Neosporosis

Neosporosis is an important infectious cause of weak calves and abortion in cattle at 4-6 months of gestation, a unique time frame among infectious causes of bovine abortion.

It is an efficiently transmitted parasite with an incidence of almost 90% within some herds.

The major economic losses associated with neosporosis in cattle are caused by reduced reproductive performance, diagnostic investigation, and the cost of replacement if affected cows are culled1.

There is some evidence that seropositive bovine herds achieve lower milk production than seronegative ones2.

Etiology

Neospora caninum is a protozoan parasite, closely related to Toxoplasma gondii, that has emerged as a major cause of reproductive failure in cattle worldwide3.

  • Definitive hosts: Dogs and coyotes
  • Intermediate hosts: Clinical form of neosporosis that is found in: cattle, goats, sheep, deer, and horses4
neospora caninum life cycle

Adapted from Dubey JP. 2005. Neosporosis in cattle. Vet Clin North Am Food Anim Pract 21:473-483.

There are two methods of within-herd transmission:

Horizontal Route

Cows become infected by the ingestion of protozoan oocysts, which are shed by a definitive host (canids).

Vertical Route

Infection is passed from mother to fetus during gestation, which can be healthy but congenitally infected, which contributes significantly to persistence within a herd by propagating the infection to successive generations.

Once it infects an adult cow or bull, a calf, or a fetus, it is maintained as a life-long infection.

Vertical transmission is the major route of transmission, but elimination of vertical transmission may not be enough to eliminate the infection from a herd, because horizontal transmission may still occur5.

Impact on Reproductive Function

Depends on several factors:

  • Gestational age of the fetus at the time of infection
  • Immune status of the dam

Clinical Consequences of Infection During Pregnancy Include:

  • Abortion of the fetus
  • Birth of a weak calf, sometimes showing neurological signs
  • Birth of a clinically healthy but persistently infected calf6

Abortion occurs in mid-gestation, usually between the fourth and sixth month, with no clinical signs of disease in the dam. The aborted fetuses are usually autolysed with no gross lesions, and placentas are not retained.

Clinical Presentation

Abortion may occur as an epidemic or sporadic event and occur at any time of the year.

  • A majority of abortions occur from 4-6 months gestation, with moderate autolysis of the fetus.
  • 95% of calves born to positive dams will be born infected but normal.
  • An infected calf may be born alive with neurological signs, birth defects, and/or born weak and unable to stand.
  • Cows can transmit the infection to their offspring in several consecutive pregnancies or intermittently.

Diagnosis

  • Histopathology and immunohistochemistry of aborted fetuses.
  • Serology of the dam or the fetus:
    • Indirect Fluorescent Antibody Test (IFAT)
    • Enzyme Linked Immunosorbent Assay (ELISA)
    • Direct Agglutination Test (DAT)

The ELISA has sensitivity of more than 95% and specificity of more than 97%, making it a good test for the determination of infected animals and herds, particularly with multiple or strong positive results, or both5.

Best tissues to sample: brain, heart, liver, placenta, and body fluids or serum.

Although lesions of neosporosis are found in several organs, fetal brain is the most consistently affected organ, with the most characteristic lesion being focal encephalitis characterised by necrosis and nonsuppurative inflammation3.

3 cows

Prevention

Control of neospora-associated abortions may involve:

  • Blood testing and selling the offspring of positive cows to reduce vertical transmission.
  • Preventing exposure of the cows to feed and water contaminated with feces from dogs that may contain oocysts to prevent horizontal transmission.

References

  1. Nguyen H, Nam N, Aiumlamai S, Chanlun A, Kanistanon K. (2011). Neospora caninum infection in cattle -economic loss, prevention and control. 10.13140/2.1.4396.9286.
  2. Romero JJ, Breda SV, Vargas B, Dolz G, Frankena K. 2005. Effect of neosporosis on productive and reproductive performance of dairy cattle in Costa Rica. Theriogenology;64(9):1928-39.
  3. Dubey JP. Review of Neospora caninum and neosporosis in animals. The Korean Journal of Parasitology. 2003;41(1):1-16. doi:10.3347/kjp.2003.41.1.1.
  4. Dubey JP. 2005. Neosporosis in cattle. Vet Clin North Am Food Anim Pract 21:473-483.
  5. Haddad JP, Dohoo R, VanLeewen JA (2005). A review of Neospora caninum in dairy and beef cattle--a Canadian perspective. The Canadian veterinary journal = La revue veterinaire canadienne, 46(3), 230-43.
  6. Innes EA, Wright S, Bartley P, Maley S, Macaldowie C, Esteban-Redondo I, Buxton D. 2005. The host-parasite relationship in bovine neosporosis. Vet Immunol Immunopathol. 108(1-2):29-36.