence.

Strangles

Strangles, caused by S equi subspecies equi, is a contagious bacterial infection that most commonly affects young horses. The term “strangles” refers to the clinical signs of severe cases in horses with airway or esophageal impingement caused by enlargement of lymph nodes in the throatlatch region (cervical and retropharyngeal lymph nodes). Many states consider strangles a reportable disease; other states vary in their approach to strangles outbreaks and disease reportability.

Two types of vaccine are currently available for strangles: an M-protein–based inactivated vaccine for intramuscular use and a modified attenuated live bacterial vaccine for intranasal use. The intramuscular vaccine requires a 3-dose primary series. It is labeled for annual booster administration, but twice-yearly administration is common in strangles-endemic regions and for horses at high risk.

In experimental models, both vaccine types decreased the severity of the disease.⁷ Pregnant mares in strangles-endemic regions may be vaccinated 30 days before the expected foaling date, although recent investigations have suggested that the intranasal vaccine does not produce colostral antibodies and, therefore, the intramuscular vaccine should be administered to mares in late pregnancy.⁷

After recovery from strangles, immunity persists in approximately 75% of horses for 5 years or longer.⁷ Serologic testing may be used to assess the level of immunity conferred by natural exposure or vaccination. Because natural exposure or vaccination provide variable levels of immunity, serologic testing may serve as a guideline for determining the need for current or future vaccination. Animals determined by enzyme-linked immunosorbent assay to have titers of 1:3200 or greater should not be vaccinated due to increased risk for purpura hemorrhagica and other life-threatening immune-mediated disorders.⁷

Horses that have signs of strangles should not be vaccinated.

Internal Parasites

Internal parasite control for horses focuses on reducing parasite populations within the individual animal as well as the environment. Parasite species most devastating to adult horses include small strongyles (cyathostomes), large strongyles, and tapeworms; those that most affect foals during their first year of life are ascarids (roundworms).⁸ Less clinically significant equine parasites include bots, pinworms, stomach worms, lungworms, and threadworms.⁸

Unfortunately, widespread development of anthelmintic resistance in small strongyle and ascarid populations has resulted in no single effective anthelmintic compound for horses. Thus, adult horses and foals cannot effectively be dewormed with the same products, and an age-specific deworming strategy is required for effective internal parasite control.

Deworming

In the past, common practice dictated that all horses be dewormed every 4 to 8 weeks in a program called rotational deworming. That practice led to widespread anthelmintic resistance and has since been replaced by targeted treatment programs, which are aimed at reducing fecal egg shedding by horses identified as high shedders (i.e., horses that consistently carry higher internal parasite burdens and shed more infective ova into the environment) and at maintaining refugia (susceptible populations of parasites) in horses identified as low shedders. Targeted deworming identifies high and low shedders and applies anthelmintic treatments more frequently to high shedders and less frequently to low shedders. Collectively, this practice protects refugia and maintains anthelmintic efficacy over time.

The most common method for performing fecal egg counts (FECs) in horses is the modified McMaster technique, which provides a quantitative assessment of fecal egg shedding.⁸ FECs do not necessarily correlate with parasite burden and are not useful for confirming that clinical signs in ill horses are attributable to internal parasites. Rather, FECs are a method for identifying horses that are shedding high numbers of parasite eggs into the environment and are therefore good candidates for targeted anthelmintic treatment.

The AAEP Internal Parasite Guidelines recommend that anthelmintic treatment of adult horses (> 3 years of age) focus on high shedders only, with the caveat that all adult horses (regardless of shedding status) should be dewormed at least once a year with a compound containing a macrocyclic lactone (e.g., ivermectin, moxidectin) and praziquantel.⁸ This strategy reduces overtreatment of low shedders while still providing protection against large strongyles and tapeworms. Although the specific figure varies among parasitologists, there is general agreement that only horses with FECs greater than 200 to 500 eggs/gram (EPG) require deworming.⁸

The overall goal of modern deworming programs is not to eliminate parasites from the horse or herd completely but rather to reduce the overall herd parasite load and minimize pasture contamination by reducing shedding of infective ova from persistently high-shedding individuals in the herd. The timing of treatment varies by geographic region. In general, horses do not need treatment with anthelmintics when extreme heat or cold prevents larvae survival and transmission (e.g., summer in the southeastern United States, winter in the northern United States).

Foals, however, are more susceptible to clinical illness and complications resulting from high ascarid burdens and should receive regular treatments with anthelmintics effective against ascarids until the natural transition to strongyles at approximately 6 to 9 months of age.

• The first deworming is recommended at approximately 2 to 3 months of age with a benzimidazole drug (e.g., fenbendazole) to ensure efficacy against ascarids.⁸
• The second deworming is recommended just before weaning (approximately 4 to 6 months of age).⁸
• The third treatment is recommended at approximately 9 months of age, and treatment should primarily target strongyles (moxidectin or ivermectin).⁸
• The fourth treatment is recommended at approximately 12 months of age and should also target strongyles.⁸

Assessing Anthelmintic Efficacy

Because of developing parasiticide or anthelmintic resistance, which has been documented for all classes of anthelmintics used in horses, fecal egg count reduction tests (FECRTs) should be performed on farms to identify resistance patterns and determine which medication to administer to high shedders.⁹

The FECRT involves determining a FEC by using the modified McMaster test, after which the horse is administered a single dose of anthelmintic. At 14 days later, another FEC is performed and the percentage of egg count reduction is used to determine the efficacy of the anthelmintic (TABLE 3).

FECRT equation:

[(Pre-treatment EPG – Post-treatment EPG) / Pre-treatment EPG] × 100

(Wait 10 to 14 days between pre- and post-treatment samples.)

Environmental Management

Environmental practices aim to minimize parasite burdens by reducing the number of parasites in horses’ environments with the added benefit of not contributing to development of resistance. Recommendations for controlling parasites on horse farms are as follows:

• Prevent overgrazing, and reduce fecal contamination by keeping the number of horses per acre to a minimum.
• Group horses in a pasture by age to reduce exposure to certain parasites and maximize the deworming program geared to that group. Before adding new horses to a group, perform FECs to determine shedding status/parasite burden.
• Clean and dispose of manure in the pasture at least twice a week.
• Mow and harrow pastures regularly to break up manure and expose parasite eggs to the sun. Remove horses from a harrowed pasture for 2 to 4 weeks.
• Do not spread fresh manure on pastures.
• If possible, rotate pastures by allowing other livestock to graze them.

Summary

Vaccination and internal parasite control are key components of infectious disease prevention in horses, and veterinarians are encouraged to incorporate the current AAEP recommendations into their routine equine wellness programs. All horses should be vaccinated with core vaccines and evaluated as to which, if any, risk-based vaccines they should receive. The past practice of rotational deworming has been replaced by targeted treatment programs in an effort to slow the development of anthelmintic resistance.