Toxoplasmosis

Pets and livestock: toxoplasmosis, Neospora and Sarcocystis

I work in a veterinary school, and half the questions I get are about animals. Toxoplasma has two close relatives that matter enormously to farmers and vets and that almost nobody outside the field has heard of.

By Jeroen Saeij · Updated 17 September 2026

Toxoplasma in animals

Cats. Cats are the only animals in which the parasite reproduces, and almost none of them get ill from it. Infection is usually silent, shedding lasts a week or two and happens once, and the main risk a cat poses is to the environment rather than to the people it lives with¹. Keep cats indoors and feed them commercial or well-cooked food, and they are unlikely to become infected at all. The cats page has the details.

Sheep and goats. Here the parasite does real damage: infection during pregnancy causes abortion and stillbirth, and toxoplasmosis is a major cause of lamb losses. It is the one situation in which a vaccine exists. Toxovax, a live weakened strain, has protected flocks since the early 1990s². The strain used cannot form cysts, which is why it is safe in sheep, and its being alive is why the same trick cannot be applied to people³.

Wildlife. Some species have little evolutionary experience of the parasite and are exquisitely sensitive to it, marsupials, New World monkeys and marine mammals among them. Southern sea otters along the California coast die of toxoplasmosis acquired from oocysts washed into the sea, most often from an unusual lineage called type X that can be traced to cats in nearby watersheds⁴. A rare strain called COUG has recently killed otters with a distinctive severe inflammation of the body fat⁵, in work led by my UC Davis colleague Karen Shapiro.

Dogs. Dogs can be infected but rarely develop disease, and when they do it is usually alongside another illness that weakens immunity. The parasite that really matters in dogs is the next one.

Neospora caninum: the cattle parasite

Neospora caninum looks so much like Toxoplasma under the microscope that it was misdiagnosed as Toxoplasma until 1988⁶. Its life cycle is the same story with a different lead: dogs and other canids are the animals in which it reproduces and shed the oocysts, and cattle are the intermediate host that suffers.

In cattle it is one of the most important infectious causes of abortion worldwide⁶. A systematic review of the economic literature put the losses at more than 1.3 billion US dollars a year across the ten countries with enough data, two thirds of it in the dairy industry⁷. What makes Neospora so hard to control is that infected cows pass it to their calves in the womb, pregnancy after pregnancy, so the parasite persists in a herd without any dog involved once it is established. Dogs matter for introducing it: a dog that eats an infected placenta or aborted fetus can shed oocysts onto pasture and feed.

Where Neospora breaks the Toxoplasma rule

On the pregnancy page I make a reassuring argument: a woman infected with Toxoplasma before she became pregnant is, with rare exceptions, protected, because the parasite sits in dormant cysts and her immunity keeps it there. The same holds for sheep, which is why a ewe that has been infected or vaccinated is protected in later pregnancies².

For Neospora in cattle that argument fails completely, and it is the most interesting difference between the two parasites⁸. A cow can infect her calf in two ways. She can swallow oocysts while pregnant, the equivalent of a first infection in a pregnant woman. Or, far more often, a cow that has carried the parasite for years, frequently since before her own birth, has her dormant infection wake up during pregnancy and cross the placenta. Researchers call the first route exogenous and the second endogenous⁹. The endogenous route is highly efficient, it recurs in successive pregnancies, and a heifer calf born infected goes on to infect her own calves, so the parasite travels down family lines for generations¹⁰¹¹. The cow has antibodies and immune memory throughout. They are not enough.

Nobody knows why. The leading idea is that pregnancy shifts the immune system away from the inflammatory responses that hold the parasite in check, so that the fetus is tolerated, and that Neospora takes advantage of the lull¹¹. That cannot be the whole explanation, because pregnant women and pregnant ewes go through a comparable shift and their Toxoplasma cysts stay quiet. Why one parasite reawakens in pregnancy and its close relative does not, or why the cow lets it happen and the ewe does not, is an open question, and answering it would say something important about how dormant infections are controlled in general.

In dogs themselves, the disease is a progressive paralysis of the hind limbs in puppies infected before birth, and occasionally neurological or muscle disease in adults. It is not known to infect people.

Control is management rather than medicine: keep dogs away from calving areas, placentas and feed stores, avoid breeding from infected cows, and test before buying replacements. A commercial vaccine was available in some countries for a time and is no longer marketed, and the economic case for a better one has been made for over a decade¹².

Sarcocystis: horses and birds

The third relative is a large family of parasites, each cycling between a predator that sheds it and a prey animal that harbours it in muscle. Most cause no disease. Two are important.

Sarcocystis neurona is shed by opossums and is the usual cause of equine protozoal myeloencephalitis, a serious neurological disease of horses in the Americas, and it also kills sea otters¹³. Keeping opossums away from feed and water is the practical control.

Sarcocystis calchasi causes fatal brain and muscle inflammation in pigeons, first described in racing pigeons in Europe¹⁴. It has since turned up in California in cormorants and captive parrots. The California work was spearheaded by my colleague Professor Asli Mete of the California Animal Health and Food Safety Laboratory at UC Davis, with the California Department of Fish and Wildlife and with my lab as collaborators. In 2019, wildlife rehabilitation centers along the southern California coast admitted Brandt’s cormorants with head tilt, tremors and paralysis, and her team showed that Sarcocystis was responsible¹⁵. The predator that spreads it in Europe is the goshawk; in California the same collaboration found the parasite in the intestines of Cooper’s hawks and red-tailed hawks, which are likely to be the local carriers¹⁶. Bird keepers and rehabilitators should suspect it when pigeons or parrots develop neurological signs, and keep raptor droppings away from aviaries. It does not infect people.

Advice that covers all three

• Do not feed raw meat or offal to cats or dogs. Cooking or deep-freezing removes the risk.
• Keep cats indoors where you can, and keep both cats and dogs away from lambing and calving areas, placentas and dead animals.
• Protect feed and water from cats, dogs, opossums and wild birds.
• For flocks and herds, talk to your veterinarian about vaccination for sheep and about testing and replacement strategies for cattle.

Sources

1. Elmore SA, Jones JL, Conrad PA et al. Toxoplasma gondii: epidemiology, feline clinical aspects, and prevention. Trends Parasitol 2010;26:190-6. PubMed · DOI
2. Buxton D, Innes EA. A commercial vaccine for ovine toxoplasmosis. Parasitology 1995;110 Suppl:S11-6. PubMed · DOI
3. Arranz-Solís D, Saeij JPJ. New Avenues to Design Toxoplasma Vaccines Based on Oocysts and Cysts. Front Immunol 2022;13:910961. PubMed · DOI
4. Shapiro K, VanWormer E, Packham A et al. Type X strains of Toxoplasma gondii are virulent for southern sea otters (Enhydra lutris nereis) and present in felids from nearby watersheds. Proc Biol Sci 2019;286:20191334. PubMed · DOI
5. Sinnott DM, Miller M, Arranz-Solís D et al. Fatal Toxoplasma gondii COUG strain infections in southern sea otters (Enhydra lutris nereis): New insight on contributing factors and parasite serotyping. PLoS One 2025;20:e0332223. PubMed · DOI
6. Dubey JP, Schares G. Neosporosis in animals--the last five years. Vet Parasitol 2011;180:90-108. PubMed · DOI
7. Reichel MP, Alejandra Ayanegui-Alcérreca M, Gondim LF et al. What is the global economic impact of Neospora caninum in cattle - the billion dollar question. Int J Parasitol 2013;43:133-42. PubMed · DOI
8. Innes EA, Bartley PM, Maley SW et al. Comparative host-parasite relationships in ovine toxoplasmosis and bovine neosporosis and strategies for vaccination. Vaccine 2007;25:5495-503. PubMed · DOI
9. Trees AJ, Williams DJ. Endogenous and exogenous transplacental infection in Neospora caninum and Toxoplasma gondii. Trends Parasitol 2005;21:558-61. PubMed · DOI
10. Williams DJ, Hartley CS, Björkman C et al. Endogenous and exogenous transplacental transmission of Neospora caninum - how the route of transmission impacts on epidemiology and control of disease. Parasitology 2009;136:1895-900. PubMed · DOI
11. Innes EA. The host-parasite relationship in pregnant cattle infected with Neospora caninum. Parasitology 2007;134:1903-10. PubMed · DOI
12. Reichel MP, Ellis JT. Neospora caninum--how close are we to development of an efficacious vaccine that prevents abortion in cattle? Int J Parasitol 2009;39:1173-87. PubMed · DOI
13. Dubey JP, Howe DK, Furr M et al. An update on Sarcocystis neurona infections in animals and equine protozoal myeloencephalitis (EPM). Vet Parasitol 2015;209:1-42. PubMed · DOI
14. Olias P, Gruber AD, Heydorn AO et al. A novel Sarcocystis-associated encephalitis and myositis in racing pigeons. Avian Pathol 2009;38:121-8. PubMed · DOI
15. Bamac OE, Rogers KH, Arranz-Solís D et al. Protozoal encephalitis associated with Sarcocystis calchasi and S. falcatula during an epizootic involving Brandt's cormorants (Phalacrocorax penicillatus) in coastal Southern California, USA. Int J Parasitol Parasites Wildl 2020;12:185-191. PubMed · DOI
16. Rogers KH, Arranz-Solís D, Saeij JPJ et al. Sarcocystis calchasi and other Sarcocystidae detected in predatory birds in California, USA. Int J Parasitol Parasites Wildl 2022;17:91-99. PubMed · DOI