Neosporosis

Also known as

Neospora abortion

Description

Several cattle diseases are caused by protozoa, and some are transmitted via the fecal-oral route. An immature stage of the organism is shed in feces of the infected or carrier animal, and picked up orally by a susceptible animal. Some protozoal diseases require two hosts in a predator-prey relationship, such as canine and grazer.

Neospora caninum is a microscopic protozoan parasite with worldwide distribution. Many domestic animals (dogs, cattle, sheep, goats, water buffalo, horses, chickens) and wild and captive animals (deer, rhinoceros, rodents, rabbits, coyotes, wolves, foxes) can be infected. Neosporosis is one of the most common causes of bovine abortion, especially in intensively farmed cows and dairy animals. Neosporosis abortion also occurs in sheep, goats, water buffalo, and camelids, although they may be less susceptible than cattle.

Coccidia, neospora, sarcosporidia and cryptosporidia are all fecal-oral organisms, and cyst-forming. With coccidia and cryptosporidia the cyst is in the intestinal tract and the result is diarrhea. Sarcosporidia form cysts in muscles, including heart tissue.  Neospora form cysts in brain tissue and create inflammation in the heart of a developing fetus, usually killing it. In the aborted fetus there is a lot of swelling around the heart.

Neosporosis causes abortions and premature birth in cattle and is similar to sarcocystosis in that it has a 2-host life cycle between predator and prey. Neospora protozoa can be spread to cattle when carrier dogs or wild animals (coyotes, raccoons, foxes) defecate in cattle feed or on a pasture.

Neospora is the leading cause of abortions in many dairy herds, and these problems are now seen in beef herds as well. This disease has probably been prevalent in cattle for a long time, but this protozoan was discovered only a few decades ago. Researchers have found it to be a common cause of abortions in several countries. Healthy cattle seem to handle the infection fairly well, but protozoa may get the upper hand if a cow gets a large dose of canine fecal material in her feed.

These abortions are often misdiagnosed as something else.  If blood samples are taken, however, some herds are all positive; all the cows have been exposed, yet only a small percentage abort, so there are other factors involved. One study in Canada, for instance, found cows exposed to BVD virus abort more frequently from neospora than cows not exposed to BVD. The viral disease may hinder the immune system and put the cow at greater risk for neospora abortion.

These protozoa affect the fetal brain and heart. Some calves are minimally affected and some are born alive and normal. Congenitally infected calves may be born weak or with neurologic deficits. However, most congenital infections are subclinical.

If a calf is sluggish at birth you don’t know whether it’s due to a slow birth and oxygen deprivation or neospora in the brain. If a cow aborts from neospora, it’s usually during mid to late gestation (4 to 6 months). Unless she’s culled she may have a normal calf the next year, but there’s a chance she’ll pass the infection to her calf, which may remain infected for life.

A certain percentage of those calves will later abort when they become cows, due to neospora--either on their first or second calf. This might explain why dairies often have low level abortion episodes--just low enough they don’t worry about it; then along comes another small outbreak. There are 2 patterns--low level of abortions year round, and mini-outbreaks. In some dairies the outbreaks are not small; they might have 100 cows abort at once.

Blood samples from a pen of pregnant heifers may find they are all positive; they’ve all been exposed, but only a small percentage will actually abort.

Signs

• Abortion or weak calves at birth

Cause

Neosporosis in cattle herds may have endemic (continual) or epidemic abortion patterns, but it is also possible for a herd to have high infection prevalence without a noticeable abortion problem. Both endemic and epidemic transmission patterns in cattle are associated with the presence and number of dogs in and around those farms.

Dogs (canids) are the definitive hosts for N. caninum, which means they are necessary for the parasite to complete its lifecycle. Oocysts are shed in the feces of an infected dog, and after as little as one week under favorable climate conditions they develop (sporulate) and become infective.

Dogs can shed infectious oocysts for a variable period after infection and some of these may survive in the environment for a while. These sporulated oocysts can contaminate feed and water supplies of cattle.

When a cow becomes infected from dog feces, the infection spreads to various tissues in the cow, and if she’s pregnant it invades the placenta, where damage to the placenta or transmission through the placenta to the fetus can occur. This vertical transmission from cow to fetus is considered the main route of infection in cattle, with the same cow able to pass infection to multiple offspring; she still carries the infection in following years.

Abortion may be a result of the primary damage to the placenta and/or the inflammatory response of the cow’s immune system. As a result of the immune response, the immature protozoa can transform into a slowly dividing dormant stage and remain latent until the immune system of the cow is suppressed.

At that time the infection can recrudesce (get going again). The immature dormant stages in tissue cysts can be consumed by dogs (eating the placenta, or meat from the cow’s carcass) and then complete the life cycle of the parasite in the dog.

Dogs shed oocysts in their feces after eating tissues of infected animals. Gray wolves, coyotes, and dingoes are also definitive hosts, and many other wild canids are suspected. Neospora oocysts have an impervious shell that enables them to survival in soil and water for prolonged periods even after the canine feces have decomposed.

Intermediate hosts (cattle) become infected by ingesting oocysts. Cattle do not shed oocysts and thus do not transmit infections to other cattle, but latent infection may endure permanently in their tissues--transmitted to canids when the dogs or other canines eat bovine body tissues.

Dogs become infected by eating infected cattle (including placentas) or deer and can also become infected by consuming raw meat, barnyard chickens, and a variety of wild animals that may carry oocysts in their tissues.

Endemic (continual low-level) abortion in a herd of cattle is mainly associated with recrudescence of latent organisms during pregnancy, followed by transmission to the fetus, although occasional transmission from dogs or other canids may compound the problem.

Epidemic abortion is usually a consequence of sudden large-scale transmission to pregnant cattle, presumably by ingestion of a mixed ration or water that has been contaminated with infected canine feces. The use of mixed rations in dairy herds may account for the greater prevalence of neosporosis in dairy cattle than in extensively grazed beef cattle.

A farm dog is often the culprit. Dogs often follow a feed truck and defecate in fresh feed, in a feed bunk, or on a freshly opened bale of hay. The most common risk is when feed is piled on the ground where carnivores may defecate on it and the contaminated feed is mixed in a ration to put in feed bunks. The entire herd may be exposed to neospora and be at risk for abortions.

There may be a continual low level of abortions year round in a dairy herd or during a calving season due to cows infected before they themselves were born, or occasional outbreaks of abortions in which a group of cows might lose their calves within a weeks’ time, due to contaminated feed.

The outcome of new infection or recrudesce of infection in a cow depends on the stage of pregnancy when the infection occurs. In a non-pregnant animal, infection results in persistent infection. Although infected cattle may conceive at the same rate as non-infected animals, infection/recrudesce in the first trimester of gestation may result in early embryonic death.

Infection/recrudesce in mid-trimester can result in abortion or birth of a weak or brain-damaged live calf (characterized by incoordination), whereas infection in the last trimester will result in a seropositive weak or brain-damaged calf or a normal calf. Since the greatest risk of abortion is in mid to late gestation, pregnancy diagnosis undertaken before 5 months gestation in endemically-infected herds may lead to an overestimate of the number of pregnancies that will actually go full-term.

Prevention

Currently, there are no available Neospora vaccines for cattle or dogs. A vaccine was available at one time, but was withdrawn due to questionable efficacy.

With uninfected herds, strict biosecurity should be maintained to prevent disease entry, and any animals purchased should come from herds known to be free of neospora, or tested before they calve for the first time. All abortions should be carefully checked (including tests on the dam) to monitor herd disease status.

With neospora, there’s always a carnivore in the life cycle. This makes it difficult to control, since it is not always easy to keep dogs or wild carnivores out of the cattle’s environment. It may be impossible to keep dogs/wild canines out of pastures and pens, but you can fence off or protect stored and piled feed. Since stored feed used in mixed rations is the most common route for infection, protecting your feed may reduce the risk considerably.

Infected cattle are assumed to be infected for life, so with infected herds, the goal is to prevent abortion and reduce the risk of vertical transmission. A control policy would be based on knowing the prevalence of infection through testing, especially if the abortion rate is high.

Control of N. caninum infection can be achieved in infected herds by blood testing all dams pre-calving or all calves soon after birth and identifying the dams of the seropositive calves as infected animals. These cows can be culled or bred for beef production only.

Culling infected cows annually and not breeding replacements from seropositive offspring will reduce the infection in a herd rapidly if the transmission by dogs can be limited.

Infected cattle might not need to be culled automatically, however, since other factors such as milk production, genetic merit, etc. may make them worth keeping--since not all pregnancies in a seropositive cow will end in abortion despite the increased risk.

A seropositive cow is no direct threat to her herdmates, though she will be a potential reservoir of infection (for dogs) after calving—and those dogs can then infect other breeding animals.

It is common for dairy and beef herds to have a small percentage of infected cattle. Reducing the risk of Neospora transmission is a useful goal, but complete eradication from a herd is usually impractical.

It is easier to prevent contamination of feedstuffs used in mixed rations. Large dairies might consider erecting dog-proof fences around the area in which feedstuffs are stored outdoors, with automatic gates installed to facilitate daily traffic of machinery. Smaller dairy farms may be able to protect feedstuffs within barns, grain bins, and silos.

In addition to protecting feedstuffs, herds with endemic neosporosis may consider not retaining heifer calves born to seropositive cows, thus reducing the number of infected replacement heifers that enter the breeding herd. The seropositive cows could be bred using beef semen. For seropositive cows with valuable genetics, use of embryo transfer to Neospora-seronegative recipient cows could be considered.

Dead cattle, offal from home slaughter, aborted fetuses, and placentas from calved cows should be discarded in ways to prevent ingestion by dogs. Do not feed raw meat to farm dogs and minimize access by dogs onto pastures grazed by breeding stock.

Blood tests can detect antibodies for the protozoa; a positive test means the animal is infected. Because neosporosis is only one of many causes of abortion, diagnostic efforts should focus on all possible causes. Aborted fetuses should be submitted to a veterinary diagnostic laboratory, together with placenta and a serum sample from the aborting dam.  

Treatment

There is no specific treatment for neosporosis in cattle.