Systemic toxoplasmosis in weaned pigs in Pennsylvania, USA, with a brief review of the literature

Abstract

Toxoplasma gondii is an important apicomplexan parasite in veterinary medicine, with swine prevalence varying due to age, geographic distribution, and production program. Samples from a 6-wk-old pig from a small backyard farm with multisystemic disease concerns were submitted to the Iowa State University–Veterinary Diagnostic Laboratory. Gross findings included severe necrotizing enteritis with pseudomembrane formation and edematous, non-collapsing lungs. Histologic findings were severe necrotizing enterocolitis and random multifocal pulmonary and hepatic necrosis with numerous intralesional protozoal cysts consistent with T. gondii. Immunohistochemistry, reverse-transcription real-time PCR (RT-rtPCR), and serologic tests were positive for T. gondii; concurrent influenza A virus, porcine reproductive and respiratory syndrome virus, porcine circovirus 2, African swine fever virus, and classical swine fever virus were ruled out by RT-rtPCR. Given the worldwide distribution and zoonotic potential of T. gondii, the possibility of infection in swine should be considered.

Toxoplasma gondii is one of the most well-studied parasites due to its medical and veterinary importance. It was discovered in 1908 and its significance noted as the cause of abortion in sheep in 1957.^9,14,15 T. gondii has a worldwide distribution, a wide host range, and is the only member of the genus Toxoplasma. ⁹ The 2 major routes of transmission of T. gondii are ingestion of food or water contaminated with oocysts in feces of the definitive host (felids) or the ingestion of tissue cysts in undercooked meat. ⁷ After ingestion, tachyzoites multiply and encyst in several tissues, particularly in the brain.^1,6

Toxoplasmosis can be congenital or acquired after birth in humans and other species ⁷ ; the disease may have the greatest economic impact in sheep production. ¹⁰ Swine infections are usually subclinical ⁵ ; however, transplacental infections of the sow may result in abortions, premature births, births of weak piglets, and neonatal deaths. ¹⁷ Toxoplasmosis has been observed in both young and adult swine, with disease severity depending on the number of oocysts ingested. Clinical signs in growing pigs include fever, lethargy, weight loss, diarrhea, and respiratory distress. ⁵ The National Animal Health Monitoring System has shown an overall decline in the apparent prevalence of T. gondii in swine since 1990 in the United States to <1%, with infections restricted to small pig establishments.^8,17

In September 2024, serum and tissue samples from a single euthanized 6-wk-old pig, from a small backyard farm in Pennsylvania, were submitted to the Iowa State University–Veterinary Diagnostic Laboratory (ISU-VDL; Ames, IA, USA) along with serum samples from 6 cohort swine on the premises. Farm mortality had not been substantial until this submission, except for a litter with 75% piglet mortality. The farm had 6 sows in a farrow-to-finish style production setting with animals penned in an open-roofed pole barn bedded with straw. The premises also had feedlot cattle, 5 cats, and a reported rodent infestation. There were no significant concerns with the pigs’ feed, general health, and body condition.

Enteric, respiratory, and systemic clinical signs were reported on the submission form, with gross field autopsy findings of pleural and peritoneal adhesions, thickening of the small intestinal wall, and interlobular pulmonary edema. Fresh and formalin-fixed samples included lung, heart, liver, kidney, spleen, lymph node, small intestine, and colon. Lung, lymph node, intestine, and colon were cultured for routine swine pathogens at the ISU-VDL; formalin-fixed tissues were routinely trimmed, paraffin-embedded, and sectioned at 4 μm for H&E staining. Supportive assays included: T. gondii immunohistochemistry (IHC) using a rabbit polyclonal antibody (PU125-UP, 1:400 dilution; BioGenex) with secondary polymer OMap a-Rb HRP, a commercial serum ELISA (IVD Technologies) for the detection of T. gondii–specific IgG [sample:negative (S:N) ratio ≥1.0 was considered positive], and a T. gondii reverse-transcription real-time PCR (RT-rtPCR) assay performed from a paraffin-embedded tissue scroll of small intestinal tissues.

As part of the diagnostic investigations, individual RT-rtPCRs were performed for influenza A virus (IAV) and porcine reproductive and respiratory syndrome virus (PRRSV) on the lung; porcine circovirus 2 (PCV2), African swine fever virus (ASFV), and classical swine fever virus (CSFV) assays were conducted on the spleen.

Gross evaluation of the fresh tissue samples at the ISU-VDL showed severe necrotizing enteritis with pseudomembrane formation and edematous, non-collapsing lung. Histologically, the small intestinal mucosa was diffusely necrotic with numerous tissue protozoal cysts with smooth 0.5-μm eosinophilic capsules densely packed with 1-μm crescentic, basophilic bradyzoites suggestive of T. gondii in the tunica muscularis and mucosa. The necrotic areas had numerous neutrophils admixed with cellular debris, fibrin, and indistinct basophilic bacterial colonies (Fig. 1). The liver had random, variably sized areas of hepatocyte necrosis with loss of cord architecture and associated hemorrhage with infiltrating lymphocytes, plasma cells, and macrophages (Fig. 2). Protozoal cysts were also present in multifocal areas of alveolar septal necrosis in the lung (Fig. 3). T. gondii RT-rtPCR testing of the scroll was positive at an early cycle threshold (Ct) of 16.8, and IHC strongly labeled protozoal cysts and tachyzoites (Fig. 4).

Figures 1–4.

Microscopic, multi-organ lesions of toxoplasmosis in a domestic pig. Figure 1. Segmental necrotizing and suppurative enteritis (asterisk). The mucosa is thickened by numerous neutrophils and cellular debris admixed with basophilic indistinct bacterial colonies. Inset: the tunica muscularis containing a tissue cyst with Toxoplasma gondii bradyzoites. H&E. Figure 2. Multifocal hepatocellular coagulative necrosis (arrows) throughout the parenchyma. Inset: a hepatocyte containing intracytoplasmic T. gondii bradyzoites. H&E. Figure 3. Tissue cysts containing T. gondii bradyzoites (arrowheads) within alveolar septa and areas of alveolar septal necrosis. H&E. Figure 4. Strong immunoreactivity for T. gondii in both the tissue cysts and numerous free tachyzoites in the small intestine. Immunohistochemistry.

There were no significant bacterial isolates from cultured tissues from the euthanized pig, and RT-rtPCR tests for IAV, PRRSV, PCV2, ASFV, and CSFV were negative. Serologic results from the evaluated pig (sample 1) and screening of the other animals and sows from the herd revealed that 5 of the 6 other animals tested were positive for T. gondii IgG antibodies (Table 1).

Table 1.

Toxoplasma gondii ELISA results from swine herd-level serum screening.

Sample	Age	S:N result	Interpretation
1	Juvenile	3.07	Positive
2	Juvenile	2.19	Positive
3	Juvenile	0.79	Negative
4	Juvenile	1.81	Positive
5	Adult	1.57	Positive
6	Adult	2.51	Positive
7	Adult	2.83	Positive

S:N = sample:negative ratio.

Multi-organ inflammation and necrosis in our case are consistent with previous descriptions of toxoplasmosis in pigs.^11,17 Outbreaks and prevalence of T. gondii have been associated with outdoor and free-ranging pigs, possibly exposed to cats.^16,20 T. gondii in pigs has been reported worldwide, and seroprevalence varies with age, geographic distribution, production program, and system.^2,18,20 In the United States, the prevalence varies from 1% in conventional pig farms with high hygiene standards, ¹³ to 25% in free-range farming, ¹² and 27.7% in wild boars ¹⁹ ; however, clinical toxoplasmosis in pigs is rare. ¹² Isolated cases have been documented in piglets with intestinal necrosis, pneumonia, and diarrhea. ⁴

The contribution of T. gondii–infected pigs to the overall epidemiology of toxoplasmosis remains unknown; there is no evidence of toxoplasmosis in humans linked to eating pork in the United States. ¹² However, clinical toxoplasmosis and blindness have been associated with ingestion of undercooked pork in Asia. ³ Maintaining cleanliness and promoting healthy practices are the best measures for the prevention of protozoal disease. ⁹ Limiting exposure to domestic and feral cats is key to preventing and controlling toxoplasmosis in pigs.^2,7,12,16 Toxoplasmosis should be considered when investigating multisystemic disease in pigs, in light of the potential for zoonotic disease.