Lawsonia intracellularis infection in horses (Proceedings)

Lawsonia intracellularis is the causative agent of equine proliferative enteropathy (EPE). Although GI disease caused by the Lawsonia organism has been identified and characterized in other species for some time – most notably as the cause of proliferative ileitis in swine – the causative organism was not identified as Lawsonia until 1990. The clinical entity in horses was first described in case reports published in 2000. The presenting picture is so distinctive it is unlikely that the disease existed in its present form earlier. Since first being described, the annual number of cases and numbers of farms with multiple horses affected has increased.

Whether this is due to a true increase in the number of animals affected or due to an increased awareness is not known. Equine proliferative enteritis has been diagnosed in horses throughout the United States. In addition to swine, multiple native wildlife species harbor Lawsonia in their GI tract and shed the organism. Many of these, such as rabbits, other rodents, and deer share pastures with horses. The vast majority of equine cases have no history of exposure to swine or swine facilities, making spread from wildlife reservoirs the most likely source of the organism in horses. Subclinical shedders have been documented among swine and rabbits. The presence of adult carrier horses cannot be ruled out, but serologic, PCR, and culture evidence does not support this.

Organism

Lawsonia intracellularis is a microaerophilic, obligate intracellular organism. It can not be cultured via standard methods, but can be grown in certain cell culture lines. It can survive for up to 2 weeks in the environment under the proper conditions. The difficulty in culturing has hampered the ability of clinicians to make a clear diagnosis, and thus slowed down research efforts. For this reason, information on the clinical course of horses with naturally-occurring and experimental infection is just emerging.

Pathophysiology

Most of what is known about Lawsonia is extrapolated from experimental infections in swine and rodents. Transmission is via the fecal-oral route. Factors such as concomitant disease, crowding, and stress increase the likelihood of an animal developing clinical signs. The stress associated with weaning is particularly associated with the development of clinical signs although unweaned foals may also develop the disease.

Once the organism is ingested, it invades the crypt cells in the ileum, where the bacteria and then the crypt cells multiply. The presence of Lawsonia within the cell stimulates division in the crypt cells, and proliferation of young crypt cells is the hallmark of the disease. As the crypt cells proliferate, the intestinal wall thickens diffusely, and a severe protein losing enteropathy results. The panhypoproteinemia is further exacerbated by the inability of the intestine to absorb amino acids due to generalized malabsorption of foodstuffs.

Clinical signs

One of the most notable features of EPE is the tight age range of affected horses. EPE is a disease of weanling-aged foals. The age range of affected animals can be between 3 and 18 months of age, but the vast majority present when they are between the ages of 4 and 8 months. The disease primarily occurs from August to December. Whether this is because it is the time most of the year's foal crop is at the age of highest prevalence or due to other factors is not known.

The most consistent clinical signs are weight loss, peripheral edema, and depression. The weight loss is often severe and appears to occur extremely rapidly. Owners report animals losing 50 to 100 pounds within days. Edema of the ventral midline, limbs, scrotum, and under the jaw may develop, although many foals are severely hypoproteinemic with no peripheral edema. Diarrhea ranging from cow pie to liquid is often present, and it may appear dark black or tarry in color. Other signs referable to GI disease are often present, including colic, anorexia, and depression.

Clinical pathology

The hallmark of EPE is a severe hypoproteinemia, particularly hypoalbuminemia. Albumin levels below 1 gm/dL are not uncommon. This level of severe hypoproteinemia observed with EPE is a singular finding. A total plasma protein of 3.5 gm/dL or less in a weanling-age foal is extremely suggestive of EPE. Other typical clinical chemistry findings such as decreased sodium, chloride, hypocalcemia, and azotemia are consistent with diarrhea and hypoproteinemia.

Low total calcium concentrations reflect the decrease in the protein-bound fraction secondary to low albumin. Ionized calcium concentrations are generally normal, and clinical signs of hypocalcemia are not present. Fibrinogen concentrations are often in normal ranges or low. When corrected for the decreased total protein concentrations, the fibrinogen may be found to be relatively increased. Complete blood count findings often include a low grade anemia. The leukon is variable, but often there is an inflammatory leukogram with an increased neutrophil count.

Leukopenia is extremely rare, and the presence of a low neutrophil count should suggest another disease process. Peritoneal fluid is generally normal and interpreted as a transudate. Glucose absorption and D-xylose absorption testing is normal. Ultrasound examination will reveal a markedly thickened and edematous small intestine. At times, the small intestinal wall thickening is segmental, so a careful and thorough scanning of the abdomen is always warranted.  EPE can not be ruled out if no abnormal small intestine is imaged using ultrasonography.

Diagnosis

Definitive diagnosis is made by either fecal PCR or positive serum serology. Fecal shedding of the organism is intermittent and stops quickly after the initiation of therapy, thus there is a relatively high percentage of false negative PCR tests. Foals may be positive on serology as soon as clinical signs appear supporting the belief that infection is present for prolonged periods of time prior to clinical signs.  Diagnosis is made on post-mortem examination by visualization of intracellular bacteria in the small intestinal crypt cells and the presence of proliferation of intestinal crypts.

Epidemiology

If EPE is diagnosed in one weanling on a farm, all other foals of similar ages should be tested for the presence of Lawsonia infection. It is not uncommon to find decreased total blood protein in asymptomatic herdmates of affected animals. Serology for Lawsonia should be submitted on all foals that have been in direct contact with the symptomatic foal, and all should be carefully examined. If a hypoproteinemic foal is treated, more severe disease may be prevented. Even if a large percentage of weanlings on a farm has evidence of disease, it is unusual for the following year's foal crop to be affected. Serial total protein measures can be conducted to monitor for early evidence of EPE, but in most instances an outbreak is a one-time event on any given property. 

A recent study of all resident foals on two breeding farms each with an index case of EPE revealed a relatively high seroprevalence rates at 30% and 34% respectively. Despite this, the number of affected foals was low at 10% and 5% respectively. PCR analysis of the feces was not helpful, as no positives were detected. This study highlights the fact that a large number of foals mount antibody titers without developing clinical disease. In addition, this study illustrates that the presence of a positive antibody titer without supporting clinical signs is not sufficient evidence to make a diagnosis of EPE.

Differential diagnosis

Other causes of GI disease and hypoproteinemia should be considered when presented with a 3-15 month horse. Intestinal parasitism, particularly Parascaris equorum infection, can cause hypoproteinemia and colic. Fecal floatation should reveal large numbers of eggs if a patent infection is present. Sand-induced enteritis, severe gastric ulceration, or neoplasia are other infrequent non-infectious causes of hypoproteinemia.

Septic diseases that must be considered include the GI forms of Strep equi or Rhodococcus equi, both of which can be tested for via serology. Peritonitis or peritoneal abscess can be ruled out by peritoneal tap. Colitis caused by Salmonella, Clostridial spp, or other bacterial species can be detected by fecal culture. Typically, foals with septic enterocolitis present with leucopenia and other clinical signs consistent with systemic inflammatory response syndrome.

Treatment

Treating a weanling with EPE is very much a good news/bad news situation. The good news is that there is no septicemia or endotoxemia in horses with this disease. There is little worry of an animal succumbing to overwhelming infection, or of spread of the organism to remote sites. When deaths do occur, it is generally a consequence of the pulmonary edema that follows severe hypoproteinemia. The bad news is that this is not a disease a weanling will “snap out of” even when appropriate therapy is instituted. The clinical course is often prolonged. Often multiple plasma transfusions or administration of other colloids such as hetastarch are needed, making the total cost of treatment quite high.

Antimicrobial treatment with an agent that has activity intracellularly is imperative. Oral erythromycin (37.5 mg/kg PO BID), either alone or in combination with rifampin (5 mg/kg PO BID), can be given. Erythromycin should be given with caution to foals that have not been weaned, as the dams of foals receiving erythromycin may develop necrotizing enterocolitis. Other oral macrolides such as azithromycin (10 mg/kg PO SID) or clarithromycin (7.5 mg/kg PO BID) may also be given. They can be more expensive, but are much more convenient to administer due to their longer dosing intervals. Other therapies that have been described include oxytetracycline (10 mg/kg IV SID), oral doxycycline (10 mg/kg PO BID), and chloramphenicol (50 mg/kg PO QID).

No matter which antibiotic is chosen, a minimum of three week's treatment is needed, and often therapy must be given for a much longer period of time. Typically signs of colic and diarrhea resolve shortly after antibiotic therapy is commenced. Hypoproteinemia often persists for weeks. Supportive therapy, particularly to increase resting oncotic pressures by increasing total protein, is often needed. It is not unusual to administer several liters of plasma on more than one occasion. Hetastarch at a dose of 4-10 ml/kg IV is a viable plasma substitute. Other supportive therapies such as intravenous fluids and parenteral nutrition may be beneficial.

Prevention

It is difficult to give firm recommendations on how to best prevent EPE as so little is known about the disease in horses. Limiting overcrowding, stress, and other diseases would seem like logical control measures, although many affected foals appear to have been given excellent care. If a foal is diagnosed with EPE, it should be isolated from other weanlings to prevent spread of the disease. Stalls and other areas can be cleaned with quaternary ammonium products in order to kill Lawsonia organisms in the environment.

Before disinfection, removal of as much organic material as possible is necessary, as the organism is viable in feces for up to two weeks. Asymptomatic weanlings on the farm should be tested specifically for Lawsonia via serology and by determination of total plasma protein concentration. There is attenuated live vaccine against Lawsonia intracellularis that is available for swine. Administration of the swine vaccine to horses will result in seroconversion and detectable antibody titers. However, it is not known whether the vaccine is protective in horses.